Vegetable gardening differs from container gardening of houseplants and bonsai trees. Whereas creating a house plant container environment is similar to building an endurance touring car, creating a container environment for vegetables is more like building an IHRA performance dragster that peaks after only a quarter mile. While I have primarily grown indoor and outdoor ornamentals in containers over the past years, I plan to do more container gardening of vegetables this summer. Not being my specialty, I have been reading a lot on container gardening and associated field studies. The following is a very general review of some of the concepts that I've seen on gardenweb and in university studies that I plan on using this summer. This post is a bit wordy, but I also wanted to share what I found with others here. Feel free to comment or question anything here. As I have one shot to get it right this summer, the more that can be put together, the better off I will be.

Container Media

50% organic and 50% drainage is suggested, although the percentage blend will vary depending on vegetable plant variety and regional or local conditions. The organic can be peat moss, pine bark, or other natural materials with large pore spaces for drainage and high cation exchange ratios for holding nutrients. The drainage material can be perlite, builders sand, or other fast-draining inert material. Weight is a consideration with perlite making containers light weight and builders sand being heavier for anchoring pots that might otherwise blow over in a wind. Take time to find the performance specifications of the container media that you use. For example, you might find that perlite has a lot of pore space for draining, but it does not hold as much water for plant use between watering. The ideal as often stated is to use a fast draining mix and water more frequently.

Goal: Use a fast draining soiless mix with ability to hold enough nutrients and water for good growth.

Porosity

While it has been stressed before that adequate pore space is needed, there has not been a lot of coverage of pore types. In a study that I read, pore space was divided into three categories. Large pores created fast drainage, but little to no water storage. Small pores did not drain well. Interestingly, small pores between small particles and small pores inside of larger particles both tended to hold water with enough tension that plants could not make much use of this water source. It was an abundance of medium pores that offered the ideal quality of good drainage and water availability to plant roots.

Goal: Use a soiless mix that has more medium sized pores than small or large.

Container Size

It is not feasible to provide containers that allow root growth that matches the root growth of in-ground garden plantings. For example, several vegetables have been shown to put out roots up to six feet down and eight feet in diameter. This obviously does not mean container gardening is impractical. Rather, studies show that bigger containers produce better harvests. Containers that were 22 to 24 inches in diameter and 18 to 24 inches in depth were found in one study to provide the best compromise for larger plants like tomatoes, egg plant, squash, etc. Plants like lettuce were grown in containers as shallow as three inches, but six inches of container media did well with the lettuce.

Goal: Use a container that is sized correctly for the vegetable that you are growing.

Root-Based Soil Collapse

While the decomposition of media into smaller particles that eventually impede drainage has been discussed, in single-season vegetable gardening root-based soil deterioration in more prevalent. As the roots colonize the container media, greater competition for both air and water will limit the growth of the plant. The roots fill in the pore space in the media, making more difficult to maintain a healthy root zone in the media. It is important to have enough media for the roots to continue to grow over the season.

Goal: Recognize that root-bound conditions can stop fruit production and either plan to re-plant the container or use a larger and deeper container.

Watering Frequency

The goal of watering is to keep a container mix evenly moist, but not overly wet. The fine feeder roots of plants can die in dry conditions, seriously disabling the plants ability to take up water. Unfortunately, this often occurs later in the season during vegetable fruit formation. Early in the season with cooler temps, watering once every 2 to 3 days may be sufficient. However, as the plant grows and temperatures climb, watering up to 2 to 3 times per day may be required. Since watering is so critical, an automated watering system should be considered. It would be easier to group containers for easier watering.

Goal: Use automation to keep soiless mix evenly moist.

Water Distribution

While a taller container does drain better, it is more difficult to evenly water. One time I saw an exhibit that should how rubber balls when dropped from the same spot would take a semi-random path bouncing down through a series of rods, with most of the balls stacking up in one particular area at the bottom of the rods. Similarly, with a wide and shallow container that is watered evenly across the top, each water drop does not have a lot of vertical drop time to move laterally through the soil. With a tall container that has a small diameter opening, each water drop has a lot of vertical drop time to move laterally as it cascades through the media. As a consequence, taking the path of least resistance, most of the water provided at the top of the container could end up on one side of the container where it runs down the inside container wall leaving the lower section of media fairly dry. Since taller containers are preferred, a better watering system needs used.

Goal: Develop better methods for watering deeper containers to allow a constant mositure level without drowning roots.

Fertilizing

One study I read showed that using a lot of compost in conjunction with a slow release fertilizer did not perform as well as compost alone. It was speculated that too much nitrogen was available at one time and burned the roots. Another study found the best results came from mixing a slow release fertilizer throughout the container media when mixed at a rate of 1/2 teaspoon per gallon, and when the vegetables began to flower they used a dilluted water soluable fertilizer applied once per week or every other week. While organic fertilizers can be used, its was suggested that synthetic fertilizer may provide a more consistent source of nutrients for container plants. With a normal watering schedule, a lot of both fertilizer and salts will be washed out of the containers.

Goal: Maintain adequate nutrient levels throughout the season without using so much fertilizer that root burn occurs with an emphasis on low dose water soluble fertilizer combined with slow release fertilizer.

Heat

Container temperatures reach much higher levels that previously thought. These high summer temperatures are not experienced in in-ground gardening where the earth buffers the soil temperature. Heat can also rapidly deplete slow release fertilizers, creating a slow surge of fertilizer. With container plants, damage to the roots and overall plant can be avoided by managing heat. Using wood boards to block the sun's rays; painting containers with reflective color paint; keeping plants out of afternoon sun; moving plants to different locations; and misting are all methods for dealing with the heat.

Goal: Take steps to maximize sunlight and minimize heat build up.