Planting underneath your landscape trees
Story by Karen Neill
Under the trees is where you will find me as we head into the hot summer months. Trees play an important role in our landscapes, providing much needed shade. Proper placement can also mean energy savings. You can reduce summer air conditioning costs by up to 35 percent simply by planting large deciduous trees on the east, west, and northwest sides of your home.
But shade is not always easy to deal with when it comes to other aspects of landscaping. That shade we so need for our homes and decks makes it difficult to choose plants that will grow underneath and the ever-expanding root system quickly takes up all the moisture, making it even more difficult. So where do you start? Mulch offers many advantages, such as weed control, moisture conservation, moderating soil temperatures, reducing erosion, and looking tidy – all while providing clean lines that enhance the landscape design. But let’s face it; mulch alone can be kind of boring.
The first question you have to ask is “How much shade do you have?” The quantity of shade varies. Densely shaded areas beneath large trees or under building overhangs presents more problems than sites in partial or light shade. Sometimes these difficult shade situations can be improved by lifting or thinning the tree canopy so more sun or indirect light can penetrate. Partially or lightly shaded areas receive direct sunlight for a small portion of the day, but that light is still quite bright. For these spots, there are many plant choices.
You also need to consider the moisture available under the trees. 50-75 percent of the feeder roots are located in the top 1 foot of soil and they will compete for all the available moisture in the area. You need to be prepared to supply supplemental water, not only at planting time, but also during periods of inadequate rainfall.
Spring-flowering bulbs and spring ephemeral plants, which last only for a short time, are two you might consider for those denser shade areas. Both of these grow and flower before trees fully leaf out in the spring. They are, therefore, able to get adequate sun and rain during their early, showy season. They then die back and do not reappear until the following spring. Because they aren’t visible year round, they can be interplanted with shade-tolerant ground covers or other perennials.
Let’s take a step back – before we discuss plants, we need to talk about soil preparation. It is critical that you avoid the temptation to create a planting ring, however large, around the base of the tree. Bringing the soil level up around the base of the tree can actually cause decay of the bark, leading to failure, or even suffocate the roots, which will also lead to failure of the tree. Remember, 50-75 percent of the feeder roots are located in the top 12 inches.
Instead, start by removing any turfgrass, weeds, and existing plants manually, and then carefully incorporating organic matter into the soil profile to a depth of 6 inches. Again this should be done manually. If the roots are too thick to dig through, move to another spot. Large swaths of plantings will actually make the trees look more natural in the landscape and you can and should tie it all together with mulch when finished.
Lastly, look locally for plants that grow naturally under trees. In areas with large amounts of shade, consider fast-spreading ground covers, but use caution. You definitely want to avoid invasive plants like English ivy (Hedera helix). One of my favorites is Pachysandra (I have never had a problem with this becoming too aggressive). Others to consider are creeping phlox (Phlox subulata), barrenwort (Epimedium spp.), green and gold (Chrysogonum virginianum), holly fern (Cyrtomium falcatum), lily-of-the-valley (Convallaria majalis) wild geranium (Geranium maculatum), sweet box (Sarcococca hookeriana var. humilis), and autumn fern (Dryopteris erythrosora).
A few perennials planted in my shady spots for their colorful foliage and some flowers include ginger (Asarum), bleeding heart (Dicentra), Hosta, coral bells (Heuchera), hellebore (Helleborus spp.), marsh lady’s tresses (Spiranthes odorata) and columbine (Aquilegia spp.). You aren’t going to get an abundance of flowering in the shade of a tree but you can still end up with a colorful display.
As for shrubs, due to the competition for moisture you will need to limit these but Rhododendron, Japanese aucuba (Aucuba japonica), and Japanese pieris (P. japonica) are all good choices. It is important to choose plants that will have foliage that will look good all season.