Plants growing under trees fight for water, nutrients, light

Gardening under trees can be a challenge for gardeners. Perennials living in the root zones of trees must compete for light, water and nutrients. Most of a tree's roots responsible for water and nutrient uptake are within the top 12 inches and may extend well beyond the canopy's drip line.

Rototilling, creating raised beds or root pruning to make a new garden bed may damage the tree's root system and cause the tree to decline or die. It may come to the decision: the tree or the garden under it. But if you know how to cope with the dry soil, shade, root competition and changing light conditions, you can have a healthy tree and a beautiful shade garden too.

Not all trees are created equal. Each type has specific light, soil and moisture requirements. Some trees, such as oaks, are extremely sensitive to environmental change like soil compaction, change in soil depth, and damaged or cut roots. Many trees, such as Norway maples, have very dense canopies that block sunlight and deflect rain. Selective pruning to thin the canopy will increase the amount of light under the tree.

Black walnut trees are especially challenging for gardeners. The roots of black walnut trees produce juglone, a chemical compound that is toxic to many plants. The toxic zone averages 50 to 60 feet from the trunk but can extend up to 80 feet. There is a long list of perennials that grow just fine under black walnut trees including ajuga, astilbes, bellflowers, coral bells, geraniums, hostas, Jacob's ladder, Solomon's seal and sweet woodruff.

It can also be difficult getting plants to thrive under evergreens like spruce and pine trees. In addition to a lack of sunlight and water, falling evergreen needles may increase the acidity of the soil. Consider planting acid-loving plants are azaleas, rhododendrons, hydrangeas, lily of the valley and sweet woodruff. Hostas, hardy geraniums, bleeding heart and Japanese painted fern also grow nicely under pines, but may need supplemental watering.

When planning a new garden under trees, begin by removing whatever grass is still there. Pull sparse patches of grass by hand. Instead of stripping turf which can damage the tree's surface roots, try smothering grass with several layers of wet newspaper covered with a couple inches of composted wood mulch. It may take several weeks for the grass to die, but rich soil will be the reward.

To keep root disturbance at a minimum, it's best to plant perennials rather than annuals. Resist the urge to add large amounts of soil under the tree to make planting easier. Adding more than a couple inches of soil can reduce water and oxygen supplies to the tree's roots.

Plant the smallest plants possible. Smaller plants mean smaller holes and less disturbance to tree roots. Be flexible in plant placement. If you come upon a large root, move the plant over a few inches. Start planting at least 12 inches away from the trunk of the tree and work outward.

Choose plants that tolerate dry soils and partial shade. Choose perennials like goatsbeard, wild ginger, barrenwort, deadnettle, and Solomon's seal. Others to consider include white wood aster, Jupiter's Beard, Virginia Bluebells and pachysandra.

Spring ephemerals and bulbs are good choices for planting under the canopy of deciduous trees because they grow and flower before trees leaf out in the spring. Good choices include rue anemone, trout lily, mayapple, bloodroot, celandine poppy, trillium and Virginia bluebells.

Interplant these with late-emerging perennials and grasses to hide their foliage as they die back. Native grasses and sedges can give the appearance of a meadow when massed together. Consider tufted hair grass, Gray's sedge, Pennsylvania oak sedge and bottle brush grass.

Water the entire area thoroughly when the planting is finished, and water weekly if conditions are dry. Be prepared to spot water plants that require more moisture. Top dress the garden each spring with a couple inches of organic matter to replenish the soil.

• Diana Stoll is a horticulturist and the garden center manager at The Planter's Palette, 28W571 Roosevelt Road, Winfield. Call (630) 293-1040, ext. 2, or visit planterspalette.com.