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. 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 replacing the soil. 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. 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. 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. 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. -- Never leave lids off garbage cans, indoors or out. -- Keep compost piles far from the house. -- Rinse bottles and other containers before putting them in recycle bins. What to Plant and When You can still plant trees and shrubs this season. If we have an early freeze in November and you are unable to put them in the ground, heel in their roots. Buy several bags of shredded hardwood bark mulch or compost, and pile the organic material liberally around the ball or container. Be sure to water deeply. You can plant most spring-flowering bulbs now. Install them about three times as deep as their height. Tulips can wait until November. Keep cool and dry until planting. A Greener Thumb About 37 percent of baby-boomer potential home buyers said they wanted an 'environmentally friendly' home, according to a survey by the National Association of Home Builders and the MetLife Mature Market Institute. Well-selected plants can complement the ecological attributes of these properties to further reduce their carbon footprint. The Agriculture Department estimates that one acre of forest absorbs six tons of carbon dioxide and releases four tons of oxygen, enough to meet the needs of 18 people. That same acre will remove 13 tons of particulate pollution and potentially harmful gases. A young, healthy tree equals 10 room-size air conditioners operating 20 hours daily. Lerner is president of Environmental Design in Capitol View Park, Md., and author of 'Anyone Can Landscape'(Ball 2001). Contact him through his Web site, www.gardenlerner.com. 