Watch Our for Spider Mites

A friend told me the leaves of her Ficus benjamina are covered with black soot. I suspected that the plant had been infected with a soft-scale insect that exudes a honeydew substance that breeds sooty mold. However, upon examining the plant, I saw it was severely infested with spider mites.

If you moved your houseplants outside for summer, there is a good possibility that they are infested with spider mites.

Spider mites are tiny insects about the size of dust particles. On houseplants, look for them on flower and vegetative buds, in the axis of leaves and on the undersides of leaves. Most often you will see their tiny web before you discover the mites.

An easy method of detecting them is to place a clean white sheet of paper under the leaves and branches and striking them rapidly with your hand. If you see red or green particles moving on the paper, your plant has a heavy population of spider mites.

The best method of control is to keep the plant outdoors and spray it thoroughly with a strong jet of water, which will remove half to three-quarters of the spider mites.

As soon as the foliage has dried, spray with a four percent concentration of horticultural oil, summer oil or Volck oil — about one teaspoon of oil per quart of water. Thoroughly spray the entire plant, especially the underside of the leaves and the buds in the axis of the leaves.

These oils are safe to use even in the home, and they will kill not only the adult spider mites but also the eggs. The problem is that the oils leave a greasy residue, so it is best to spray the plant outdoors. The oils will also give a gloss to the foliage.

Keeping a Happy Smoke Tree

Q: Because I love trees with great fall color, I just purchased a native smoke tree, Cotinus obovatus. Michael Dirr’s book describes it as living in limestone soils. How should I amend the soil when I plant it, and do I continue to amend as long as I live where I’ve put it? How do you amend soil for an established tree? Also, I need to prune off one branch to establish a strong central leader. It had a small broken twig when I bought it, and it seems to bleed a lot of sap. Any suggestions on when to prune it?

–Lucy Goszkowski, Annapolis 

A: Cotinus likes a pH near 6.5 to 7. Have your soil tested from samples taken at a depth of six inches from eight to 10 core samples taken within the drip line of the tree. Without soil test results, I cannot be of any help.

Once the tree becomes well established, the only method for maintaining the proper pH is by top-dressing with limestone.