Friday, November 19, 2021

Don’t move firewood, It Bugs me! It really burns me up.

By Jim Janks, Gunnison County Advanced Master Gardener

I have a bumper sticker on my truck that says, “Don’t move firewood, it bugs me.”  I really don’t know if those who read it understand what is trying to be said, but if you care about our natural and urban forests, it may be wise to take an interest in the meaning behind the slogan. 


So, what do I mean by not moving firewood and why would moving firewood bug me in the first place?  Let me give you a hint.  Did you know that an insect introduced into the United States from Asia has caused the death of an estimated twenty million Ash trees, five million of which are in the Southeast corner in the State of Michigan? This same pest has shown up in other states (Ohio, Illinois, Indiana, Pennsylvania, Maryland and Virginia) via firewood and other means.  


The culprit responsible for all this damage is the Emerald Ash Borer (Agrilus planipennis).   It is less than the size of a penny, and has a green metallic look.

Figure 1.


The Emerald Ash Borer (EAB) is a wood-boring beetle native to China and eastern Asia.  It is likely that it came to the United States via some form of wood packaging materials.  No matter how it got into the United States, it’s here.  It is up to us to do our part in helping to prevent the spread of this little critter.


This brings us right back to the bumper sticker.  If the EAB is going to get into Colorado one of the ways it is likely to come is via firewood. I once heard a lecture from Dr. Whitney Cranshaw of Colorado State University who said, “The Great Plains acts as a natural barrier against the Emerald Ash Borer.”  So the most likely way that the EAB will get here is if people bring it with them.

Colorado is a popular vacation and recreation site.  People come to hunt, fish, camp, and enjoy other outdoor activities and when they do, some people bring their own firewood. If they are from any of the infected/quarantine areas, the wood they bring may also bring the Emerald Ash Borer. It is critical for us to be informed and to inform those whom we know that visit Colorado not to pack firewood and haul it to Colorado. It is best to leave their firewood at home and when they get here buy wood locally. One reason for bringing in their own firewood is to save expenses. Let’s admit that it is probably less expensive to bring your firewood in and not buy wood locally, but at what cost?


Before I come to the end of this article, let me make one last observation. Although this article has focused on the Emerald Ash Borer as a potential threat, the EAB is by no means the only pest that needs to be considered when it comes to moving firewood.  There are other bugs and other diseases out there that are not native to our State or County.  These things too could be just as threatening to our ecosystem.  It is also important to realize, in this new global economy, that there will be new potential threats to our environment. There will always be the possibility of new and exotic pests arriving to create new problems for us to deal with.  Not moving firewood may seem a bit trivial or unnecessary, but to save our Ash trees and other trees, it is important that we be proactive in our stewardship. 


There is an Ash tree in Hotchkiss that I visited once a few years ago and I was impressed with its size and beauty.  The first time I walked over to it I tried to put my arms around the trunk.  The best I could do was to get my arms about half way around.  It would be a shame to see this majestic wonder of life die because someone was careless or indifferent in moving firewood.


Don’t move firewood.  It bugs me!

Friday, November 5, 2021

A Helpful Tool for Watering Efficiently

By Kristina Hughes, Clear Creek County Master Gardener 

’Tis the season to start thinking about fall and wintering watering. And in Colorado, it is always the season to think about how to water effectively and efficiently. A few years ago, I was tasked with watering a large planter full of perennials. I directed the hose at the planter for what felt like a good long while. I went back the next day to find that the plants were wilted! I stuck my finger in the soil and it felt moist. I was perplexed. 

I thought about it for a while and then dug out a tool I had hardly ever used: a soil moisture meter. It had prongs that reached 8 inches deep, much deeper than the tip of my finger could reach. When I stuck it in the planter the potting mix was very, very dry. Ah hah! This time I watered the planter until the moisture meter told me it was moist 8 inches down. The plants were much happier. 

Since then, I have frequently used my soil moisture meter to make sure the soil is moist where many of the plant roots actually are, 6-8 inches down. 

And I have been surprised a few times. I was working in a native plant garden this season. We had gotten a lot of moisture in late spring, but then went through a dry spell and the soil seemed to be dry in this garden. I used the moisture meter to double check and found that the top 2-3” of soil was indeed very dry, but deeper down it was still plenty moist. And when I looked at the plants (natives, every one), they looked fine even though they had gotten no rain or water in weeks. (A testament to native plants and a thick layer of mulch!) 

