Planting under trees poses different challenges

Gardening under trees can be tricky! Plants living in the root zone must compete for light, water and nutrients.

Most of the tree's roots that are responsible for water and nutrient uptake are within 12 inches of the surface and can even extend well beyond the canopy's drip line.

To give your new shade garden an advantage, you can choose to till, create a raised bed or root prune the tree, but these methods can damage the root system of a mature tree, causing it to decline or even die. Sometimes you must decide, which is more important to you, the tree or the garden under it.

Nevertheless, 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 (soil compaction, change in soil depth or grade, damaged/cut roots, paving, etc.), and can be damaged by the amount of water needed to maintain a lush lawn. Frequent watering near the root crown creates a moist environment favorable to insects and root diseases.

Many trees, such as Norway maples, have very dense canopies that block sunlight and deflect rain. Selective pruning to thin out the canopy will increase the amount of light under the tree. Pruning will also need to be repeated as the tree grows.

Black walnut and butternut trees can be a challenge for home gardeners. The roots of these trees produce juglone, a chemical compound that is toxic to some plants. The toxic zone averages 50 to 60 feet from the trunk, but can extend up to 80 feet. Some plants that grow well under black walnut trees are arisaema triphyllum or jack-in-the-pulpit, dicentra cucullaria, also called Dutchman's breeches, osmunda cinnamomea or cinnamon fern, pulmonaria, which goes by lungwort, and tradescantia virginiana, known as spiderwort.

A more extensive list can be found at www.ohioline.osu.edu/hyg-fact/1000/1148.html.

It can be especially tough getting plants to grow under evergreens, such as spruce and pine trees. In addition to a lack of sunlight and water, falling evergreen needles may increase the acidity of the soil to a level that most plants can't tolerate. Some 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 well under pines, although for best success, they may sometimes need supplemental watering.

To keep root disturbance at a minimum, it's best to plant perennials rather than annuals under trees.

Do not add large amounts of soil under the tree to make planting easier. Adding more than a few inches of soil can reduce water and oxygen supplies to existing roots, which will lead to an unhealthy tree.

Choose plants that will tolerate dry soils and partial shade. Spring ephemerals and bulbs are good choices for planting under the canopy because they grow and flower before the trees leaf out in the spring. Interplant these with late-emerging perennials and grasses to hide their foliage as it dies back. Native grasses and sedges can give the appearance of a meadow when massed together. Taller-growing shrubs should be planted outside the drip line.

Some suggestions

Grasses/sedges - Deschampsia caespitosa, called tufted hair grass; carex grayi or Gray's Sedge; carex pennsylvanica, known as Pennsylvania oak sedge, and hystrix patula, commonly called bottle brush grass.

Wildflowers - Aster divaricatus, the white wood aster; aster lateriflorus or Lady in Black, also called woodland aster; centranthus ruber, which goes by Jupiter's beard; hepatica americana, which you might find as round lobed hepatica; mertensia virginica, called Virginia bluebells;, and pachysandra procumbens or Alleghany spurge.

Ferns - Dennstaedtia punctilobula, which you might know as hay-scented fern; dryopteris spinulosa, called toothed wood fern; osmunda claytoniana, named interrupted fern, and polystichum acrostichoides or Christmas fern.

Spring ephemerals - Anemonella thalictroides is more commonly called rue anemone; you might know erythronium americanum as trout lily; podophyllum peltatum goes by mayapple; the common name for sanguinaria canadensis is bloodroot; stylophorum diphyllum is celandine poppy; trillium, and viola cuculata or marsh blue violet.

Perennials - Aruncus dioicus is known as goatsbeard; asarum canadense you might find as wild ginger; epimedium goes by barrenwort; lamium is a popular name for dead nettle, and polygonatum biflorum is more frequently called giant Solomon's seal.

Care

Follow these steps when planting and caring for your garden:

1. Determine the size and shape of the area to be planted by using a flexible garden hose or rope. Spray paint the edge for a more durable outline.

2. Pull sparse patches of grass by hand. Avoid stripping the turf because this can damage the fine surface roots of the tree.

The least invasive way to kill unwanted turf is to smother it with five to six layers of wet newspaper covered with 1 to 2 inches of composted wood mulch. Avoid using fresh wood chips because microorganisms take up valuable nutrients as they decompose. It may take several weeks for the turf to die back using this method, but you will be rewarded with rich soil.

3. Think small when purchasing your plants. Smaller plants need smaller holes, which means minimal disturbance to the tree's roots.

4. If you come upon a root more than 1 inches in diameter, move your planting spot to avoid damaging it. Start planting at least 12 inches away from the trunk and work outward.

5. When you are finished planting, water the entire area well. Water weekly if conditions are dry, and spot water plants that need more moisture. Avoid fertilizing the first year because it encourages green growth over root growth. Top dress the planting bed each spring with 1 to 2 inches of composted material to replenish the soil.

• Diane Cunningham is a horticulturist at The Planter's Palette, 28W571 Roosevelt Road, Winfield, IL 60190. Call (630) 293-1040.