Q. I was wondering if there is a trick to container gardening. I have been using potting soil, along with plant food, and my plants keep wilting. What am I doing wrong?
A. There are two things you should be aware of regarding watering and containers in our climate. Don't use small containers and never let the soil get too dry before you irrigate again.
Small containers do not work well in the desert. They just don't hold enough water and their soil volume is small so it heats up rapidly. Plants use a lot of water in a desert environment. This means they have to draw upon a large reservoir of water in the soil. Large plants in small containers use the water in the soil quickly.
When the soil dries, it shrinks. In containers the soil dries from the outside, inward. The outer surface of this container soil can be very dry or hydrophobic, meaning it will repel water. When the soil shrinks in the container, it pulls from the sides of the container leaving a large crack. The combination of this large crack between a very dry soil and the inside of the container wall makes a nice channel where water can flow. Water applied to dry container soil flows down the inside of the container wall without wetting the soil. Of course you see it run out the bottom and think you have wetted the soil when in fact the hydrophobic soil surface is still dry.
Plant water use can be 25 to 40% higher in our hot, desert climate than in more moderate climates. You have to use larger containers to compensate for this higher water use or water more often.
Another problem. If containers are left exposed to the sun, the soil in the container dries rapidly and can get very hot, sometimes reaching temperatures around 160°F (70°C).
Most plant roots are not as tolerant to heat as aboveground parts. Why should they be? Soil can be a great temperature buffer. Large soil volumes provide a better cushion against high soil temperatures than smaller ones. Also, wet soils are slower to get hot than drier soils so set your irrigations clock to irrigate containers just prior to the heat of the day.
Here are some other suggestions about growing plants in containers in our desert environment.
· When wetting a very dry container soil, use a couple of teaspoons of liquid detergent in a gallon of irrigation water to help the water penetrate the dry soil. Add the detergent after the bucket is full of water.
· Always make sure water can drain from the container directly out of the bottom to keep salts moving through the soil profile. Our tap water coming from Lake Mead has lots of salt in it, about 1 ton of salt for every 320,000 gallons (1.28 million liters).
· When watering, let about 20% of your applied water (1/5 of the volume applied) run out the bottom. This helps flush salts out of the container. This is called a leaching fraction.
· Replenish container soil regularly. If container plants are annuals such as vegetables, replenish one fourth to one third of the soil volume each time you plant. If the container plant is a perennial such as a fruit tree, remove and replant the tree every 2 to 3 years. When replanting, prune the roots as well as the top to bring it back into scale with the container.
· Shade the container from direct sun during the day time. You can do this by placing the container inside a larger container. This is called double-potting.
· Use an inexpensive soil moisture meter made for houseplants to give you a rough idea if the soil is wet or dry. Otherwise lift or push the container. Containers get much lighter when it is time to water. Lift or push it after you finish watering as well. This helps reassure you that the soil is wet.
· Fertilize lightly once every one to two months during the growing season.