I have to say that the soil moisture meter has become one of my favorite tools for gardening in Colorado. It helps me assess the conditions deep in the soil without having to dig holes After all, half of most plants’ mass is underground where we humans don’t see what is going on. 

The inexpensive soil moisture meters available at hardware stores (the kind I’ve been using) are definitely less robust than the expensive scientific instruments used by commercial operations, but I have found mine to be very useful especially when combined with other observations from the garden. 

So that’s another tool which can help with water management in your landscape. Remember to support your woody plants with water during long spells of warm, dry weather in the fall and winter (see the PlantTalk article on winter watering below for detailed instructions). PlantTalk 1706 - Fall & Winter Watering An assessment of Portable Soil Moisture Meters from University of California Cooperative Extension

Friday, October 22, 2021

Winter Prep Tips for Gardens and Trees

Kristina Kasik, Master Gardener, Gunnison, CO

We’ve already seen a few cold nights that have us forgetting about gardens and thinking about winter: splitting and stacking wood for the stove, buying better tires, prepping the plow, waxing those skis in anticipation of the slopes.  But the growing season is not exactly over, not just yet! There are things you can do still, before the snow flies and the ground freezes solid. 

This is the perfect time to plant.  No, I’m not crazy.  Fall is a great time to plant seeds and bulbs, and it’s sow easy!  Yes, pun intended.  Most of the perennial flowers that do well in our valley, such as penstemon and columbine, need that cold snap of winter to tell the seed when to germinate come spring.  Many annuals are the same, and putting those seeds down now will reward you with hardier plants that require less pampering than store-bought starters. 

Cosmos are a good example. Several gardening websites call them tender annuals, meaning they should not be sown until after danger of frost. Yet the cosmos I planted from seed three seasons ago keep coming up from the seed they produce and drop each fall. The seeds seem to “know” when to start growing and the resulting seedlings can tolerate light frost, unlike transplants. The blooms will be later in the season than transplants, but if you’re really itching for early color, let’s talk bulbs.  

As long as the soil is not frozen you can plant bulbs for your early spring crop of tulips, daffodils, alliums, crocus, hyacinths, and more. I find nothing more rewarding than seeing the hardy colorful blooms appear, even when there may still be snow on the ground.  Ideally bulbs should have a few weeks before the ground freezing to establish themselves so they will have a good strong start in the spring.  Let’s not forget about garlic, either!  This is the one “veggie” crop that you plant just like flower bulbs: in the fall. 

Don’t forget to water. Yes, water. This is a semi-arid climate, after all. Those bulbs you just planted need some water to get some roots going before the big freeze. The trees and shrubs you planted last year, the perennials you planted in the spring?  Yep, they want a drink, too.  

Sunny, windy days plus lack of rain wil wick moisture from the soil and from plant roots. The leaves may be gone, but that tree is not dead. After the leaves have fallen but before the ground freezes (not just above-ground frost - don’t worry about that) is the perfect time to give those trees and shrubs a deep watering, especially evergreens. 

Since evergreens keep their needles all winter, they are even more prone to water loss because of all that extra surface area. Younger or newly planted trees need more water than those that have had several years to establish a large, deep root system.  Not sure when to water or not?  Try to remember this: after the leaves, before the freeze, days above 45 degrees (I swear I just made that up). 

On a related note to watering, you can do a couple of things to prevent water loss leading to damage. Trees may benefit from some wrapping protection, especially those young evergreens. All winter long the sun’s radiation and the wind are pulling moisture from evergreen needles. Trees planted from a container or balled and burlapped typically have a much smaller root system than a wild tree of the same size would have. 

More top to less root means more water loss. Wrap the entire tree with burlap or a similar material, or create a screen on the south, southwest, and windward sides of the evergreen. Wraps should be removed in the spring but screens can stay in place for another season. 

Thin-barked trees such as aspen can suffer from frost cracks and sunscald.  Frost cracks will appear as vertical cracks in the bark, where sunscald will leave a sunken appearance where cells under the bark have died.  Both, in short, are caused by extremes of heating and cooling, and are seen on the south and southwest-facing sides of the main stems of very young trees. 

Both can be prevented by wrapping the trunk with a light colored paper, such as crepe paper, from about November through April. Start from the ground up to the first branch, overlapping the paper in a diagonal fashion.  This is only necessary for the first couple of years, until your new tree builds up a thicker bark layer. 

Mulching is another great way to prevent water loss and adds a layer of insulation to roots from extreme temperature changes. Snow is a perfect mulch, but since we can rarely predict when or how much snow we will get, mulching is a good idea.  

