Green Scene

Wisdom From the Gardener's Notebook

Native plants like this Virginia sweetspire are desirable plants to install.
Native plants like this Virginia sweetspire are desirable plants to install.
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By Joel M. Lerner
Saturday, October 3, 2009

One of the great joys of writing this column is collecting all sorts of interesting tidbits about gardening. It's time to clean house and share some unique stats and entertaining facts from the gardener's notebook.

The Do's and Don'ts of Mulch

Many people mistakenly think that piling on ornamental hardwood bark is the best way to improve soil. In fact, the proper method for conditioning soil is by spreading two inches of compost over the earth. If you like the look of dark, aged, double-shredded hardwood bark, lay a half-inch veneer over the compost.

Over-mulching, in fact, can be bad for your garden. Francis Gouin, a retired horticulture professor at the University of Maryland, has found exceptionally high levels of manganese where excessive layers of shredded hardwood bark mulch had been spread. One of the worst cases of manganese toxicity was in Columbia, where beds had as much as six inches of mulch. Manganese is good for plants in trace amounts, but too much can spell trouble. Plants display pale leaves and stunted growth, then die. Ordinarily, lime can remedy high manganese levels, but sometimes the only cure is totally replacing the soil. Note: these symptoms can be traced to many other problems. Be sure to get a soil diagnosis from your local Cooperative Extension.

When buying mulch, look for the Mulch and Soil Council product certification logo. The council tests and certifies that companies have complied with industry standards, verifying that the product does not contain wood treated with chromated copper arsenate, an insecticide found in treated lumber. (For more information, go to

As you prepare to treat and dress the soil in your garden, remember that one of our most precious resources can be cultivated right from your own yard. About 24 million tons of leaves and grass clippings end up in landfills each year. This valuable organic matter could be put to far better use as compost. If your county doesn't have a composting program, build a pile in your yard. The brown leaves and wood chips can be up to 90 percent by volume -- green is supplied by grass trimmings and leafy herbaceous plants. A little compost or soil, moisture, and air are the catalysts to get the process going.

Keep Pests in Check

Not everything with six or eight legs is a pest. Lady beetle, lacewing, aphid lion, trichogramma wasp and other parasites that kill caterpillars and larvae are beneficial insects. Refrain from using even the most benign insecticide, unless you must.

Outdoor flying insects are drawn to brightness. To keep them away, install a dimmer switch on your outdoor lights. You can also mount light fixtures on tall branches to draw insects to them.

Mosquitoes can be kept in check by policing any standing water on your property. Fill low spots with soil, change water in birdbaths and dog bowls, and clean roof gutters. For fountains and pools without fish, use Bt israelensis Mosquito Dunks.

Some food-based products can help keep bothersome bugs under control. For example, almond oil painted on wood discourages carpenter bees from boring into decks to lay eggs, while garlic spray repels Japanese beetles and aphids.

To avoid pest issues indoors, the makers of Pest Bomb extermination products offer the following useful guidelines for homeowners:

-- Be careful when bringing in firewood. You might bring a pest inside that you don't want.

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