The so-called “self-watering” or sub-irrigated container has been a great boon to those of us who want the satisfaction of growing our own food but don’t have space for rows of tilled earth or even raised beds.
Of course these containers don’t actually water themselves. You still have to water. And as the plants get large in July and August, I have to water my pot crops more frequently.
Watering is a special challenge in container vegetable gardening because a full-grown, fruiting tomato or pepper plant will rapidly soak up all the moisture that is available in a limited soil volume. So the general rule is: The bigger the pot, the better. I wouldn’t think of growing tomatoes in anything smaller than a 5-gallon bucket.
Sub-irrigated containers help by providing water at pretty much the rate at which the plant can absorb it. The potting mix — it needs to be good stuff, highly organic, fluffy, absorbent but freely draining – sits on a grid or mesh within the container, slightly above the water level of a reservoir. A column of soil that reaches down into the reservoir wicks water up into the soil of the root zone. The consistent moisture supply allows the plant to keep growing without ever suffering the trauma of dry roots.
But the real genius of these things is that you can’t overwater either. The reservoir has an overflow, so surplus from rain or from watering just flows harmlessly away. The plants’ roots can never sit in waterlogged soil, as they might in the garden during a rainy spring or summer. And that substantially reduces the risk of many fungus diseases.
I’ve used several brands of these containers in various shapes and sizes. Now I’ve taken to constructing my own from various found materials, using the biggest pots I can for tomatoes. Smaller pots are good for herbs, or for baby greens, which need only a couple of inches of depth.
But don’t go too big if you want to be able to move containers. If you are fortunate enough to have a greenhouse, movable containers (some come with casters) can greatly extend the season. Start plants in the greenhouse weeks before the last frost date and move them out to the garden once it’s perfectly safe. In late September, move the pot back into the greenhouse and you can keep harvesting long after your neighbors’ tomato plants are in the compost pile.