Perennial flower beds will appreciate a 2 to 4-inch layer after the last of the plants have died and turned brown. Those fallen leaves you raked up can be used here, or straw, or a bark or wood chip mulch. Newer trees and shrubs can be mulched as well, but never right up to the trunk or stem, that can hold too much moisture and cause rot. Make a ring around the tree, starting about 6 inches away from the main stem and end at the outermost branch tips. It's as simple as that.

Is any of this information interesting?  Like planting and growing and want to learn more? Never gardened before and want to start?  Consider taking the Colorado Master Gardener class this winter.  To learn about the Colorado Master Gardener program visit



Friday, October 8, 2021

Battling with Rodents in My Garden

By Ed Powers, Jefferson County Master Gardener 

Like many of you, I have battled pests and varmints in my garden. The last 2 years my vegetable gardens have been very disappointing. The previous 7 years I had gardens that produced very little. My major villains are mice, voles, rabbits, chipmunks and ground squirrels. Many do not eat the plant but wait until the fruit is ripe and feast, including tomatoes, squash, rutabagas and beets. Of course, deer and elk eat the whole plant. So last year I did research to try to solve the challenge of hungry varmints in my garden. I ran across a product called a solar powered rodent repeller. Several companies sell these under other names but all are solar powered varmint repellers.
It uses state-of-the-art technology and chases away many types of varmints. It generates a sonic pulse that penetrates the soil and can be heard above ground. Most rotate every 30 to 33 seconds and pulses from 2-3 seconds. They are waterproof but must be pulled before cold weather sets in. The device is environmentally friendly as it eliminates the need for poisons and traps keeping people and pets safe. It can radiate in all directions and can be heard and felt by a number of rodents that live above ground and underground. When fully solar charged, it works at night and when cloudy. I put out four, 20 feet apart to cover not only my gardens but my flowers and trees. I also put flower pots on my deck.

The results have been amazing. The first two weeks we saw some activity but no damage to our plants. The varmints I saw seemed to be confused and ran away from our gardens. None of our cucumbers, squash, tomatoes, rutabagas or beets show any damage and the fruit is great. Deer and elk are not affected by the pulse. I am doing research to see what I can do to change that.

Friday, September 24, 2021

Joe Pye Weed

By Cherie Luke, Jefferson County Master Gardener since 2014 

Eupatorium purpureum which has officially been changed to Eutrochium purpureum, aka Joe Pye weed, is a North American native in the Asteraceae family. There are about 40 species in this genus. The plants range in height from 2’-3’ for Little Joe, to up to 10’ tall and 3’-4’ wide, so give them plenty of space in your garden. They can be planted in average soil and grow well in alkaline soils. Blooms are a beautiful rosy/purple color and bloom from late summer to fall. Plant in full sun to part shade.

The common name, Joe Pye, came from a Mohegan Indian who was an herbalist that lived in the late 1700’s in Massachusetts. Joseph recommended Eutrochium purpureum for the treatment of typhoid fever. These plants induce sweating and were useful in a number of ailments. If you haven’t been to Denver Botanic Gardens lately, I highly recommend you make a reservation to visit. The gardens are still timing entries. The Joe Pye Weed is spectacular! And so are the rest of the gardens!

Friday, September 10, 2021

The Amazing Monarch Butterfly

By Lisa Mason, Horticulture Agent, Colorado State University Extension, Arapahoe County

Monarch butterflies are an iconic species in the United States! These bright orange and black butterflies are known for their migration in North America. How do these small creatures make the 3,000-mile journey every year?


While the bright orange butterflies can be hard to miss, Colorado has a variety of orange-colored butterflies. You can identify a monarch butterfly by the black veins on the wings in addition to the bright orange color. They also have white spots on the edges of the wings. The wingspan usually ranges from 3-4 inches long. You can identify if the butterfly is a male or female by looking at the hind wing. If the butterfly is a male, it will have one black spot on each hind wing along one of the center veins. If the butterfly is female, she will not have a spot.

Monarch caterpillars have contrasting black, yellow, and white stripes on their body. Caterpillar size varies depending on what instar, or stage of growth the caterpillar is in.

