Well-Dressed Garden: Gardening under the trees
Gardening under trees can be frustratingly difficult. A tree's blissful shade limits the selection of plants gardeners can grow, though weeds never seem to have any trouble moving in. But if you work at it, you can have a beautiful, healthy tree and a lovely shade garden, too.
Instead of trying to fight the naturally shady conditions under trees, take advantage of them, says Linda Chalker-Scott, an extension specialist and professor at Washington State University. A simple ring of mulch that extends out to the drip line (the outer edge of a tree's canopy) will make your whole garden look sharply manicured and benefit your trees tremendously, Chalker-Scott says. In her own garden, she puts down a generous 8-inch layer of mulch every year, but her rule of thumb for home gardeners is a 4-inch layer, which is enough to keep annual weed seeds from germinating in the soil under trees.
A ring of mulch gives a clear a signal not to cut across the yard or park too close to trees, so it helps reduce soil compaction. A generous layer of mulch also provides nutrients to trees as it decomposes. Mulch provides a habitat for beneficial insects and microbes, and it reduces the need for herbicides and fertilizers. Mulch can be free, too: Chalker-Scott advocates using "arborist mulch" from tree-trimming crews. Often, all you have to do is ask tree-trimmers to drop off their wood chips in your driveway.
Arborist mulch is usually fairly coarse, with chips no less than about half an inch in diameter, to allow air and water to move through. Don't worry, Chalker-Scott says -- this mulch will not suffocate roots or invite termites to invade your property. For the health of your trees, place mulch around them in a donut shape, not like a volcano, she says. The mulch ring should have high outer edges and taper down to a thin layer around the trunk in the center.
A sweep of grass under a tree isn't out of the question -- but you have to be realistic. "You can definitely grow grass under a tree, just not a manicured turfgrass," Chalker-Scott says. "A mixture is better than monoculture," she says. Instead of trying to plant bluegrass, rye, fescue or any other single-turf species under a tree, look for a grass mixture formulated for shade, which will weather conditions under a tree without looking scruffy. Make violets and clover welcome, if they appear, and set your mower's cutting height up a bit -- in shade, grass will naturally grow a little taller than in bright-sun conditions.
To give turf under shade trees a chance to grow and thrive, avoid walking on the grass. If it simply never flourishes and you find you need to plant fresh seed every year, try another approach.
Low-growing groundcovers, such as liriope, ajuga, violets and pachysandra, are all well-suited to the dappled light under the canopy of trees. Groundcovers chosen for your climate and conditions are easy to establish and will not compete aggressively for the water and nutrients in the soil. They also protect tree trunks from mowers and string trimmers, which Chalker-Scott calls "instruments of doom" for your trees.
If a path through your garden leads through an area with trees, a bed of gravel and a series of stepping stones will direct traffic and limit soil compaction. Mulch or groundcovers on either side of the path will give the area a tailored look.
Mature trees are garden treasures, and they deserve special recognition. A luxurious ring of mulch around a large tree might not be feasible, and a dense canopy may make it impossible to grow even the most shade-tolerant groundcovers. In such a situation, a handsome garden bench or a tree seat custom built around a great old tree will transform the scene. From the house, the quiet tableau will invite you out, even if only in your imagination, every time your glance falls on it. And when you are out in the garden, a bench under a tree is the perfect place to escape from the sun and set your tools down for a moment, a spot where you can linger and listen to the wind and the birds and forget about the busy world in the bright sun outside.