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Woodland Gardens

Chapter 4: Gardening in the Shade

by FM Mooberry

Success with a woodland garden requires some attention to the basic requirements of light and soil, proper plant selection and planting and some commonsense maintenance. It is not difficult and is well worth the effort.

As a first step to starting a garden, inventory what is already growing on the site. Are the trees deciduous or evergreen? Is there an understory of shrubs? If not, you may want to plant one. A close examination of existing growth may show desirable species that you want to save. If invasive non-natives have taken over, they must be controlled before you establish your garden. Hand weeding is safest because it will do the least damage to plants you want to preserve, including trees and shrubs.

Light and Shade

Available light determines the difference between an open meadow and a woodland garden. Woodland plants require partial or filtered sunlight for most of the day. Trees and shrubs provide the sheltering canopy but you need to understand what kind of light you have, and how to manipulate light and shade to achieve the optimum conditions for what you want to grow.

Different plants require or tolerate different degrees of shade. Gardening books provide guidelines for different species based on broad categories of dense, dappled or partial shade.

Light is also influenced by topography. For example, a south-facing slope receives much more light than a northern slope. What time of day your woodland garden receives direct sunlight is also important. Temperatures increase in the afternoon. Many plants require protection from the heat and the sun's burning rays at this time of day.

Sun and shade patterns differ with each woodland. Study and record the ones in your garden. The morning patterns will be different from those in the afternoon. What's more, because the sun's track changes during the year, spring sun and shade will be different from summer's. Shade patterns at the edge of the woodland should also be noted. Edges allow light to penetrate farther into the woods, even under large canopy trees. Record your findings in a notebook, along with simple sketches; these will become important tools for the planning of your garden, refreshing your memory as the process may continue over a considerable period of time.

Manipulating Shade

Even the most shade-tolerant plants often benefit from a bit more light. Too much shade can be corrected by pruning existing vegetation to allow light into the garden. I suggest that you prune on a sunny day when the trees are in full leaf and you can see the results immediately. If you're doing the work yourself you have the luxury of pruning over several growing seasons. Taking down a quarter of the branches at a time will avoid an overly severe thinning. Removing some of the major branches of large trees is a dangerous job best left to professionals. To keep down costs, you'll probably need to have this type of thinning done all at once.

The quickest way to provide additional shade if you have too much sun is to plant young trees and a dense secondary layer of shrubs. It is important to choose shrubs that will thrive in full sun when they are first planted. These sun-loving shrubs will decline in vigor as the trees grow and the shade canopy increases. Many different kinds of shrubs are available at local garden centers and nurseries. Most shrubs are fast growing and can be pruned up to provide a quick shade canopy. As your larger trees mature the shrubs can be pruned, thinned or removed altogether.

Soil

In addition to light and shade the selection of appropriate plant species should be guided by the drainage and soil characteristics in your yard. Instead of trying to change existing conditions, match your plants to the site-it's not only easier on the environment but also a lot less work for you. Most woodland plants will benefit from a rich soil that insures good air circulation, water and nutrient retention and drainage. Shade-loving plants also favor loose-textured soils, which promote deeper, more drought-resistent roots. Organic matter is the key. It modifies sandy soils to improve moisture retention and breaks up clay soils to add structure and reduce moisture retention. As it breaks down, the organic matter also slowly releases plant nutrients. Unlike herbaceous plants, trees and shrubs become established and grow better in the original soil, without amendments other than a surface layer of compost.

A simple test will determine the composition of your soil. Put 2 cups of soil taken from the top 6 to 8 inches of your garden into a clear glass quart jar. Fill with water, cap and shake vigorously. Wait 24 hours. The organic material will be the layer floating on the surface; the next layer is clay; the third layer, silt; and the bottom layer, sand. This profile will show you what basic materials you have and their relative proportions. A perfect soil for a woodland garden would have a small percentage of clay and sand and a large proportion of silt and organic matter.

Test the site for drainage by filling a hole with water. If there is still standing water after six hours, choose plants that thrive in wet soil.

Soil chemistry, particularly soil pH (the degree of acidity and alkalinity), is another consideration. For best success, look to your local woods for ideas on which plants are adapted to the soils in your area. The pH scale ranges from 0 to 14. The lower the number, the more acid; 7 is neutral; and high numbers are alkaline. Most eastern soils tend to be acidic, while midwestern and western soils are neutral to slightly alkaline. Most commercially available woodland plants grow best in slightly acid soil with a pH of 6 to 6.5. Azaleas, rhododendrons and blueberries thrive when the soil pH is around 5.5. Plants from regions with limestone bedrock do best in the range of 6.8 to 7.2. Red osier dogwood, staghorn sumac and native viburnums such as cranberry and arrowwood are shrubs that do well in alkaline soil.