A female monarch butterfly. Photo: Lisa Mason

Life Cycle

All butterflies including monarchs go through a lifecycle called metamorphosis that includes an egg, caterpillar, a pupa called a chrysalis, and an adult butterfly. When monarchs are caterpillars, their job is to feed as much as possible. They feed exclusively on milkweed plants (Asclepias spp.). Once they have fully grown, they will find a safe space to form a chrysalis. The chrysalis is a protective covering for the caterpillar while it transforms into a butterfly. It begins as a pale green color, then gradually turns black and orange as the butterfly gets ready to emerge. After emergence, the butterfly will soon search for a mate and the female will begin laying eggs on new milkweed plants.

A monarch caterpillar. Photo: Lisa Mason


Adult butterflies only live two to five weeks. The only exception is the overwintering generation of monarchs that can live up to nine months in Mexico. Once spring arrives, this overwintering generation will migrate north to Texas and surrounding areas. The females will lay eggs for the next generation. Once the next generation becomes adult butterflies, they continue to migrate north. After a few weeks, they will lay eggs for another generation further north. Typically, monarchs will have two to three generations throughout the summer season. Once fall arrives, the fourth generation, also known as the overwintering generation, will begin to migrate south back to Mexico.

How does each generation of monarch know how to navigate migration? For other migratory species like Swainson’s hawks, they follow their parents and large groups of hawks to the overwintering grounds in Argentina. Scientists are still researching how monarchs are able to migrate to the same location every year. Recent research suggests they use a combination of the sun’s position in the sky, landmarks like mountains, and an internal magnetic compass. Genetics may also play a role in the ability to navigate.

Monarchs have two migratory pathways in North America. The eastern monarch population migrates from Mexico up north through the Midwest and eastern US. The western monarch population migrates from the Pacific Coast of California to the states west of the Rocky Mountains. While Colorado is not one of the main migratory corridors, you can still see monarchs throughout our state.

Butterfly Mimicry

Other species look similar to the monarch butterfly include the queen butterfly (Danaus gilippus) that hosts on milkweed and dogbane plants in the Apocynaceae family, and the viceroy butterfly (Limenitis archippus) that hosts on plants in the Salicaseae family including willows, cottonwoods, and poplars.

These three butterflies look similar for one reason: mimicry! The contrasting orange and black colors serves as a warning to predators that the insect may be distasteful and potentially toxic. Milkweed is full of a compound called cardenolides. When a caterpillar feeds on milkweed plants, the cardenolides stay in the body of the monarch, which makes it distasteful and toxic to predators like birds. The predators learn to stay away from insects with the bright coloring.

For a long time, scientists suspected viceroy butterflies mimicked monarchs in a form of Batesian mimicry meaning that the viceroy appeared toxic and distasteful to predators based on coloring and wing shape, but they were not actually toxic or distasteful. Further research indicates that the monarch, viceroy, and queen butterflies may exhibit Müllerian mimicry, meaning all three can be distasteful or toxic to predators and they mimic each other. More research is needed to fully understand this mimicry relationship between the butterflies because variations in the butterfly’s colors, wing shape, distastefulness, and toxicity vary among different regions and caterpillar host plants.

A viceroy butterfly can be differentiated from a monarch by the black, circular line through the hind wing that is perpendicular to the other black veins. 

A queen butterfly has more white spots on the hind wings than a monarch. Colors can be variable but often they are a darker orange color than monarch butterflies. Photos: Lisa Mason

Supporting Monarchs and Other Butterflies

You can support monarchs and other butterflies by providing food, habitat, water, and space in your landscape. Each species of butterfly has a different caterpillar host plant, for instance, monarchs rely on milkweed plants for caterpillar food. Black swallowtail caterpillars feed on dill and fennel. All adult butterflies will visit a variety of other flowers for nectar. Plant flowers that have different bloom times so you will have flowers all season. Butterflies need sunny areas and places to shelter from wind and weather. Planting a variety of trees and shrubs can help provide sheltering areas. Be mindful of pesticide use because they can harm caterpillars and butterflies.

Click here for more information on attracting butterflies to your landscape.

For more information on monarch butterflies, visit

To learn more about butterflies in Colorado, read this CSU Co-Horts blog post called The Fascinating Lives of Butterflies


Friday, August 20, 2021

My Yellow Rose Of Colorado

By Ed Powers, Colorado Master Gardener

I have written two previous blogs about my love of cactus in which I included information about Colorado Cactus identification and information on the cactus garden I tried to complete.  I am still working on that garden.  In one of those previous blogs, I called our prickly Pear Cactus the Yellow Rose of Colorado because it has one of the most beautiful yellow flowers I have ever seen.  

Potted Prickly Pear Cactus early in summer.  Middle of plant is a columbine that is two years old and grown from seed.

