Bay Gardener by Dr. Francis Gouin

White Birch Belongs up North

Southern Maryland gardens want native yellow river birch

Arbor Day has just passed, and it’s the season of tree planting.

At a recent lecture, one of the planters asked why she was having so much trouble growing white birch trees in her yard. She lamented that she had replaced three in 10 years. Where she came from, she said, white birch trees grew like weeds. I concluded that she came from either New England or from northern New York or Pennsylvania.

White birch trees originate in regions of long, cold winters and moderate summers. In Southern Maryland, we do not have ideal growing conditions for white birch. Many of our winters are mild, and often the trees will not receive adequate cold to resume growth early in the spring. Most summers are much too warm for optimum plant growth.

The white birch, left, is a denizen of the north, wherease the heritage birch, right, prefers our milder climate.

Another environmental factor that causes extensive damage to white birch is the frequent orange and red air-pollution alerts of summer months. The white birch tree is extremely sensitive to air pollution, which often results in severe defoliation during an important growth period. I have seen many white birch trees loose half or more of their foliage during July and August. Losing such a high percentage of leaves and having to produce new leaves inhibits the accumulation of important plant metabolites essential for good growth and survival.

The combination of environmental factors weakens the trees, making them more susceptible to invasion by cankers, foliage-feeding insects and trunk borer. The National Arboretum in Washington, D.C., has tested different species and cultivars of white birch without satisfactory results.

A good substitute for white birch is the heritage birch. Heritage is a yellow river birch native to this area. Its bark, while not as white, is an attractive exfoliating light yellow-brown. Since it is propagated by cuttings, it can be grown as a single individual tree or a clump by simply planting three or more rooted cuttings together. It is susceptible to air pollution damage, but it is otherwise better suited to Southern Maryland conditions.

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How to Treat New Rhododendron

Q We just bought some rhododendrons and wondered about mulch. Ron chipped a lot of forsythia and vines and wondered if he could use those chips. I was concerned. We also have bags of leaves a couple of years old.

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A Plant them shallow, mulch them with leaves, and water them only twice weekly for the first month. Water them well.