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Fertilizing container plants

Q: By midsummer, my seasonal container plantings have yellowing leaves, minimal flower production, and stunted growth. What could be the problem?

Judith McKelvie, Wauwatosa, WI

A: Byron Martin, owner of  Logee's Greenhouses, Inc. , in Danielson, Connecticut, replies: Poorly flowering plants with stunted growth and yellowing leaves most likely need fertilizer. With each watering, both macro and micronutrients leach out of container soils, which are usually only soilless mixes. Fertilizing your containers periodically during the summer growing season to replenish nutrients will help avoid these problems.

Container plantings respond well to a number of different fertilizers, and the best method is to feed plants often with a low concentration of fertilizer. Soluble salts, the most common form of fertilizer, come in either a balanced, water-soluble mix or a slow-release, pelletized form. For a balanced, water-soluble fertilizer such as 15-15-15, dissolve 1/2 to 3/4 teaspoon in a gallon of water and apply on a weekly basis. The pellets, which release small amounts of nutrients to plant roots at each watering, can be added right into the potting mix before planting your containers, or sprinkled on the soil’s surface if the container has already been potted. 

Organic fertilizers carry a broad range of trace minerals, also known as micronutrients. Fish emulsion, seaweed concentrate, greensand, animal manures, and bonemeal are all examples of organic fertilizers. One drawback of organic fertilizers is that the percentages of nitrogen, phosphorus, and potassium vary widely from type to type. For example, organic fertilizers high in nitrogen like fish emulsion and bat guano will create exaggerated foliar growth if used on a weekly basis, so it’s best to alternate their use with balanced soluble salts during the growing season for optimum growth.

Yellowing between the veins of the leaves is a symptom of chlorosis, a condition that results from a lack of available iron in the container’s soil. Magnesium is a macronutrient that specifically acts to free up the iron in soil to make it more available to plant roots. Applying an Epsom salt solution at the rate of one tablespoon to one gallon of water once or twice over the course of the summer is an easy way to supply the magnesium that container plants need to stay green.

From Fine Gardening 63, pp. 14