You can purchase simple soil-testing kits from local garden supply outlets. You can also get a complete soil chemistry profile from your county's Cooperative Extension office for a small fee.

Planting

Spring is a great time to plant because temperatures are cool and rainfall is abundant. Roots have a chance to become established before it gets hot and dry. Fall is also a good time to plant; even after air temperatures drop, the soil remains warm, promoting rapid root growth. Transplant in the morning or evening to avoid stress from strong sunlight and the usually heavier midday winds.

The planting hole should be two to three times the diameter of the root ball but no deeper than the roots. Cover the plant with soil to the level the plants grew at the nursery, which you'll be able to see by a change of color on the stem. (Rhododendrons are shallow rooted, so take care not to set them in too deeply.) Water to eliminate air pockets and settle the soil. Don't step on the roots; rather, firm the soil with your hands. Use the excess soil to build up a small dike around the plant to retain water. Make sure your new transplants receive about an inch of water a week, whether from rain or hand irrigation, until they are established. Apply mulch to help keep the soil moist and keep down weeds.

Plants, especially trees and shrubs, are usually available in three forms: container grown, balled-and-burlaped and bare root. Container-grown plants. Container-grown plants are the most popular because they can be planted at any time during the growing season. Buy specimens in leaf so you know they are healthy. Try to avoid plants that are rootbound-that is, whose roots have completely filled the container and are wrapped around the bottom; unless you untangle the roots and spread them out they will continue to grow in a circle, never branching out.

When you're ready to transplant, remove the plant by tapping the bottom of the container; then lay the container on its side and pull the stems gently. Spread out the roots if necessary and position the crown at the proper level in the hole. If you have a flat of plants whose roots are intermingled into a solid mass, use a sharp knife and a spatula and cut out plants as you would cut a cake. Balled-and-burlaped plants. Many shrubs and small trees are available as balled-and-burlaped (B&B) stock. These are dug up with a quantity of the soil around the roots, then tightly wrapped in burlap or other material for transport and handling. A high percentage of the roots in a B&B plant have been cut away but those that are left are safely protected in soil, so be careful not to break up the root ball. Remove the burlap or other wrapping material just before you're ready to plant.

Bare root plants. Bare root plants are available only in early spring and late autumn because they must be dug up when dormant. The nursery will send them at the appropriate time for your region. Put them in as soon as they arrive or, if that's impossible, "heel" them in by placing the plants on their sides and covering the roots with good garden soil. Keep them moist until planted. Soak bare-root plants in a bucket of water for two hours before planting; trim off any broken roots and spread out the roots in the hole.

Mulch

If there's any "miracle ingredient" for a woodland garden, it's mulch. Organic mulches help improve soil structure as they break down over time. Mulch also inhibits the germination of weed seeds and weed growth, reduces the stress on plants by keeping down summer soil temperatures, protects against the winter freeze-thaw cycles that damage plants by keeping soil temperatures relatively constant, and conserves water.

Nature renews the mulch in woodlands each autumn as the plants lose their leaves. Woodland gardens likewise benefit from an annual application of leaf mulch in the fall. Fallen leaves become a valuable asset rather than a disposal problem. Shred or run the lawn mower over the leaves several times before applying them.

Other traditional garden mulches include pine needles, wood chips and shredded bark. Some more exotic materials that make good mulch are locally available, including buckwheat hulls, peanut shells, ground corncobs, cocoa hulls and licorice root. Eight to ten sheets of newspaper placed under mulch is especially effective in creating a weed barrier for new gardens.

Some materials are not recommended. Avoid using salt marsh hay and southern pine straw, as they are removed from native habitats where they are needed to support the natural system. Peat moss not only is extracted from bogs but also forms an impenetrable crust that repels precious water. Hay usually contains weed seeds. Green grass clippings soon turn into a hot smelly mess; leave them on the lawn where they break down, becoming a mild form of fertilizer.

Don't overdo the mulching. Organic mulch applied too thickly allows roots to grow into it. In hot, dry weather these fragile roots can burn and die. This is especially true of trees and shrubs. A good rule of thumb is to apply 3 to 6 inches of mulch and inspect it several time a year for root growth. And keep mulch away from the base of trees and shrubs, as constant moisture in such close proximity can encourage fungus build-up, rotting the bark.

As long as you're using organic mulch annually, synthetic fertilizers will be unnecessary.

FM Mooberry writes a column on native plants for The Kennett Paper and is a designer of native plant landscapes, including gardens at the Brandywine River Museum, Chadds Ford, PA. She founded the annual Native Plants in the Landscape conference held at Millersville University, Millersville, PA.