The scientific name is Opuntia macrorhiza.  They are usually yellow but sometimes purple.  The pads and fruit (spent flowers) are edible. The pads are modified branches. And of course anyone who has dealt with the cactus is aware of their spines. 

Five years ago I found two pads joined on my deck, at the same time I was fighting varmints in my flower gardens on my deck.  So I decided to do a test.  I planted the pads in an urn pot that had flowers. It grew very slowly for the first four years and never flowered.  However the varmints left the pot alone.  I grew many different annuals in that pot, including a columbine I started from seed. 

This year something changed.  Where before two to three new pads were added each this year, this year we added 10 new pads.  Before we had no flowers, this year we had 28 flowers. Something else occurred, the first flowers bloomed for one day, the later flowers lasted 2-3 days and again they were a bright yellow.

I compared the cactus in the urn to wild ones in the yard that were about the same age as my urn flower.  It was 3-4 times the size of wild ones.

So I must have converted my wild two pads to a large beautiful potted plant.  My family really enjoys our Yellow Rose of Colorado.

Friday, August 6, 2021

Time to Take a Walk Through the Wildflowers

By Susan Carter, CSUE Tri River Area Horticulture and Natural Resource Agent

With drought through much of Western Colorado, and lower snow amounts in many areas, snow melt was earlier this year than normal.  We start our CSU Extension Native Plant Master courses at lower elevations in the Tri River Area and head higher, beginning in April and typically into August.  This year during the first two classes we noticed the plants were done!  Many shriveling from drought and others already going to seed.  So once we heard and saw that the wildflowers on the Grand Mesa were blooming, we moved that class earlier, and boy are we glad we did.

As I am writing this article, it is actually raining- YES! That will help the flowers.  Some of the flowers on the Grand Mesa seemed to be a super bloom or stellar bloom at that.  The five nerved sunflower was one of those that was amazing.  It is called five nerved as it has a mid-vein and four side veins.  The flowers always face to the east, so this is a good one if you get lost. 

Subalpine meadow. Photo by Susan Carter

If you are new to going out into nature, as many people are due to COVID-19, know the rules before you go. Pick up after your dogs, because it can hurt wildlife and takes the enjoyment out of looking at plants with a big pile of dog poo there. Try to stay on trails especially if the soils are fragile or the plants are rare.  We do go off trail in some instances to learn certain plants, but we try to avoid stepping on plants, and step on rocks when possible. Keep your dogs on a leash, this protects them and the wildlife. Don’t dig up or pick flowers.  Save them for others to enjoy, and so the plants can reproduce.  If we all pick or dig, it does end up having a big impact on the plants.  Just look up how Washington DC had to come up with a program to keep people from picking cherry blossoms, it was killing the trees. If you want to dig, visit the entity that the plants are growing on, for example BLM or USFS.  They have inexpensive permits for digging or collecting forestry products, and this allows them to record and realize if some areas are being overused so they can protect plant populations.  And keep your foraging to a minimum.  The wildlife needs it to get through the drought and winter. Visit

Brephidium exile, butterfly, on Aster
So back to the wildflowers, it is so fun to see all the colors and to see all the pollinators.  At higher elevations flies and bumblebees are the two main types of pollinators.  I learned something about fireweed this year.  The flower only lives two days.  The first day the male parts of the flowers mature, and the bumblebee likes to sleep on this flower overnight.  The second day the female parts mature, and guess what, the bumblebee just spread the pollen on the flower while he is there overnight.  Makes you wonder all the different ways that plants and pollinators help each other.  Some flowers, like the white flowered pea and evening primrose, turn a different color once they are pollinated.  The evening primrose can even be pretty as they go from white to pink or yellow to orange.  This tells the pollinators to move on.

While walking through the wildflowers, take time to look down.  We have so many small flowers that you might not notice.  An example would be Rock Androsacea, Easter daisies or the trailing daisy.  It is also good to appreciate that some plants like the Monument plant (also known as Elk weed because the elk like to eat it), only bloom once in their life time!  So there is much to appreciate while you are taking a walk through the wildflowers.  Tread Lightly.

Note Ladybug on Fleabane, Susan Carter photo

Rock Androsace, photo by Susan Carter

From All of us on the Grand Mesa, Take care and enjoy. 

Friday, July 23, 2021

Homegrown Arugula in January!

