Thursday, October 22, 2015

Fall Planting Season Is a Good Time to Revisit Your Local Climatic and Site Conditions For Proper Plant Selection and Placement - By Pete Biggam

For many of us, fall is a fine time to plant many perennials. The soil and air are cooler and sunlight is less intense, so the weather's less stressful for newcomer plants. Competition from weeds isn't likely to be a big problem, either.
In many of our high altitude areas, rainfall becomes more regular, too, which helps provide the moisture the perennials need to start good root growth. Yes, the perennials will soon head into winter dormancy, but fall planting often gives these perennials a head start over their spring-planted counterparts.
In spring, the fall-planted perennials should be raring to grow, larger and more robust.

Prior to planting, you may want to confirm your local plant hardiness zones and take a look at your local site conditions to see if you may have some beneficial (or detrimental) microclimatic conditions that may allow you to utilize better adapted plants in these areas.

USDA Plant Hardiness Zone Maps

The 2012 USDA Plant Hardiness Zone Map is the current standard by which gardeners and growers can determine which plants are most likely to thrive at a location. The map is based on the average annual minimum winter temperature, divided into 10-degree F zones.

If your hardiness zone has changed in this edition of the USDA Plant Hardiness Zone Map (PHZM), it does not mean you should start pulling plants out of your garden or change what you are growing. What is thriving in your yard will most likely continue to thrive.

Hardiness zones are based on the average annual extreme minimum temperature during a 30-year period in the past, not the lowest temperature that has ever occurred in the past or might occur in the future. Gardeners should keep that in mind when selecting plants, especially if they choose to "push" their hardiness zone by growing plants not rated for their zone. In addition, although this edition of the USDA PHZM is drawn in the most detailed scale to date, there might still be microclimates that are too small to show up on the map.  is a relatively new reference website that contains several interactive maps and tools to assist gardeners, botanists, farmers and horticulturalists. By entering a ZIP code, users can find not only the new USDA Plant Hardiness Zone, but also the first and last frost dates, heat zones, drought conditions and annual climatology for their area, that the USDA Plant Hardiness Zone website does not provide.

Here’s some examples of plant hardiness zones and climatic data for Idaho Springs, Evergreen, Bailey, and Conifer, and you will notice that even those some of these areas have the same plant hardiness zones, they have different first and last frost dates which you should account for in your plant selection and garden management strategy.


Microclimates, which are fine-scale climate variations, can be small heat islands—such as those caused by blacktop and rock outcrops —or cool spots caused by small hills and valleys. Individual gardens also may have very localized microclimates. Your garden soils could be somewhat warmer or cooler, or drier or moister than the surrounding area because it is sheltered or exposed. You also could have pockets within your garden that are warmer or cooler than the general zone for your area or for the rest of your yard, such as a sheltered area in front of a south-facing wall or a low spot where cold air pools first.

No hardiness zone map can take the place of the detailed knowledge that gardeners pick up about their own gardens through hands-on experience.

The graphic below depicts the effect of aspect and solar radiation on soil temperature and soil moisture that can result in contrasting microclimates on your property.

Many species of plants gradually acquire cold hardiness in the fall when they experience shorter days and cooler temperatures. This hardiness is normally lost gradually in late winter as temperatures warm and days become longer. A bout of extremely cold weather early in the fall may injure plants even though the temperatures may not reach the average lowest temperature for your zone. Similarly, exceptionally warm weather in midwinter followed by a sharp change to seasonably cold weather may cause injury to plants as well. Such factors are not taken into account in the USDA PHZM.
All PHZMs are just guides. They are based on the average lowest temperatures, not the lowest ever. Growing plants at the extreme of the coldest zone where they are adapted means that they could experience a year with a rare, extreme cold snap that lasts just a day or two, and plants that have thrived happily for several years could be lost. Gardeners need to keep that in mind and understand that past weather records cannot be a guarantee for future variation in weather.
Other Factors to Consider
Many other environmental factors, in addition to hardiness zones, contribute to the success or failure of plants. Wind, soil type, soil moisture, humidity, pollution, snow, and winter sunshine can greatly affect the survival of plants. The way plants are placed in the landscape, how they are planted, and their size and health might also influence their survival.
Hopefully after revisiting your local climatic and site conditions you can develop a gardening strategy that will accommodate these factors.

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