What to plant in dry shade

Dry shade is usually under a mature tree, where the canopy blocks both sun and rain and the roots take what water does fall. The plants that cope best there are slow, tough woodland natives and their relatives: in much of the eastern and central US that means barrenwort (Epimedium), bigroot geranium, Pennsylvania sedge, Christmas fern, Solomon's seal, white wood aster and hellebores. The list is short because most shade plants expect moist soil. Even these need regular watering until they are established, and they are best planted small, between the tree's roots.

Why dry shade is so hard

A plant in this spot is short of three things at once.

• Light. A dense canopy (maples, beeches, many evergreens) lets through very little.
• Water. A full canopy intercepts a large share of light rain before it reaches the ground, and the soil near the trunk can stay dry long after a storm. This "rain shadow" is worst on the side of a house wall under a roof overhang.
• Room. Most of a tree's absorbing roots are in the top foot or so of soil and spread well beyond the canopy. They outcompete anything new for water and nutrients.

Shallow-rooted trees such as maples (especially Norway and silver maple) and beech make the hardest version of the problem. Oaks and other deep-rooted trees are usually easier to plant under.

Prepare the ground without hurting the tree

The instinct is to dig out the roots or build a raised bed. Both can harm the tree.

1. Don't add deep soil. Keep any new layer over roots to an inch or two of compost or mulch. Deep fill reduces the oxygen roots get and can damage the tree over several years.
2. Don't cut major roots. Roots thicker than about an inch are part of the tree's structure and water supply. Work around them.
3. Plant small. Use plugs, 4-inch pots or quart pots. A small root ball fits between roots with a hand trowel; a gallon pot means hacking out a big hole.
4. Improve the planting pocket. Mix a little compost into each hole rather than the whole bed.
5. Mulch with leaves. Shredded leaves or fine bark, 2 inches deep, kept away from the trunk. In a woodland, fallen leaves are the natural mulch; leaving some in place feeds the soil.
6. Call 811 before you dig (or visit call811.com). Utility lines often run through the same corner of the yard as the tree.

A short, honest list

These are among the most reliable dry-shade plants for the eastern half of the US, roughly zones 4 to 8. Most are widely available. Check each for your zone, and check your state's invasive list for anything not native.

In the West, dry shade is a different problem with a different list. Under California oaks, for example, summer water can harm the tree, and local natives adapted to dry summers do better. Your state extension service or a native plant society will have a regional list.

Plants to avoid

Some groundcovers sold for dry shade are too good at it. English ivy (Hedera helix), periwinkle (Vinca minor and V. major) and goutweed (Aegopodium podagraria) are listed as invasive in a number of states. English ivy also climbs trees and can add weight and hold moisture against the bark. Check your state's list before buying any fast-spreading groundcover.

Watering through the first year

Even drought-tolerant plants need help to get established, and in dry shade the tree is competing for every drop.

• Water at planting, then roughly weekly for the first growing season whenever it hasn't rained an inch, more in heat.
• Water slowly, with a soaker hose or a hose on a trickle, so it soaks in rather than running off.
• In the second year, water only in long dry spells. From the third year, most of the plants above get by on rainfall in their usual range, though a deep soak in drought helps both them and the tree.

Plant in early fall where you can: soil is still warm, rain is more frequent, and the tree is using less water as it heads toward dormancy. Early spring is the second-best time.

Set expectations

A dry-shade bed fills in slowly. Plan on two to three years before a groundcover of epimedium or sedge closes up, and use mulch for the gaps in the meantime. Spacing a little closer than usual (say 12 in rather than 18 in for small perennials) speeds things up if your budget allows; see how many plants do I need for a garden bed for the sums.

FloraVista's list asks how many hours of direct sun your bed gets and only picks plants rated for that light: start with a photo.