Planting under a Tree

Photo/Illustration: Bonnie Blanchette

You aren’t a fool for wanting to put a garden beneath a mature tree. It’s natural for a gardener to close his or her eyes and transform this patch of hard earth, sparse weeds, and knobby roots into a shady nook lush with hostas, astilbes, and jack-in-the-pulpits. And natural is good when it comes to gardening.

As with many endeavors, there is a right way and a wrong way to establish a garden under a tree. My training as a professional horticulturist has taught me that caution is required when planting beneath mature trees to avoid damaging their roots. I also know that the plants I choose will need to cope with dry soil, shade, root competition, and ever-changing moisture and light conditions. Even if you aren’t a horticulturist, being aware of a tree’s root system and cultural requirements allows you to create a garden where new plants and the tree will thrive.

Meet your tree's needs first

Not all trees are created equal. Each requires specific light, soil, and moisture conditions to survive and remain healthy. As you begin to plant your understory, make every effort to work with the situation you have. Some tree species, such as oaks (Quercus spp. and cvs., USDA Hardiness Zones 4–9), are extremely sensitive to major soil disturbance. Massive undertakings to alter the grade of the landscape or to change soil pH under a tree are difficult and often impractical. Adding a layer of soil that is more than 2 inches deep, for example, can reduce moisture and oxygen availabilities and hinder gas exchange to existing roots, causing trees to suffer or even die.

A tree’s root system and canopy also determine how easy or difficult it will be to install a garden under a tree. It can be particularly troublesome to work among the extensive surface roots of shallow-rooted trees such as maples (Acer spp. and cvs., Zones 3–9) and elms (Ulmus spp. and cvs., Zones 3–9). The dense canopies and umbrella-like habits of trees such as conifers, Norway maples (Acer platanoides and cvs., Zones 3–7), and lindens (Tilia spp. and cvs., Zones 3–9) not only block sunlight but also deflect rainfall. Only the toughest plants have a chance of surviving in such conditions.

Will my tree tolerate root disturbance?

Some trees are more agreeable than others about giving up some of their ground. You can still plant beneath trees that are sensitive to having their roots disturbed, but your plan may have to be scaled back or carried out over a few years. Here are groupings of some common trees according to how indulgent they will be of your efforts at their feet.

Be careful when disturbing
Beeches (Fagus spp. and cvs., Zones 3–9)
Black oak (Quercus velutina, Z 4–8)
Buckeyes (Aesculus spp. and cvs., Z 3–9)
Cherries and plums (Prunus spp. and cvs., Z 3–9)
Dogwoods (Cornus spp. and cvs., Z 2–9)
Hemlocks (Tsuga spp. and cvs., Z 3–9)
Larches (Larix spp. and cvs., Z 1–8)
Lindens (Tilia spp. and cvs., Z 3–9)
Magnolias (Magnolia spp. and cvs., Z 3–9)
Pines (Pinus spp. and cvs., Z 3–10)
Red oaks (Quercus rubra and cvs., Z 5–9)
Scarlet oaks (Quercus coccinea and cvs., Z 5–9)
Sugar maples (Acer saccharum and cvs., Z 4–8)

Tolerates some disturbance
Hickories (Carya spp. and cvs., Z 4–9)
Hornbeams (Carpinus spp. and cvs., Z 3–9)
Ironwood (Ostrya virginiana, Z 5–9)
Redbuds (Cercis canadensis and cvs., Z 4–9)
Red maples (Acer rubrum and cvs., Z 3–9)
River birches (Betula nigra and cvs., Z 4–9)
Sourwood (Oxydendrum arboreum, Z 5–9)
Spruces (Picea spp. and cvs., Z 2–8)
Swamp white oak (Quercus bicolor, Z 4–8)
Sycamore (Platanus occidentalis, Z 5–8)
White ashes (Fraxinus americana and cvs., Z 6–9)

Is easygoing about disturbance
Arborvitae (Thuja spp. and cvs., Z 2–9)
Crab apples (Malus spp. and cvs., Z 3–9)
Ginkgos (Ginkgo biloba and cvs., Z 5–9)
Hawthorns (Crataegus spp. and cvs., Z 4–9)
Honeylocusts (Gleditsia triacanthos and cvs., Z 3–7)
Kentucky coffee tree (Gymnocladus dioica, Z 5–9)
Pecan (Carya illinoinensis, Z 5–9)
Poplars (Populus spp. and cvs., Z 3–9)
Silver maples (Acer saccharinum and cvs., Z 4–9)
White oak (Quercus alba, Zones 5–9)
Willows (Salix spp. and cvs., Z 4–9)

Dogwood Photo/Illustration: Jennifer Brown

Redbud Photo/Illustration: Nancy Rose

Honeylocust Photo/Illustration: Michelle Gervais

Start with small plants to reduce soil disturbance

It’s best for the tree if you disturb the soil only where you are installing new plants. If turfgrass is growing—or attempting to grow—under your trees, it needs to be removed. Avoid stripping the grass, which not only is backbreaking work but also damages a tree’s fine roots. Try instead to smother the grass with five or six sheets of wet newspaper, topped with a layer of organic mulch 1 to 2 inches deep. The downside of this method is that it may take two to three months to kill the grass. Chemicals such as glyphosate (Roundup) will kill the grass faster and allow you to plant sooner, but it’s important to avoid spraying herbicides on the tree because they can be absorbed through the bark.

