Fertilizing Container Plants

"Landscape plants grow in soil, and even the worst soils ordinarily have some nutrient value," says master gardener Paul James. "But container plants grow in sterile potting mix--artifical soil--which contains next to nothing in terms of nutrients."

So why not just put garden soil in your containers? Because it's too dense, and once it settles in a container, it packs so tightly that plant roots have a tough time penetrating it. Also, garden soils can harbor all sorts of pests and diseases. Potting soils address both problems--they're light and fluffy, so roots can easily grow and spread in them, and they're sterilized to eliminate pests and diseases.

The lack of nutrients in potting mixes means that container-grown plants need routine doses of fertilizer to remain healthy throughout their growing season, whether it's just for a few months, or in some parts of the country, year-round.

The most popular fertilizers are various synthetic products. They include the familiar dry, granular fertilizers, crystalline products that dissolve readily in warm weather, control-release formulations, and liquid concentrates. James reviews each:

James: Frankly, I don't recommend the dry, granular fertilizers on container plants, largely because the risk of burning your plants is considerable. They are also the least efficient form of fertilizer because only a small fraction of their nutrients are actually made available to plants. The bulk of the nutrients are lost due to processes such as leaching, volatilization and fixation.

Synthetic crystalline fertilizers

James: Synthetic crystalline fertilizers, especially those made specifically for container plants, are far more forgiving and efficient, and once dissolved in water, they can used to drench the mix and feed the foliage. The same is true of ready-to-use liquid concentrates. However, in both cases, make sure to read the instructions carefully and apply no more than the recommended dosage.

Actually, when using synthetic fertilizers, I suggest you use only half of what the manufacturer recommends just to be on the safe side, and fertilize more frequently. And drench the potting mix with water before you apply synthetic liquid fertilizers to reduce the risk of burning plant roots. Let the excess water drain, then pour on the fertilizer solution.

Time-release fertilizers
James: Controlled-release or time-release fertilizers are the simplest, most convenient synthetics on the market, and they're relatively foolproof. The fertilizer is encapsulated in a plastic material that allows water in to slowly dissolve and release nutrients over time, anywhere from one to nine months. These products can be added to the potting mix at planting time, or lightly scratched into the top inch or two of mix after planting.

The rate of nutrient release with these products depends on the temperature. At 40 degrees F, for example, nutrient release stops, whereas at 70 degrees, it reaches its ideal release rate. I don't use time-release fertilizers for the simple reason that they're synthetic, but I do consider them to be the best of the synthetic fertilizers.

Liquid organic fertilizers

James: What I do use are various liquid organic fertilizers, especially compost tea and manure tea made from worm castings, as well as fish emulsion and kelp meal.

These all-natural fertilizers provide plants with all the major nutrients they need, namely nitrogen, phosphorous, and potassium, plus various micronutrients and organic compounds such as amino acids and enzymes not found in synthetic fertilizers. And they can be used both to drench the soil and to feed the foliage.

What's more, the risk of burning plants with fertilizers is virtually non-existent. In fact, the only real downside to using them is their smell, which thankfully tends to disappear after a day or two.

When to fertilize

James: The answer depends on the plant, the season and the temperature. In cooler weather many plants grow at a slower rate, and don't need to be fed as often. But as temperatures rise and growth rates increase, plants may need to be fed as often as once a week.

However, the problem with a regular feeding schedule is that the nutrient needs of plants vary enormously. Many culinary herbs, for example, need very little supplemental feeding, whereas heavy feeders such as bananas need regular and hefty doses.

So I suggest you fertilize container plants every two weeks, using half the amount recommended on the product label. If within a month, your plants appear stunted or their leaves begin to yellow, then you can increase the frequency to every seven or ten days but maintain the half-rate dosage.