I want to start a rain garden in an area of my yard that sometimes is boggy but not always. The area is mostly sunny and the soil is sandy. I want plants that are native and not overly aggressive. Do you have any suggestions for a garden plan?

Polluted runoff is a big problem in urban areas where much of the ground is covered with hard surfaces such as roofs, streets, parking lots and sidewalks. Before development, rain and snow melt seeped slowly into the earth. Now water flows quickly across hard surfaces, picking up pollutants—from organic particles, pesticides, fertilizers, gas, oil and other types of residue—before dumping into storm drains.

Once in the storm sewer system, the water flows into local lakes and streams, thus polluting our water supply. Rain gardens are designed to direct polluted runoff into a low, vegetated area, where the pollutants can be captured and filtered.

The features of a rain garden aid in this biofiltration process: a shallow basin depth, gentle side slopes, soil that allows infiltration, and vegetation that traps sediment and sediment-polluting runoff. Vegetation shields the soil surface from raindrop impact while the root mass holds the soil particles in place. Improved water quality results from the nutrient removal process as the water and pollutants come into contact with roots and microbes in the soil. Plants, trees, and groundcover absorb up to 14 times more rainwater than a grass lawn.

The basic design components of a rain garden are a grass filter strip, a shallow surface water ponding area, a bioretention planting area, a planting soil zone, an underdrain system, and an overflow outlet structure.

A shallow ponding depth—approximately six inches—is preferred, underlain by two to four feet of depth for the planting soil zones. A strip of turf or groundcover at the top edge of the rain garden slows water as it flows into the garden and filters sediments. Water should infiltrate within 4 to 6 hours.

Ideally the soil is composed of a blend of 20 percent organic matter, 50 percent sandy soil, and 30 percent topsoil. This blend will naturally filter the rain as it runs into the rain garden. Some clay is desirable, because clay particles absorb heavy metals, hydrocarbons, and other pollutants. However, the clay content should not exceed 10 percent of the total. Clay soils hold water well, but high clay concentrations may cause poor drainage. Sandy soil permits water percolation, but very sandy soil is too permeable. If the soil is too sandy and will not hold water for any length of time, you may wish to add composted organic matter to the soil to increase moisture holding potential.

A soil pH of 5.5 to 6.5 is ideal for pollutant removal by microbial activity. A mulch layer on the garden surface aids in the decomposition of organic matter and helps to remove metals. It also helps to suppress weeds. Shredded hardwood mulch is best, because it resists flotation and has a greater surface area for binding metals in runoff. Soils in the landscape holding areas should not be compacted because this inhibits the water moving through the soil.

The way to make a rain garden, or any garden, appear “well kept” is to keep the edges tidy. Tall plants and grasses tend to flop over, so if you want a neat silhouette, you will want to stick with short species. Plants that can tolerate standing water and fluctuating water levels are typically planted in the center of the rain garden, while those at the outer edges grow in slightly drier conditions. Your local nursery can make recommendations as to what plants would be suitable for your needs. For more information on building a rain garden you can check out the website from Rain Gardens of West Michigan (www.raingardens.org).

A couple quick tips: Make sure that you locate downspout rain gardens at least ten feet away from the foundation of your house, so you don’t inadvertently direct water into your basement. Also, even if you are constructing a simple system with a rake and shovel, be aware of underground utility line locations.

When can I start putting a pre-emergent herbicide such as Preen on my garden beds?

There are two types of chemical weed controls: post-emergent and pre-emergent. Post-emergent herbicide kills weeds that are actively growing. A pre-emergent (Preen being one of these) prevents weed seeds from germinating, but does not kill existing weeds.

Pre-emergents work in the top 1 to 2 inches of the soil by creating a chemical barrier that prevents cell division in developing weed seeds. Different weeds germinate at different times of the year, and pre-emergents last anywhere from 3 to 6 months, depending on the brand you use and local conditions. The rate at which pre-emergents biodegrade in the soil and lose their effectiveness will depend on temperature, soil type, and moisture conditions of the soil. They will need to be reapplied more often in hot, humid conditions.

Depending on the brand, some pre-emergents are applied over the top of mulch, and some are applied to the soil before mulch is applied. To be effective for the spring season, apply pre-emergent in late winter or early spring, before the soil warms up.

While pre-emergents will not harm your plants if they come in contact with them for a short period of time, always wash or brush off stray granules from plant foliage to minimize damage or discoloration that could be caused by long-term direct contact with the chemical. Never use a pre-emergent herbicide in your vegetable garden, and be careful around areas where you intend to sow grass seed. If you spill a little in an area where you intend to plant grass, it will not grow.

The October 2006 “Tree Tips” column about oak tree problems discussed anthracnose and that a spring spray would help control that disease. What kind of spray would be best for my 2 very tall trees?

Since your trees are “very tall,” it is likely you cannot reach them while standing on the ground to prune or to spray. As such you need to call a professional. Also, the equipment used to help prevent anthracnose diseases in oaks requires the power of commercial grade hydraulic sprayers to propel the protectant.

Call a certified arborist who can examine your oak trees for problems, make an accurate diagnosis, and determine if they are in need of a preventive spring spray. A reputable arborist will be licensed by the state not only in tree knowledge but also in chemical application. They have the proper equipment and protective gear that are necessary when dealing with these products. They know the correct mixture and application regime to accomplish what is needed.