By Lindsay Graves, Eagle County CSU Extension Master Gardener

Homegrown arugula in January? At 6,700’?  Yep! With some low tech season extension techniques, the hardiest salad greens will overwinter in much of Colorado, so you can enjoy tasty treats in winter and early spring harvests before many gardeners can even plant!

Once you become accustomed to the taste and heft of homegrown leaves, you won't want to go back to store bought.

Let’s consider the challenges:

  • Space

    • Harvested in fall but planted in summer. Our space is typically filled with summer veggies. Where will the greens go?

  • Cold

    • Certain vegetables have adaptations to help them survive.  Some concentrate sugars in their tissues to lower freezing point; some have extra space between their cells so ice crystals don’t cause as much damage.

  • Light

    • Between Nov 15 and Feb 1, days are too short for much growth, so the plants are in basically a waiting state.  When growing fall greens, we have two options: grow them to near full size and hold for fall + early winter harvest, or grow them to a small size and overwinter and resume growth in the spring for late winter + early spring harvest.  

  • Water

    • When the ground is frozen, plants can't drink water. Our dry winds pull moisture out of their tissues.  

While formidable, the challenges can be overcome.  Let’s check out the solutions.

  • Planning

    • Incorporate a patch of cold season greens in your planting plan. They can follow the first planting of greens, peas, or garlic. You'll want to plant in mid July for fall harvest and early Sept for spring harvest (in my garden at 6,700’).

  • Starts vs Seeds

    • Using starts is a great idea! Planting a 4-6 week old plant vs a seed saves you time and space. It can be hard to source seedlings in summer, so start your own. Seeds will work too though if you don't want to grow starts.

  • Season Extension

    • Row covers provide a more even temperature and allow plants to start growing earlier in the morning and later into the evening, so they can size up quickly. Add a row cover after day time highs drop below 85. Add plastic over the row cover when heavy snow threatens (so it doesn't rip the row cover). Plastic must be vented daily until the highs are below 60.

  • Suggested Varieties

    • Lettuce- winter density

    • Mache-any

    • Tatsoi-any

    • Arugula-Speedy

    • Spinach-Space for fall harvest

    • Winter Bloomsdale for spring harvest

The Fall Greens Gardener’s Year-round Checklist


  • Add cold season greens to your planting plan

  • Order cold tolerant varieties with your seed order


  • Clean your seed starting kits thoroughly after their first round of use

  • Replenish any seeds starting supplies that were used up


  • Harvest something to make space for greens

  • Apply a rich compost (like worm castings)

  • Sow greens (in ground or in starting kits). Make last sowings by Sept 15.

  • Cover (row cover after day highs below 85, plastic after day highs are below 60)


  • Harvest greens on warm days when leaves are not frozen. They will resume growth after Feb 1.

My cabbage in October

My lettuce in October

Friday, July 9, 2021

Keeping Your Aspen Trees Healthy

By Eric McPhail, Gunnison County CSU Extension

Everyone enjoys watching the aspen leaves as they glitter in the wind and change colors. But aspens do have enemies. Two of the most common are cankers and Eriophyid mites.  


The term canker describes an area of dead cambium (living cells just beneath the bark) and bark, usually on the tree trunk and usually elliptical in shape. Aspen cankers display great variety.  They can kill individual twigs, branches, and portions of trucks when it succeeds in girdling those parts. They kill many aspen each year in Colorado, and affect all sizes and ages of trees.

Cytospora is probably the most common canker-causing fungus. Look for orange-stained areas of bleeding bark, small orange tendrils of a jellylike material on the bark, and large patches of dead bark. Sooty-bark cankers give the trunk a “barber pole” appearance. On close inspection, the stripes or areas of dead, black bark crumble in one’s hand to a sooty powder. Black cankers are large, black swollen or flared areas on the trunk that contain concentric rings. This feature gives them the nickname of “target cankers.” 

Black canker

Canker fungi travel from tree to tree in interesting ways—some have their spores disseminated by wind, others by rain, still others by insects—and humans do a good job as by well by not cleaning their pruning shears between cuts!

Prevention is much more profitable than attempts at control. Avoid wounding the aspens by carving on them which means almost certain death to the tree! Sufficient watering also keeps the trees from being under stress, which makes it more susceptible to insects and disease.

If a small canker is found on a branch, prune it off. Although pruning is preferred during the dormant season, if you must prune during the growing season, be sure to sterilize the pruning shears by soaking them in a 10% Clorox® solution, by spraying the blades with Lysol® disinfectant, or by using some other alcohol product after each cut

Small trunk cankers can sometimes be cleaned up by removing affected tissue down to sound, unstained wood. Seek help from a professional arborist if unsure of how to perform this surgery. Do not attempt to remove large trunk cankers. Currently, research by the Colorado State University Forest Service shows the usefulness of fungicidal sprays and wound paints to be in question. 


