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Gardening with Allen Wilson: Fertilize plants to keep them healthy

By Allen Wilson
Published: July 16, 2019, 6:00am

Last year my container plants started to go downhill about the end of July. How can I keep them attractive until the end of summer?

Your experience is very common. We give our containers fertilizer when we plant them and are very attentive in watering to get them started. As the weather warms in July and we are busy with other summer activities, we sometimes forget to water container plants.

By this time most of the fertilizer is either used up or washed out of the soil in containers. Even if you used slow-release fertilizer it is probably mostly gone by now. Slow-release coated fertilizer such as Osmocote is the best to use now. One application will probably last for the rest of the summer.

One of the earliest signs that tell us plants need fertilizer is lower or older leaves start to turn yellow and then fall off. When plants are short of nitrogen they transfer it from old growth to new growth. Supplying fertilizer keeps the older foliage active longer.

Plants growing in the ground have a lot more soil from which roots can draw nutrients. We water less often than containers so less fertilizer is washed beyond the root system. However, vegetables and annual flowers planted in the ground use a lot more fertilizer than more mature plants. Most long-term vegetables like corn, tomatoes, peppers, squash, and cucumbers can use an additional application of fertilizer now.

Annual flowers such as petunias, impatiens, begonias and alyssum will make an additional surge of growth if given more fertilizer now. If you have planted flowers near shrubs or trees, you will find that their roots grow into the areas where flowers are planted and share in fertilizer applied to them.

I usually recommend applying fertilizer to trees, shrubs, perennial flowers and other permanent plants in the early spring. If you did not give them any fertilizer this spring you could apply some now.

If you did not use a lawn fertilizer containing slow-release nitrogen in the spring, you will probably notice a loss of dark green color in July or August. A light fertilizer application (about half rate) now will turn them green again.

Iron is as important as nitrogen in producing dark green color. Always make sure your lawn fertilizer contains iron.

I sometimes use a liquid application of iron sulfate on my lawn. Three ounces of iron sulfate dissolved in a quart of warm water, applied with a hose application sprayer, will turn a lawn dark green almost overnight. The nice part of using iron sulfate is that it changes grass color without stimulating growth. Be sure to avoid applying iron sulfate to concrete (or wash it off as soon as possible) because it will stain concrete a rust color. Iron sulfate is available from most full-service nurseries and garden stores.

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