Thursday, September 24, 2015

Experiments with Containers by Kristina Hughes, Apprentice Master Gardener, Clear Creek County

I live where the plant-eating critters abound and I don’t have a fence, so I’ve been relegated to using pots on my second-story deck. We mountain gardeners have to be resilient and creative, and each year I keep trying new techniques to improve the performance of my potted plants. Since my first frost date is approaching I’ve started evaluating this year’s strategies and I have some thoughts I’d like to share.

Last year’s experiment
Moisture management is a big issue in our semi-arid climate and it is especially critical with container gardening. Last year, in every pot I used a plastic insert to hold the soil inside each decorative outer pot (see photo). I also put rock mulch (I used 1/2” gravel from my driveway, and washed it) on top of the potting mix in each pot. I found I had to water only one time per week, even during our hottest and driest spells.

Decorative outer pot contains a plastic insert
This year, I tried a mixture of techniques since I only had a few plastic inserts and I had a bunch of new lovely pots. 

The potting mix
One strategy was to increase the moisture-holding capacity of the potting mix itself. Most potting mixes are formulated to improve drainage and aeration. They often contain a lot of peat and they can dry out quickly. For soils in the ground, we are taught that compost increases the moisture-holding capacity of porous soils, so I applied that concept to my pots. I added extra compost to my potting mix in an attempt to improve the moisture-holding capacity of the mix. To ensure good drainage, so I also added perlite (the little white balls that you see in most potting mixes, they look like styrofoam). (1.5 cu ft bag potting mix + 1.5 cu ft bag compost + 8 qt bag perlite, mixed well in a wheel barrow)

I believe the compost improved the moisture-holding capacity of the soil. However, I did not fill any of my pots with un-amended potting mix, so I had nothing to compare to. Maybe next year I’ll do a side by side comparison, so I know for sure.

Good drainage
While it’s important to retain moisture in our dry climate, we also can have torrential downpours and I know people whose potted plants have drowned, so I also wanted to provide good drainage. As I mentioned above, I made sure to add extra perlite to my amended potting mix. I also made sure every one of my pots had ample holes in the bottom. And to be sure the holes drained freely, I raised the pots off the deck by using 1/2” wooden blocks (see photo). During our wet spell early in the season, I didn’t lose any plants to drowning, so I consider this aspect to be a success. But there’s always next year and whatever crazy weather it brings, so time will tell.....
1/2” wooden spacer elevates pot and keeps drain hole free of obstruction

Self-watering containers
The most successful aspect of my experiments this season involved the use of self-watering containers. I tried several configurations and was pleased with all of them. I got inspiration for DIY self-watering pots from an excellent book that a fellow Master Gardener, Debbie of Gilpin County, brought to one of our classes: The Vegetable Gardener’s Container Bible by Edward C Smith. I’m sorry I didn’t take pictures while we were making the DIY versions, but you can get an idea of what the reservoirs look like by doing an internet search for ‘self-watering container reservoir.’ My key criterion was having a large reservoir for water.

I used a commercially pre-made self-watering container (Earthbox) and compared it to a DIY version in a 10 gallon plastic tub (see photo). The commercial version has a reservoir that holds 3 gallons of water to serve 15 gallons of soil mix and my DIY version was a little smaller, but had a similar ratio of reservoir size to soil volume. They both had plastic covering the top of the soil, so no water was lost through evaporation. I have several tomato plants in them. They worked really well and I found filling the reservoirs once a week to be adequate. I’ve gotten my first fruit from my russian tomato plant and have lots more on the way!
DIY self-watering pot using a 10-gallon plastic tub

Another comparison
I did another comparison between two larger pots. One pot was filled only with soil mix and the other pot had a 1 gallon DIY reservoir in the bottom half. 

The large pot containing only potting mix had some problems. I watered it regularly and faithfully until water ran out of the bottom. But my plants (especially the basil and tiny ageratum) were perpetually wilting. It took me awhile to realize that, even though I had been watering frequently during the dry spell, part of the soil mix had completely dried out and most of the water was running out of the pot instead of being absorbed into the soil. So I embarked on a re-wetting program, watering in small amounts many times per day until the soil mix was moist all the way through. It’s a good thing that most of the plants in that container were drought tolerant!

Without a doubt the self-watering pot had superior performance. I still needed to water regularly from the top, because the very top 2” of the soil dried out, but it was much more forgiving. My moisture-loving canna and dahlia did very well and never wilted even during the hottest, driest part of the season. The soil in the DIY self-watering container never dried out completely.

Next year
So, with information from this year’s experiments I have some ideas to use for next year’s pots. I loved the self-watering containers and will expand my use of them. I think the use of mulch is a good idea and I will incorporate it into my plans for next year. And I would like to compare the moisture-holding capacity of plain potting mix versus potting mix amended with compost and perlite.

As the first frost approaches I have some consolation that next year’s pots will be even better than this year’s!

PS I got inspiration for the DIY self-watering pots from an excellent book shown to me by a fellow Master Gardener, Debbie of Gilpin County: The Vegetable Gardener’s Container Bible by Edward C Smith. And further great advice on all aspects of container gardening can be found in The Bountiful Container by McGee and Stuckey. 

Most importantly, the CSU website has great information that helped me adapt to our regional conditions the information that I found in the books. I particularly recommend: 
Container Gardens - Fact Sheet 7.238  ( )
Climate Summaries by region in Colorado - Garden Notes 740  (

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