Q. I enjoy container gardening and plant in medium to large plastic pots, however, I’m concerned about the roots when the temperatures get hot. Will the heat permeate through the plastic pots and burn the roots? Other than buying clay pots, what else can I do?
A. Yes, this is a legitimate concern. It’s a bigger issue for those plants growing in the same container for several growing seasons than with new plantings. Overtime, roots reach the sides and fill the container while the insulating soil layer disappears. Plastic containers have a thin wall and the heat transfers directly through it when hit by the sun, damaging any of the roots next to the wall. I’d be concerned with established Japanese Maples, Camellias, Citrus, Roses and other ornamentals. Containers with established plants are difficult to keep moist during hot, dry weather. There is little soil to hold moisture and lots of roots so water stress occurs quickly. The late afternoon sun is the most critical as this is when the temperatures are peaking. The heat factor is influenced by the color of the container. White or other bright colors will reflect heat while earth tones and dark colors absorb the heat. The containers are cooled when the plants are watered but the moisture evaporates quickly or it can heat up if the stand for any period of time. My rule of thumb for watering container plants is very simple: When the temperatures are over seventy-five degrees, water daily. Be sure to fill the containers the brim and empty the water from any saucer, better yet, remove the saucer altogether. It’s almost impossible to over water plants in containers when the water flows out the bottom. Root rot and other problems are more likely to occur from the constant wicking action when water is left in a saucer not to mention the West Nile Virus concern from the breeding mosquitoes. The best time of the day to water plants is in the morning before eleven. With ninety degrees or higher temperatures, a second application may be necessary in the late afternoon to those plants in the heat of the day sun. It’s advisable to apply a wetting agent such Grow More E-Z Wet or similar product to increase the water flow through a root bound plant. Besides frequent watering, here are some other things you can do. Move those containers in the afternoon sun to a shadier location when the temperature is over ninety degrees. These plants can survive temporarily in shady conditions since our typical heat spell only last a couple of days. Plant dollies or walkers with wheels make this a simple task and are available at your favorite garden center. Another option is to place the plastic pot inside a larger container, the so-called pot in pot method. You need to leave at least a two-inch space between the containers that is then fill with small bark to increase the insulation. With seasonal color or vegetables containers, frequent watering is sufficient.
Buzz Bertolero is Executive Vice President of Navlet’s Garden Centers and a California Certified Nursery Professional. His web address is www.dirtgardener.com and you can send questions by email at dirtgarden@aol.com or to 360 Civic Drive Ste. ‘D’, Pleasant Hill, Calif. 94523
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