Mites feed on all parts of the trees. They are narrow, very tiny (1/100 inch long), translucent, and have four legs toward the front of their bodies. Adults appear only as a speck through a hand lens. Many mites cause woody swellings or galls, especially around the buds of aspens. Infestations usually occur in early summer, immediately after tree buds open. Under ideal conditions, development from eggs to adult takes 14 days.

Although galls are conspicuous and unattractive, they rarely do any real damage to plants. Furthermore, once galls start, formation is largely irreversible. Under most circumstances, control is not recommended. Heavy infestations that occur repeatedly over several seasons may slow the growth of the plant or make the appearance unattractive.

Most galls are produced by insects that move to the trees as new growth develops in the spring. They can be controlled only with sprays, such as Sevin or Kelthane, that cover the leaves during the egg-laying period. Repeat applications often are needed.

For more information, contact your local Colorado State University Extension Master Gardener Program or visit Colorado State Forest Service - insects and diseases at

Friday, June 25, 2021

A Wasp-Benefit Analysis – Part II: Social Wasps

By Lisa Mason, Arapahoe County Extension 

In case you missed it, click here to read Part 1. We covered the purpose of wasps in the ecosystem and answered questions on the Asian giant hornet that made headlines in 2020. Part II will cover social wasps, their role in the ecosystem, and possible control methods if they become a nuisance.

Wasps sometimes get a bad reputation because they can sting and are sometimes a nuisance. We have one species of wasp that can be particularly aggressive: the western yellowjacket. Don’t let one or two species of nuisance wasps ruin your opinion of all wasps. Wasps are a diverse group of insects that provide important ecosystem services such as pest control.

Social Wasps

Social wasps are probably the most familiar wasps to people because they are easily seen in the yard and landscape. Social wasps live in a colony together. They have a similar lifecycle to a bumble bee (Bombus spp.).  A new colony is started each year by a fertilized queen that survived the winter. She will lay several generations of female workers throughout the season. Towards the mid-to-end of the summer, she will lay eggs that are male wasps and potential queens. The males and potential queens will leave the colony to find a mate. Once cold temperatures arrive, the current colony will die except for the newly mated queens.

The cycle will repeat and the following spring, when the new queens begin a new colony. Social wasps always build a new colony each year. They never reuse old nests, which is important to note if you’re looking to control nuisance wasps. Social wasps make their nests out of chewed up wood, creating a paper nest. Social wasps also feed on insects like caterpillars, providing important pest control in our backyards. The western yellowjacket is a scavenger feeding on carrion and human sources of food such as trash.

Let’s discuss five species of social wasps that are found in Colorado. Understanding the life history of social wasps can help you control them if they become a nuisance in your landscape, and build appreciation for their complex social biology, along with the pest control services they provide.

A western yellowjacket. Photo: Lisa Mason

Western Yellowjackets

Western yellowjackets (Vespula pensylvanica) are a native, social wasp that you will find at your family BBQ, picnics, trash cans, etc. They are very common in urban landscapes and can become a nuisance. Like social wasps, they create a new colony each year. The paper comb nest is usually underground or in a cavity that is well-protected. While yellowjackets are commonly seen, their nesting site can be difficult to find.

Yellowjackets can be aggressive, especially when defending their nest and are responsible for 90% of the insect stings in Colorado. They are scavenging insects feeding on carrion, dead earthworms, garbage, human foods including meats, and sweet, sugary foods. They also will feed on honeydew, a sweet substance excreted by aphids and soft scale insects.  Scavengers are the clean-up crew for ecosystems and play an important role in the food web. Yellowjackets tend to get more aggressive in the fall as food can be harder to find.

If yellowjackets tend to be a nuisance in your landscape, you can purchase a yellowjacket trap available at hardware and garden stores. The traps contain heptyl butyrate which yellowjackets are attracted to. Traps will be most effective if they are placed outdoors in the early spring to capture the overwintering queens before they start their new colonies. Nest removal can be a dangerous task and difficult because their nests are so well-protected. Insecticide treatments often aren’t effective because it is difficult to get the insecticides inside the colony. Hiring a professional is often necessary. Remember, the colony will only last for one season. 

