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Solving the Horrors of Dry Shade

Few plants tolerate the triple trouble of dry soil, root competition and lack of sunlight found under big shade trees. The going gets especially tough when tree roots are close to the surface, such as beech, birch, tulip poplar, honeylocust and many oaks and maples.

Unfortunately, the plant most people try to grow under these substantial trees – lawn grass — is one of the least suited.

Grass and big trees make lousy botanical neighbors. Tree roots eventually out-compete grass roots for moisture and nutrients, while the growing leaf canopy each year shuts off more and more of the light that grass craves.

On the other hand, research from Illinois’ Morton Arboretum recently found that grass around tree trunks inhibits the growth of tree roots — at least in the early years. Morton’s researchers found that maple trees surrounded by an 8-foot-diameter of bark mulch had four times more roots in the top 6 inches of soil than trees with grass up to their trunks.
The moral: Scrap the grass and look to better competitors.
The dry shade and root competition under trees makes tough going for a lot of smaller plants. One option is eliminating grass and just mulching under the tree canopies.

Variegated Solomon’s seal is an example of colonizing plant that can be used in a mass under shade trees.

One option is to just mulch the ground underneath. Three inches of wood mulch and an occasional weed patrol is enough to keep it looking clean — barren, but clean. Your tree probably would vote for this no-compete plan.

A second option is laying flagstone over the mulch and using the canopy as a sitting area. Add a bench or a few chairs, maybe a potted plant or two, and you’ll go from a dusty wasteland to a useable space. Consider: The same shade that discourages grass is a plus for people.

A third option is switching from grass to a different ground cover that’s better suited to dry shade and root competition.

Pachysandra — either the native Allegheny spurge (P. procumbens) or the faster-growing and more common Asian type (P. terminalis) — is a common and inexpensive choice. Both are evergreen perennials that are tall enough (6 inches) to choke out creeping weeds but polite enough not to creep into the surrounding lawn or up the tree.

Lilyturf (Liriope muscari) is another tough competitor in dry shade. This perennial is a grassy-looking, 1-foot-tall, semi-evergreen that gets purple flower spikes in late summer. All they ask is a once-a-year, end-of-winter whackback to clear last year’s growth for new shoots.

A grossly under-used, under-tree perennial is barrenwort (Epimedium sp.), which gets red-tinged, heart-shaped leaves and dainty, hanging, bell-like flowers in spring. These are slow to get going but make an attractive, weed-choking planting by the third or fourth year.

Yet another under-used beauty is leadwort (Ceratostigma plumbaginoides), a 10-inch creeper that gets true-blue flowers in late summer followed by glossy scarlet foliage before the leaves drop for winter.

Then there are the many new introductions of evergreen hellebore hybrids (Helleborus hybridus), a gazillion choices of hosta, several nice species of heart-shaped, evergreen hardy gingers, a 1-foot-tall variegated colonizer called Solomon’s seal (Polygonatum odoratum var. thunbergii ‘Variegatum’), and an ever-expanding array of silvery- and dark-leafed coral bells (Heuchera sp.)

Some people like the idea of picking one species and massing it out for a grass-substitute effect. But there’s no reason why you can’t pick two or more of those dry-shade perennials and mix and match them for a more “gardeny” look.

Examples: use an inner ring of blue hosta surrounded by an outer ring of liriope, or plant taller hellebores and Solomon’s seal around the inside with a perimeter planting of shorter barrenwort or leadwort.
For an even more gardeny approach, add a few shrubs and evergreens.
Toughest to get going are the species that people tend to try first because the labels say, “Best in shade.” Plants such as azaleas, rhododendrons and mountain laurels may tolerate shade, but they’re notoriously difficult to grow when young, container-grown plants are retrofitted into beds where the trees have had a huge head start.

Mix and match several dry-shade-tolerant perennials under trees, such as this combination of hosta, liriope and barrenwort.

Better choices on the evergreen front are:
1. Dwarf boxwoods.
2. The fragrant, glossy spreader, sweetbox (Sarcococca hookeriana var. humilis).
3. The trailing juniper look-alike, Russian cypress (Microbiota decussata).
4. Two holly look-alikes: mahonia (either Mahonia aquifolium or M. bealei) or variegated falseholly ‘Goshiki’ (Osmanthus heterophyllus).
5. Actual hollies, such as the blue hollies ‘Blue Prince’ and ‘Blue Princess’.
6. Cherry laurel (Prunus laurocerasus).
7. Spreading English yew (Taxus baccata ‘Repandens’).
8. Or the deer-resistant yew look-alike, Japanese plum yew, (Cephalotaxus harringtonia).

For flowering shrubs, consider Virginia sweetspire (Itea virginica). These are surprisingly dry-shade-tolerant once established (their fall foliage is stunning).

Three others that hold their own in dry shade are most St. Johnswort (Hypericum sp.), the native dwarf fothergilla (also known as “witch alder”), and the native smooth hydrangea (Hydrangea arborescens).

If the trees are limbed up enough to give even a bit of sun, try viburnums (especially the native arrowwood and smooth witherod types) and native oakleaf hydrangea (Hydrangea quercifolia).

Before planting anything, it helps immensely to loosen the soil as best as you can among the tree roots. Then work 2 or 3 inches of compost or quality topsoil into the native soil. This will give your young plants a little elbow room to get started before having to do battle with the more elaborate tree roots.

Don’t slice into any index-finger-size and up tree roots if you can help it. Plant around them. Disrupting or digging up some of the smaller mats of “feeder” roots is less of a problem. These usually regrow quickly without harming the tree.

The critical part is keeping the soil consistently damp after planting until your new plants take hold. Be vigilant with the water the whole first year and then every few weeks in droughty weather.

George Weigel