Home & Garden

Showy oak-leaf hydrangea

Be on the lookout for problems with gardenias

Gardenias dazzle with their beautiful, fragrant flowers. But they also disappoint when insects set in and foliage develops odd colors. As the season opens, here's what to watch for and how to deal with common problems.

White flies. This common sucking insect is obvious when clouds of tiny white specks of adult white flies erupt off the plant. You'll see eggs and nymphs on the undersides of leaves. They excrete a sticky substance that leads to a black sooty mold on the tops of leaves.

A horticultural oil such as All Seasons Spray Oil, or one of the summer oils specifically formulated for use in warm weather will help, but you must spray the undersides of leaves carefully. Repeat as directed on the label.

Yellow leaves. This is likely due to magnesium deficiency. It typically appears on the oldest leaves. Symptoms include yellowing of margins on oldest leaves, curling edges and yellowing between the veins of the leaves.

Fix this problem by improving production of chlorophyll and use of fertilizer components with the application of Epsom salts (magnesium sulfate).

The Epsom Salt Council, an industry organization, recommends 1 tablespoon per 9 square feet of root zone, applied every two to four weeks. Epsom salts are highly soluble and rainfall will work it into the roots.

Yellow leaves with visible green veins. This is iron deficiency, usually most evident on the youngest leaves, and is easily corrected by the addition of chelated iron to spray on the foliage or over the root zone. More than one application is required. It stems from a high soil pH that prevents the plant from absorbing iron.

Yellow-green leaves that drop. A deficiency in nitrogen from inadequate fertilizer can lead to foliage that turns yellowish green, then drops prematurely in summer. This also stunts the growth of the plant. Look for veins or undersides of young leaves with reddish-purple cast. Correct this nutritional deficiency with a fertilizer formulated for acid-loving plants such as Holly-Tone.

Overgrown plants. Gardenias surprise people with the size they attain — rather rapidly. The best time to prune gardenias is immediately after the main flush of bloom in early summer. When a plant needs only modest shaping, do this by cutting flowers for the house. Make your cut just above a lateral branch or bud lower on the stem.