The following is an excerpt from Fresh Food from Small Spaces: The Square-Inch Gardener’s Guide to Year-Round Growing, Fermenting, and Sprouting  by R. J. Ruppenthal. It has been adapted for the web.
The strawberry pot is one of the most aesthetically beautiful planters available for urban gardeners. These curvaceous containers fill urban space very efficiently, since they can either sit in the corner of a balcony or hang from the eaves outside a window. Unfortunately, they also epitomize what is wrong with most of the container planters on the market. If you have ever used a strawberry pot, you will know that it is impossible to grow productive strawberries from these containers, and that even smaller food plants like herbs will be very limited in size. All those little planting holes give the container an attractive appearance, but those little plants in the little holes do not have room enough to grow. Their roots become so quickly entangled that water does not reach all of them, and there is little growth potential for the plant’s foliage.
Some years ago, an enterprising gardener came up with a good solution for the watering problems in strawberry pots: Take a short length of plastic pipe, drill some holes in the sides, and bury this in the soil vertically down the center of the pot, surrounding it with soil. This watering tube would deliver more moisture to the lower plants’ roots, enabling deeper watering with less water. Perhaps without thinking about it, this enterprising gardener also found a way to deliver more air to the plants’ roots through the vertical feeding tube. This watering tube idea worked so well that the plans were printed in a number of gardening magazines and shared among urban gardeners around the world. This may have signaled the beginning of the so-called “self-watering” planter.
Unfortunately, most strawberry pots are way too small for productive vegetable gardening. You might get a handful of strawberries, chives, or baby lettuce leaves out of a strawberry pot, but you will not make much of a dent in your family’s produce needs. Productive container gardening requires that you use larger planters. Fortunately, the concept of the self-watering container has continued to spread, and several gardening companies have refined the concept into a much more productive and modern form.
Although you can use any type of container for planting vegetables, I highly recommend that you use one of these modern self-watering containers to maximize your small garden’s production and simplify its care. These containers are constructed so that there is a water reservoir below the regular pot space, and the plant is able to soak up the water from below through a soil wick. The design mimics nature, where plants are forced to grow their roots downward toward the water table, and it makes for very healthy plants. If you fertilize the top of the soil only, plants will grow a bi-level root system to get only the fertilization and water they need. This makes for incredibly happy vegetable plants that will outproduce those grown in other containers and often will outproduce plants grown in the ground. Self-watering containers also have the potential to solve any problems related to lack of oxygen and overwatering. Plant roots get plenty of oxygen, because there is usually a space between the soil and the water reservoir, providing very healthy airflow. And, by their very design, self-watering containers eliminate the problem of overwatering; the soil only soaks up as much water through capillary action as the plant can drink. It is often possible to add enough water for several days at a time, allowing you to take a short trip away from home without having to rely on a friend or neighbor to water your plants.
It sounds too good to be true, but it’s not. The only catch with self-watering containers is that most of the ones you can buy at your local garden center are still too small for productive vegetable growing. For example, the only type of self-watering container that my local garden center stocks is a round, 12-inch-diameter plastic pot with a tapered base. These are built for flowers or herbs, but not vegetables. Just try growing a tomato or squash in one of them. (I have.) For one thing, round pots are not very space-efficient. Even worse, they don’t hold enough soil for a mature root system. In several of these round pots, I attempted to grow compact tomatoes and zucchini squash, which were severely stunted due to the inability of their roots to spread. These plants did produce a few fruit, but in the same amount of horizontal balcony space, I could have grown just one plant in a deeper square or rectangular container and achieved at least five times the vegetable production. The same is true for the shallow self-watering window boxes that are sold by seed catalogs and Internet garden supply stores; they may be nice for growing a few flowers, but even a strawberry or tumbling tomato vine would be limited by the shallow depth.
There are two possible solutions: buying a bigger pot or making a one yourself. If you can afford to buy, there is a commercial product available called the Earthbox, which is perfect for growing most varieties of vegetables. Its dimensions are 29″L x 13.5″W x 11.5″H, and it holds about 2 cubic feet of soil. According to the manufacturer, one box can grow either two tomato plants, four cucumbers, six cabbages, eight lettuces, ten spinaches, or other varieties and combinations. The trouble is that it costs more than $50 for one unit, a prohibitive price for many small-scale growers. If you can afford this, then buy as many Earthboxes as you can fit on your porch, balcony, driveway, patio, or other sunny space; this will provide a nearly trouble-free growing system that achieves truly exceptional results with most vegetables. You can order these at www.earthbox.com and the Web site also has a nice diagram of the Earthbox design and components. Gardener’s Supply Company at www.gardeners.com sells a similar-sized Self-Watering Planter with fewer features for around $35. Others I have seen are too small for serious vegetable growing.