Digging In - Advice on Exposed Roots, Burning Bush

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By Scott Aker
Special to The Washington Post
Thursday, February 12, 2009

Q My neighborhood is almost 20 years old. I have noticed a lot of maturing trees developing exposed roots, including some in my yard. Is this because trees tend to be planted in mounds aboveground and the roots extend out to the surface? Is this harmful to the trees?

A The presence of surface roots is related to soil type and tree species. Some trees, such as beech and silver maple, tend to have large roots near the surface regardless of the soil conditions. Others, such as white oak and hickory, rarely have many large roots near the surface. However, site conditions may force the roots of almost any tree to remain shallow. If the soil is wet or compacted, roots cannot find the oxygen they need. Only a handful of trees will put roots in soil that has little oxygen; bald cypress, silver maple and willow are examples.

One other cause of surface roots is soil erosion, and another is the natural growth of the root. As with branches, roots get thicker with age, so a young root just below the soil surface may end up entirely aboveground.

When a house or subdivision is built, the soil is typically compacted as it is graded by heavy equipment. This stabilizes the soil, but it also limits the ability of trees, shrubs, even turf to grow.

Trees that develop surface roots out of need are no less healthy than other trees, but their roots may be more prone to physical injury.

To establish trees in compacted soil, landscapers plant them above the soil level. It is better to work organic matter into the top eight to 12 inches of soil before planting, though this is rarely done. Often, the new landscape consists almost entirely of plants that are tolerant of compacted soil. Red maples, Bradford pears and plane trees will grow in such conditions and are commonly used.

I planted three winged euonymus, or burning bush plants, about four years ago in my front yard, near a cherry tree. They get early sun and then shade later in the day. However, they never develop the bright red fall color for which they are famous. The foliage is a dull red and yellow, and drops before getting vibrant. Is this due to lack of light, and is there anything I can do short of transplanting them?

A burning bush will develop its full brilliant red fall color only in full sun. You may have to move your plants to get the effect you want. You could replace them with other shrubs with beautiful fall color in partial shade conditions. Virginia sweetspire and oakleaf hydrangea develop good fall color in partial shade and have the added benefit of showy flowers during the summer. If you don't have room for full-size shrubs, opt for dwarf varieties, namely Virginia sweetspire Little Henry or the Pee Wee or Sikes Dwarf varieties of oakleaf hydrangea. Both hold their autumn leaves longer than burning bush. Virginia sweetspire sometimes retains its red fall foliage as late as early December.

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


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