When purchasing plants to grow under trees, think small. When you find the plant you want, buy it in the smallest size available. Smaller plants require a petite planting hole that will minimize the disturbance to tree roots. You may have to buy more plants, but you’ll have an easier time tucking them among the tree’s roots.

Avoid damaging the trunk and any thick roots

Most trees have large major roots that extend several feet into the soil to anchor the plant against buffeting winds. The majority of a tree’s roots, however, are small woody roots and fine-hair roots that grow within the upper 12 to 18 inches of soil and extend far beyond the tree’s drip line (the farthest reach of its branches). These roots are responsible for absorbing water and nutrients from the soil.

If you encounter a root larger than 1-1/2 to 2 inches in diameter while digging a hole for a plant, move the planting hole a few inches away to avoid slicing through the root. You will sever mats of small tree roots when digging, but they will regenerate fairly quickly. Continue planting as you would for any other bed, spreading out the new plants’ roots as much as possible to ensure good contact with the surrounding soil. To avoid wounding the tree bark—an open invitation to insect and disease problems—start planting at least 12 inches away from the trunk and work outward.

When all plants have been installed, water the entire area to settle them and the soil. Then spread a 2- to 3-inch-deep layer of organic mulch, such as wood chips or bark chips, to conserve moisture and keep weeds down. The moisture that mulch can hold against a tree’s bark is conducive to rot and disease, so be sure to keep the mulch at least 12 inches asay from the base of the tree.

Help your new plants compete with the tree

To ensure that your plants succeed rather than merely survive in their new spot, some aftercare is required. Water weekly when rainfall is inadequate. Continue to monitor soil moisture until the plants are established. Because they are competing with a mature tree for this precious resource, you may have to spot-water the plants if it appears that your tree is getting to the moisture first.

Avoid fertilizing for the first year after planting because it encourages more top growth than root growth, where new plants need to spend their energy if they are going to make it. If you suspect nutrient deficiency, get a soil test first to confirm your diagnosis. If needed, a general, slowrelease, balanced fertilizer, which benefits your large trees as well as smaller plantings, can be broadcast and watered in well.

Annually applying a 2- to 3-inch-deep topdressing of organic matter (such as compost, shredded leaves, or well-rotted manure) to the soil in spring will add sufficient nutrients. In natural woodlands, plants grow in soils annually replenished with organic litter, such as leaves and twigs, which then decomposes and enhances the soil. Because we tend to remove this debris as it falls, applying the topdressing of organic material replicates nature without damaging tree roots. Over time, you’ll find that this organic matter provides many benefits. It naturally enriches the soil by adding nutrients and enhancing aeration and moisture-holding capacity. It loosens heavy clay, improves drainage, and allows your new plants’ roots to become established. Organic matter also encourages the activity of earthworms and other beneficial organisms that mix and aerate the soil.

By following this procedure, you’ll find your tree more amenable to sharing its territory, and the image of the shade garden you had in mind will begin to appear before your eyes.

Plants for dry shade

When landscaping under mature trees, you will increase your chances of success by choosing plants that are suited to your site conditions. Here is a list of plants that can grow in the reduced light and moisture available under many trees.

Shrubs
1. Carolina allspice (Calycanthus floridus and cvs., Zones 5–9)
2. Cutleaf stephanandras (Stephanandra incisa and cvs., Z 3–8)
3. Ninebarks (Physocarpus opulifolius and cvs., Z 3–7)
4. Snowberry (Symphoricarpos albus var. laevigatus, Z 3–7)
5. Winterberries (Ilex verticillata and cvs., Z 5–8)

Perennials and grasses
6. Black snakeroot (Cimicifuga racemosa, Z 3–8)
7. Columbines (Aquilegia spp. and cvs., Z 3–8)
8. Foamflowers (Tiarella spp. and cvs., Z 3–9)
9. Japanese forest grasses (Hakonechloa macra and cvs., Z 5–9)
10. Japanese painted fern (Athyrium niponicum var. pictum, Z 5–8)
11. Lungworts (Pulmonaria spp. and cvs., Z 2–8)
12. Siberian irises (Iris spp. and cvs., Z 3–9)

Ninebark

Winterberry

Columbine