August gardening: Caring for perennials and controlling magnolia scale

Continue to groom your perennials and annuals by removing yellow foliage and spent flowers. Make note of perennials that have flopped so that you can install a support system next spring before the plants need it.

Gentle pruning of more vigorous perennials can help keep your border from looking overgrown. Cutting leaves off at the base can reduce the size of the plants without having them look pruned. You may want to allow certain dried flowers to remain standing on plants such as astilbe and coneflower for fall and winter interest.

The purple coneflowers in my home garden are putting on an amazing show now. Goldfinches visit my coneflowers in fall and eat the seeds.

The gladiolus produces a large, showy flower spike that lasts for several days whether in the garden or in a vase. This is one of the plants that got me interested in gardening as a child. To get the most out of a bloom for indoor decoration, cut when the lowest blossoms have begun to show color. Place the cut stem in water as quickly as you can. Stagger the planting of gladioli early in the season to provide flowers over a longer period.

This is the time of year to check closely for magnolia scale — an insect about the size of a pencil eraser. Typically found on the underside of magnolia branches, the scale is responsible for creating an overall thin condition on the tree.

The scale exudes a clear, sticky honeydew after feeding on the plant. Black sooty mold is a fungus that grows on the honeydew. Gardeners often notice the black mold on or under the magnolia before noticing the actual scale. The presence of this black mold on other types of trees can also indicate problems with other insects. The mold itself is not the real problem.

On small trees, simply pick off the scale and squash them. Other control measures include spraying with summer-weight oil or applying an insecticide when the scale is in the active crawler stage, generally in early September. For trees with a history of magnolia scale, a late-winter to early spring application of dormant oil will provide good control for the scale. If your tree is affected, provide extra water during summer to minimize stress.

I have been seeing fewer magnolias with scale so far this year. There is no reason to treat a magnolia for magnolia scale if you haven’t seen any indication that scale is present.

• Tim Johnson is director of horticulture at Chicago Botanic Garden, chicagobotanic.org.