I began my first containerized vegetable garden about eighteen months ago. Shortly after harvesting that crop, I began getting e-mail requests for information about growing food on patios and balconies. Amazing! The Principle of Serendipity guides me to assist the Universe’s intentions. It seems I have been appointed to help those who don’t own land grow some of their own food. Okay, Universe, I accept this mission. Besides, container gardening is so interesting that I am delighted to expand my research.

Containers are small

City dwellers who live on small lots often grow food in enclosed raised beds filled with soil or a purchased growing medium. Apartment dwellers with balconies, patios, or bright sunny windows try to grow food in pots. All are forms of container gardening.

The main challenge of container gardening is that plant roots are restricted. Plants grown in the field enjoy a depth and breadth of root development that can’t be duplicated in containers. Plants feed and absorb water through newly formed cells extending back from the root’s tip for less than half an inch. The tip remains active for—at most—a few days. As the root tip extends deeper into the soil, older root tissue develops a hard skin and becomes a conductive tube that carries the moisture picked up by the ever-expanding tip. To remain well fed, a plant must constantly develop new roots. When growing outdoors in natural soil, the gardener limits root-zone competition by giving each plant sufficient space, so it won’t have to struggle with other plants for water and nutrients.

My best guess is that to raise a single lettuce plant to maturity—that is, one full-sized high-quality head—before becoming pot bound you’ll need three gallons of rich, loose soil in an ordinary plastic pot. Five gallons of growing medium will support the full development of one compact bush tomato for almost three months before it becomes pot-bound. The moral: use a BIG container and be alert for signs of stress that appear on pot-bound plants.

Fertilizer for containers

It is possible to support containerized plants so they remain healthy and continue to grow after their root development becomes restricted. There are two approaches: use a growing medium that encourages development of a denser root structure (to be discussed in next month’s article); and fertilize your irrigation water. Home gardeners routinely fertigate (my word, combining fertilizer with irrigation) using rapidly soluble chemicals with a 20-20-20 analysis, or use prepared liquid organics such as fish emulsion or liquid kelp. Gardeners also fertigate with tea made by soaking compost in a bucket and then diluting the dark liquid that is poured off; worm compost drippings also act like liquid fertilizer. Sadly, and for various reasons, none of these approaches work nearly as well as is possible.

Nearly all soluble chemical fertilizers fail to provide complete, balanced nutrition. They lack one profoundly essential element—calcium—needed in considerable quantity by almost all food-producing plants. These fertilizers also lack other minor nutrients. Soluble fertilizers may be too high in nitrogen to suit many fruiting plants. Plants grown on soluble chemical preparations alone almost inevitably become diseased or are attacked by insects. If you must use such fertilizers, mix them at one-third the strength recommended by the maker and always use this dilute fertilizer to irrigate plants. If it is a fruiting plant, use this fertilizer only while the young plant is not yet bearing fruit.

Manure tea, compost tea, or worm tea will not, by itself, create maximally healthy plant growth. These fertilizers provide complex organic nutrients similar to those that plants obtain from the biological process of living soil (but not nearly as complete). Feed containerized plants at least once a week with compost or worm tea, or with liquid seaweed. Dilute these organics to roughly half the strength recommended, because you will provide the majority of the plant’s nutrients from natural organic fertilizers mixed into the growing medium itself. These mixtures can fully nourish a plant for the first 10 to 12 weeks or until it becomes pot bound.

Plants need different kinds of nutrition when making fruit (not just tomatoes—botanically speaking, snap beans are fruit, too). During fruiting, the plant’s rate of vegetative growth slows—it will not form as many new leaves. The leaf contains most of the plant’s chlorophyll, a protein whose manufacture requires most of the nitrates the plant can accumulate. Consequently, a blooming and fruiting plant needs less nitrogen than one growing a new, leafy structure. Because fruiting involves seed formation, flowering and fruiting plants use more phosphorus and potassium than they needed when growing only leaves.

Soluble chemical mixtures designed for blooming and fruiting plants are often formulated as 5-10-10 or 10-20-20. They do a better job than a 20-20-20, but these chemical combinations share the same failing as the higher nitrogen formulations. They are usually formulated with the least costly—and thus less effective—forms of chemicals.

Start out on the right foot when potting up your plants. Make the soil itself fertile enough to grow your plants without adding anything but water while the plant is building leafy structure. That means blending your potting soil with organic fertilizer. To each gallon of soil, blend in one-half cup of my complete organic fertilizer (COF) recipe or something similar, such as Down-To-Earth’s Bio-Live or Bio-Fish organic fertilizer.

COF will release its nutrients within 90 days of mixing it into soil. Organic nutrients may not release as quickly if you use a non-soil growing medium such as commercial bagged potting soil. In that case, you may need to use liquid fertilizers, too. Add one heaping tablespoon of finely ground agricultural lime to each gallon of soil unless you are using COF, which already contains lime.

Your plants may grow longer than 90 days, so will require additional fertilizer. If you are growing leafy crops whose harvest goes on longer than a month (kale, broccoli, or a cut-and-come-again pot of salad greens) their growth rate will taper off as the COF is absorbed. When this happens,
almost any high nitrogen liquid fertilizer will work well, but only if the plant has enjoyed a few months growing on COF first and if the soil mix contains lime.

What about fruiting crops? This may surprise you, but the best fertilizers for container plants are not sold in garden centers, but in places that supply indoor growers of cannabis. And the best of these fertilizers are not the organic products, but the chemical ones. The production of cannabis under artificial lights is probably the largest containerized growing industry, planet-wide. Indoor pot growers have a profound interest in maximizing yield and quality.

In a halide shop (so-called because halide lights are used by indoor growers) you’ll find nutrient solutions developed for both soil and hydroponics. Some of the more costly liquid fertilizer concentrates found in such shops contain calcium. These expensive liquid chemicals may also contain chelated minerals. Plants are able to absorb chelates far more effectively than when they are offered simple chemical salts.

I have had extraordinary results using a concentrate made by the Canna Company called Terra (soil) Flores (flower). The results of using Terra Flores are astonishing when compared to dry chemical “bloom” fertilizer to support fruiting peppers. I suggest mixing the fertilizer into all irrigation water from the time the plants start forming fruit. However, Terra fertilizers aren’t cheap! Using the Terra Vega (for leafy growth) to produce lettuce might end up costing you several dollars worth of fertilizer to produce a single head—it’s best to use COF in the soil. But to use something like Terra Flores to support the development of tomatoes once the fruits start forming, it might cost a few dollars per tomato plant, but will increase the yield (and maybe quality, too) several times.

A final warning about using fertilizer concentrates on container plants: if you use COF fertilized soil, dilute all liquid fertilizers to about half the recommend strength and only very cautiously increase the strength beyond that. If you start to see brown tips on your leaves, immediately reduce the concentration to half-strength or use pure water for a week and then reduce the concentration to about half of what is recommended.

Fertilizer Notes: See northwestgardennews.com for Steve’s COF recipe; Down to Earth fertilizers are at www.downtoearthdistributors.com/fert_blends.html; in Seattle, visit Walt’s Organic Fertilizer. Canna fertilizer products are available in the U.S. online at greenhouse supply sites, including hydroasis.com