An underground entrance to a western yellowjacket nest. Photo: Nancy Bonita

European Paper Wasps

European paper wasps (Polistes dominula) are much less aggressive, but they often build their nests close to human activity. They are a non-native insect that has become well-established in Colorado. They first appeared in Colorado in the late 1990s/early 2000s. They prey on caterpillars and other insects and feed their young live insects. Common prey includes hornworms and cabbageworms. They also will feed on honeydew secreted from aphids. The papery comb nests are often found under house eaves, overhangs, sheds, pipes, and other hollow spaces in human infrastructure. 

If the paper wasp nest is located in an area that won’t be disturbed by people, the nest can be left alone, and the wasps likely won’t be a nuisance. The current colony won’t survive when temperatures cool in the fall. If the nest is close to human activity, there are insecticide treatments to destroy the nest. Following instructions on the insecticide label is critical. Insecticides should be applied at night when most wasps are present at the nest. The nest should be destroyed afterwards to also kill the capped larvae in the nest. The location of the nest site should be thoroughly washed to prevent any remaining wasps from building a new nest.

Traps that attract yellowjackets will not attract paper wasps. There are no effective trap methods for paper wasps.

A European paper wasp. Photo: Lisa Mason

Baldfaced Hornets and Aerial Yellowjackets

Baldfaced hornets (Dolichovespula maculata) and aerial yellowjackets (Dolichovespula arenaria) are common in Colorado but are less likely to be a nuisance around human activity. They are only aggressive when their nest is threatened. They develop a large papery comb nest usually high in large trees and shrubs. They feed on caterpillars, other insects, and honey dew. If you find a nest in your tree or shrub, it may look intimidating, but the nest can likely be left alone if the nest can be left undisturbed. These two wasps can be common visitors in our landscapes but often go unnoticed by people.

A baldfaced hornet nest. Photo: Joe Boggs, Ohio State University Extension

A baldfaced hornet. Photo: Joe Boggs, Ohio State University Extension

Western Paper Wasp

The western paper wasp (Mischocyttarus flavitarsis) is a native paper wasp in Colorado and the western US. They have a similar biology to the European paper wasp. They are capable of building paper nests close to human infrastructure and activity, but they are not nearly as common as the non-native European paper wasp. They can sting if their nest is threatened, they often prefer to “ram” into the person or animal that is threatening the nest (Snelling, 1953). Like other paper wasps, they prey on caterpillars, flies, and other pests, bring the prey back to the nest to feed the young wasps the live insects. Adult wasps may also forage for nectar on flowers.

A Note About Insect Stings

Western yellowjackets are responsible for 90%+ of all stings in Colorado. When someone says, “I was stung by a bee,” they were likely stung a yellow jacket.

Both bees and wasps can sting. A stinger is a modified ovipositor (the egg-laying mechanism in insects), so only females have the ability to sting. The purpose of a stinger is defense, and in some species, predation. Generally, insects will only sting if they are provoked or their colony is disturbed.  Both social and solitary wasps and bees can have the ability to sting, but social insects are more likely to sting because they need to protect their colony.


Honey bees prefer to forage on flowers and go about their business in their hive, but they can sting if they need to protect their hive. Honey bees can sting only once. They have a barb at the end at the end of their stinger that stays in your skin. The barb is attached to the internal guts of the bee, so when the bee tries to fly away, the guts are ripped out of the bee’s body, which kills the bee.

Bumble bees have the ability to sting but will only sting if their colony is disturbed. They can also sting more than once unlike honey bees. Bumble bees are not aggressive and prefer to forage on flowers and go about their business. Many native bees are not able to sting or will only sting if handled.


Wasps can sting more than once. Solitary wasps will only sting if they are pressed up against your skin, or you try hard to provoke them. They prefer to fly away and stay away from human activity. Social wasps can be very defensive if their nest is disturbed. They also can sting if they are away from their nest and provoked. The western yellowjacket is much more likely to sting because they are scavengers and attracted to human foods and garbage. They tend to get more aggressive in the fall when temperatures cool down and food is harder to find. Other social wasps including the European paper wasp are generally not aggressive unless their nest is disturbed. The European paper wasp tends to build nests close to human activity on buildings, sheds, and other structures, which can increase the chance of nest disturbance.

Learn More

Western yellowjackets and European paper wasps can be a nuisance to people and often attract attention, but these wasps and other social wasps represent a small part of wasp diversity. Look for a future post about solitary hunting wasps. These wasps often go unnoticed in the landscape, but provide valuable pest control services!