What to grow under those trees in your landscape
Story and Photos by Garry V. McDonald
I have few frustrations when it comes to gardening because I figure there is always an opportunity to improve a site if a particular planting scheme doesn’t work out. “A gap is an opportunity” is a personal mantra. But I admit – there is one burr that gets under my saddle. After being asked by friends to pronounce on which tree species they should plant so they can sit in the shade, I’m confronted in a few short years with “I can’t get grass to grow under my trees.” I’ve been known to peevishly retort, “Did you want a shade tree or a golf course?” (It’s a wonder I have any friends at all). Shade and turf seldom play well together.
Which begs the question: Why are some plants perfectly happy under shade while others fade away? Much research has been conducted lately on what is called the daily light integral (DLI). This is simply the total quantity of light that a particular plant species requires on a daily basis to maintain its lifestyle. Turfgrass species require a much higher DLI than woodland species: no surprise there.
If not self-imposed, it can be a challenge when deciding what to do when the landscape has become woodland and the grassy sward no longer exists. The easiest option is to remove the trees and revert the space back to full sun. Obviously, this is seldom possible because of the expense involved, aggrieved relational reactions, or even local ordinances that prevent such actions. The other choice is call in a tree “trimmer,” who more often than not, causes more problems and expense down the road when weak re-growth comes crashing down. Certified arborists who know their craft and can thin the tree canopy properly are worth their weight in gold, but are expensive and hard to find in many parts of the state. And even these actions might not create enough light for most lawn turfgrasses to survive.
This leads us back to the purpose of this epistle. What to grow under those trees once the canopy closes and shade descends. Along with the absence of light, the other confounding factor when trying to grow anything under trees are the trees’ root systems. There are two types of root systems: thick or fang-like, such as those of Magnolia and oaks (Quercus spp.), or mats of fibrous roots, such as those of elms (Ulmus spp.), or maples (Acer spp.). The former involves trying to squeeze plants between the gaps in massive roots; the latter is competition for water and nutrients from the fine fibrous roots. If a tree is massive, sometimes the best option is to mulch around the base and call it good. But, if the situation is not too dire and shade seems to be the only limiting factor, there are a number of strategies that can be employed to add beauty and interest to the shady glen.
The first strategy is to incorporate shade-loving shrubs in masses extending outward toward the tree’s dripline (the outermost reach of the canopy). If the trees are pines (Pinus spp.) or oaks, this is a great spot to plant azaleas (Rhododendron spp.). Azaleas range in shape and size: Indica–types are best for the southern part of the state while the Kurume, or more cold-hardy types, are best in the northern tier of the state. Along the same line, camellias (C. japonica), sasanquas (C. sasanqua), or sweet olive (Osmanthus fragrans) are not only handsome, they also have beautiful flowers in early spring. A couple of hydrangeas are especially effective under trees, namely oakleaf hydrangea (H. quercifolia) and our own native smooth hydrangea (H. arborescens).
If the site calls for a more open design scheme where views become important, one strategy is to plant woody, low-growing ground covers to take the place of turfgrass. Admittedly, the plant palette from which to choose in this situation is small. The plant of choice in many Arkansas landscapes is periwinkle (Vinca major or V. minor). The blackbird in the nest when using periwinkle is that it has become a complete weedy nuisance in many areas. The same goes for the wintercreeper (Euonymus fortunei). A number of juniper species (Juniperus spp.) and ornamental grasses are used as ground covers but, alas, require full sun. In the southern half of the state, Asian jasmine (Trachelospermum asiaticum) is useful as a dense ground cover that requires little maintenance and grows in full shade. Probably overused where it can be grown, given the fact that it can be grown has much to recommend it. While I’ve recently seen Asian jasmine on the market in northwest Arkansas, I’ve not planted out any for evaluation since I’ve never seen it in a commercial landscape, so rather doubtful it will survive. Lilyturf (Liriope muscari) or mondo grass (Ophiopogon japonicus), while not the most exciting plants, are effective shade-loving ground covers. Other possibilities are bugleweed (Ajuga reptans) or Japanese spurge (Pachysandra terminalis), but neither is satisfying in the long run. Pigeonberry (Rivina humilis) is a useful native ground cover, but it takes a large number of plants to cover and is almost impossible to find in the trade. As can be seen, I’m still struggling with this strategy as a solution to shady areas.
A final strategy I’ll propose is using herbaceous perennials to create a woodland garden. Here, the palette is large and adaptable. Any fern works well when massed and can range from the ephemeral maidenhair fern (Adiantum capillus-veneris) to the impressive native royal fern (Osmunda regalis) or cinnamon fern (O. cinnamomea). Although they can make too much of themselves and seed everywhere, I still have an affection for the native wood’s violet (Viola spp.). Hosta are a must and range from green to bluish green, variegated creams and yellows, in sizes ranging from miniature to giants – something for everyone. Although a little outré, hardy cyclamens add color and form to a shady spot. C. hederifolium has soft pink flowers in October while C. coum has magenta flowers and is one of the first perennials to bloom for us in the horticulture garden. Slow to spread, they are worth the wait and have survived both drought and heat. Don’t forget to add spring-flowering bulbs into the mix, such as daffodils (Narcissus spp.) or snowdrops (Galanthus spp.). They are up and out before the tree canopy becomes dense enough to affect their regeneration. Summer annuals, such as Caladium, Impatiens, or wax begonias (B. x semperflorens-cultorum) need shade and thrive under trees.
When planting under trees, the main consideration during the growing season is soil moisture content, which needs to be carefully monitored. While the plants need enough water, it’s also important that the soil doesn’t remain wet to the point that it negatively affects the tree’s health: Moderation in all things.