Cacti are commonly grown in containers, even when grown outdoors. Container-grown plants are not only at the mercy of their caretaker, but also the container into which they spread their roots. Twenty years of trial and error has led me to the conclusion that either plastic pots or clay pots fired at very high temperatures work best.
Plastic pots should be opaque, not white or semitransparent. Light that passes through a pot wall will damage roots. Algae may be found choking roots near the inner surface of a white plastic pot. Some brands of white plastic are also very sensitive to ultraviolet light, giving them only a short life. They soon become so brittle that they fall into pieces, spilling plants and soil.
Clay pots should be fired at temperatures of 2000°F (1100°C). This turns clay into a hard, waterproof substance. If porous clays are used, the inside surface may be coated with polyurethane for waterproofing. Kiln-fired glazing will also serve the same purpose.
Cacti that spend their summer outdoors, where they are may be subjected to excess moisture, will tolerate porous clay pots quite well. Excess moisture from unexpected rains evaporates more rapidly, and rainwater will leach out accumulated fertilizer salts and hard-water minerals. Wintering these same plants indoors in porous clay will, however, put them at risk. Fertilizer and hard-water salts will reach toxic levels near inner pot walls. Salts are left behind as the water carrying them migrates into porous walls and evaporates. Roots follow water until they reach container walls. Plants may not die but they will not live as well as they would, or should, in better containers.
More recently I have used a thermoformed pot with two layers. Coextrusion of polystyrene forms a container with a black inner layer for protection of roots from light, and an attractively colored outer layer. The pots are environmentally friendly because the heavier inner black layer uses recycled, postconsumer plastics. The thinner outer layer is new plastic. The use of recycled material allows these pots to be sold for about 10% less than typical solid-color ones. Black has long been considered the best possible color for maximum root protection and development. Now one can have the best of both, color on the outside to please, black on the inside to insure healthy roots.
As a general rule, deep pots are safer for plants than shallow ones. Gravity causes water to move through a deep pot with greater speed. A deep pot can drain more efficiently, thus creating a healthier environment for plant roots. An easy way to add extra drainage holes to plastic pots is to use an electric drill to add more holes to the bottom. I usually add at least four to six new holes to plastic pots. The newer thermoformed pots are an exception. They provide excellent drainage and I use them just as they are. Extra holes can be added to clay or ironstone pots with a hammer and a star drill bit. It takes some practice to learn how hard to tap the bit with the hammer. I have found that it helps to turn the pot upside down on a soft grassy surface to provide a cushion for the hammer blows.
A myth recycled from generation to generation is that breaking old clay pots and using the pieces at the bottom of new pots will improve drainage. This practice is harmful and rather than improving drainage, restricts it. All that is needed is a small piece of paper towel or tissue to prevent new soil mix from spilling out of the drainage holes. A latticework of new roots will grow to hold the soil in place long before the paper disintegrates.
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