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By Scott Aker
Special to The Washington Post
Thursday, April 9, 2009

Q I have a walking stick tree, now 10 feet tall, that is afflicted with scales that release a white powder when I scrape them off. What treatment would be effective against them? Also, I've always liked mimosa trees for their pretty pink puffballs, so I jumped at the chance of getting a 12-inch plant my friend had grown from seed. She said, however, that my neighbors might regard it as a weed tree. Is it a weed?

A Your 10-foot tree is commonly called the devil's walking stick, Aralia spinosa. It sounds as though you are dealing with a scale insect infestation on your specimen. Scale insects seldom kill plants, but they do make them unsightly. Spray it thoroughly with horticultural oil mixed according to package directions.

As for the mimosa tree, weediness is often in the eye of the beholder. Mimosas do set a lot of seed and are commonly encountered in alleys and vacant lots. I pull them up in my own garden every year. On the other hand, their lacy foliage and pink powder-puff flowers have their appeal. One selection, Summer Chocolate, has deep burgundy foliage. They are tough trees that are extremely drought-tolerant and capable of thriving in the worst conditions. They are also interesting because their leaves fold up during the night.

Mimosas do have one fault that has prevented their widespread use in landscapes. They are prone to a vascular wilt disease caused by Fusarium oxysporum. It is a soil-borne fungus that may invade through wounded roots, so you should take great care in not injuring the roots of a mimosa that you'd like to keep.

At what distance from a black walnut tree can one safely plant fruit trees?

Black walnut trees exude a compound called juglone that is toxic to many other plants. Fruit trees differ in their sensitivity to juglone. Cherries, persimmons and plums will coexist much better with black walnuts than apples or peaches. Generally, it is best to stay 20 to 50 feet from the outermost limit of the tree's canopy if you are growing fruits that cannot tolerate black walnut.

Scott Aker is a horticulturist at the U.S. National Arboretum.


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