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Appearances Can Be Deceiving

Marble chips and bluestone look good, but your plants won’t like them

I recently drove by a modest home that was well landscaped with a moderate collection of azaleas, rhododendrons and Japanese hollies in the foundation planting. The junipers in the planting were thriving, while the remaining plants appeared chlorotic with dying branches. The house was dark green, and the white marble-chip mulch highlighted the plants.
    I have no idea as to how long the mulch had been in place, but based on the lush growth of the junipers and the declining appearance of the other species, it must have been in place for a year or more.
    Marble chips may look good as mulch, but they are not compatible with species that grow best in mildly acid soils. Marble chips are nothing more than a form of limestone, which means that as they weather from acid rains and the top dressing of fertilizers, they release lime that will raise the pH of the soil.
    Junipers grow best in an almost neutral soil, while azaleas, rhododendrons and Japanese hollies perform best in soil having a pH below 6.0. This explains why the junipers were thriving and the other species were declining.
    When we moved to our home in Deale, the foundation plantings around the house had been mulched with blue stone. Bluestone is made of calcium as well as other contaminants. The first thing I did was remove it all. Within a year, I saw improvement in the growth of the large Japanese hollies it had mulched.
    Another problem with bluestone is that it may contain toxic metals such as nickel or chromium. I have found numerous plant growth problems when paths of crushed bluestone have been used in landscapes. As the bluestone weathers, the calcium — as well as these toxic contaminants — are released. I have seen several old boxwood plantings ruined following the placement of crushed bluestone paths within their root zone.

Keeping a Smoke Tree Happy and Colorful

Q    Because I love trees with great fall color, I 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?
    –Lucy Goszkowski, Annapolis

A    Cotinus likes a pH near 6.5 to 7.    Once the tree becomes well established, the only method for maintaining the proper pH is by top-dressing with limestone. But without soil test results, I cannot help you further. Take eight to 10 core samples from a depth of six inches within the tree’s drip line and have them tested.

Ask Dr. Gouin your questions at [email protected]. All questions will appear in Bay Weekly. Please include your name and address.