Q. Windstorms have destroyed box elders in my yard. I am going to try some new plantings. Does a ginkgo tree require full sun or partial shade? What kind of fertilizer and soil does it prefer? I have the same question about a trio of arborvitae that should reach 10 feet as a screen for a parking lot. B.Q., Wisconsin
A. Arborvitae are a good choice, says Deborah Brown of the University of Minnesota Extension Service. You will have to shear them annually once they approach seven or eight feet, however. Otherwise, they will grow far taller than 10 feet.
If you let them tower out of control, it will be difficult, if not impossible, to prune them back heavily and still have them look good.
Arborvitae can thrive in partial shade to full sun, but if they will be in a really sunny location, choose the variety called Techney - it is less likely to brown out from winter sun.
Because arborvitae grow best in acidic conditions, work peat into the soil. Plant them in early autumn, then spread three or four inches of woodchips or shredded bark mulch on top.
Water thoroughly at planting time, then every week or so until the ground freezes. If rainfall is abundant, you may not need to water at all until next spring or summer. There's no need to fertilize until spring; when you do, use a plant food made for acid-loving shrubs and trees.
Ginkgoes, which are resistant to most pests and diseases, have the potential to reach 50 to 80 feet, but are fairly slow growing. They do best in full sun, but can manage in less. They're not fussy about soil pH, but prefer a somewhat moist, sandy soil. Just make sure you've purchased a male tree; females produce fruit that is foul-smelling and messy.
You can plant this tree now, watering and mulching as you would the arborvitae. Keep the mulch an inch or so away from the trunk to ensure good air circulation. Fertilize early each spring to encourage faster growth. You might want to have your soil tested to determine which fertilizer would be best for your circumstances.
If the ginkgo tree's mature height is too tall for you, Ms. Brown recommends two shorter trees as excellent for your area: Amur maple (Acer ginnala) and Tartarian maple (Acer tartaricum). Other good choices are Japanese tree lilac and Russian olive, although some people don't like Russian olive's irregular shape or the thorny twigs it often drops, she notes.
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