Planting Under Mature Trees

Are you thinking of joining the trend of incorporating more native understory plants to your yard? This article is for you. Your goal may be to reduce your lawn maintenance by removing some, or maybe you want to fill in some space that is just empty mulch. Maybe your goal is to bring more color, or more wildlife to your yard. Maybe you are looking to increase your biodiversity, or reduce your storm water runoff. Maybe you want to achieve all of the above. Either way, planting more understory plants can be a challenge for homeowners. Roots may be in the way, soil may be compacted or dry, or maybe you just don’t know what does well in shade.

Understanding the Challenges Beneath Mature Trees

Before planting, it’s important to understand the environment beneath mature trees. Large trees have extensive root systems that compete with smaller plants for water and nutrients. Tree roots tend to be wider and shallower than many people realize. Most tree roots grow in the top 18 inches of soil, and can reach well beyond the tree’s canopy edge. Digging large planting holes can damage important structural roots, so avoid scraping, cutting, or otherwise damaging any woody roots. If you need to cut through the small, fine, fibrous roots, that is ok. You may find that you need to edit the location of some plants in order to make things work.

Some people try to get around this issue by building up soil on top of shallow roots, but this can also lead to less oxygen reaching the tree roots, which want to be near the surface. If you feel like you need to add soil on top of roots, add no more than three inches of soil, and do not compact the newly added soil. Instead, consider buying smaller plants to start out with. Many nurseries now have plug trays available, and some species can be direct sown in the ground. If plugs or seeds aren’t an option, opt for the smallest plants you can find. Purchasing smaller plants is also more economical. If you find that digging near your tree is not presenting any issues, then larger plants are still a good option.

One note on placement of understory plants- give your tree’s trunk some space. As trees grow, trunks expand in size, as do buttress roots. Your new shrubs and perennials will also expand with time. Give everyone the breathing room they need. Keep in mind that you (and your arborist) will need to be able to walk up to the tree, take measurements, and assess the base of the tree. It is all too common that people plant right up against the base of the tree without realizing how large everything will be. Over time, plantings completely cover the root flare, holding excess moisture against tree bark, which can open it up to decay. Giving your tree a few feet of breathing room will keep it happier.

Another challenge is that the trees will be competing with your new plants for water, and the canopy above can limit the amount of rain that reaches the ground below. This means you will need to water your new plantings very diligently, even if it does rain. When you plant smaller stock, the root system of your young plants will also be smaller, and they can dry out very quickly, so you will need to stick to a strict watering schedule of every other day for the first few months to give the roots a chance to develop. Young plants typically need some supplemental water for the first 2-3 seasons they are in the ground. Even the most drought-tolerant plants need supplemental water for the first couple of years until they become established in their environment. Keep in mind that the tree roots nearby that will be taking some of that water, so you may need to water more than you think. It is always a good practice to check how wet or dry your soils are before watering, and familiarize yourself with what parts of your yard tend to stay wet for longer, or tend to dry out quickly. Adjust your plant selection to match these trends.

Choosing the Right Plants

Success begins with selecting plants that are adapted to grow in the environment you already have. Look for species that naturally grow in woodland environments, where they evolved alongside large trees. You may even want to walk your local forests or nature preserves to look for inspiration. Here in Madison, we have a few wonderful woodland gardens and nature preserves that put understory species on beautiful display. There are guides available through some nurseries on companion plants that can help narrow down your selection.

Your soil conditions are the most important consideration for picking plants. If you have dry, sandy soil, you will need different plants than if you have clay soils, or if you occasionally have standing water in your yard. The pH level of your soil is another important aspect to consider. Acid-loving plants in alkaline soils will always be stressed. If you are struggling to figure out what your soil conditions are, there a few easy tests to estimate your soil texture, pH level, and drainage available. You can also collect a soil sample and send it to the Soil and Forage Lab at UW Madison.

Below is a list of some of our favorite understory plants.

Small understory Trees and Shrubs:

• Eastern Redbud (Cercis canadensis)
• Pagoda Dogwood (Cornus alternifolia)
• Witchhazel (Hamamelis spp)
• Beaked Hazelnut (Corylus cornuta)
• American Hazelnut (Corylus americana)
• Chokeberry (Aronia spp.)
• Musclewood (Carpinus caroliniana)
• Serviceberry species (Amelanchier spp.)
• Ironwood (Ostrya viginiana)
• Bottlebrush Buckeye (Aesculus parviflora)
• Summersweet (Clethra spp.)
• Grey Dogwood (Cornus racemosa)
• Bush Honeysuckls (Diervilla spp.)
• Ninebark (Physocarpus spp.)
• Viburnums (Viburnum spp.)
• Common Sweetshrub (Calycanthus floridus)
• Oakleaf Hydrangea (Hydrangea quercifolia)
• Sweetspire (Itea virginica)
• Elderberry (Sambucus spp.)

*These plants are all native to North America, but may not historically grow in southern Wisconsin.

 Understory Perennials:

• Wild Geranium (Geranium maculatum)
• Wild Ginger (Asarum canadense)
• Pennsylvania Sedge (Carex pennsylvanicum)
• Common Wood Sedge (Carex blanda)
• Woodland Phlox (Phlox divaricata)
• Woodland Aster (Eurybia divaricate)
• Blue Wood Aster (Symphyotrichum cordifolium)
• Common Violet (Viola sororia)
• Trillium varieties (Trillium spp.)
• Jack in the Pulpit (Arisaemia triphyllum)
• Columbine (Aquilegia canadensis)
• Virginia Bluebells (Mertensia virginica)
• Zig Zag Golendrod (Solidago flexicaulis)
• Spikenard (Aralia racemosa)
• Dutchman’s Breeches (Dicentra cucullaria)
• Maidenhair Fern (Adiantum pedatum)
• Cinnamon Fern (Osmunda cinnamomea)
• Mayapple (Podophyllum peltatum)
• Jacob’s Ladder (Polemonium reptans)
• Giant Solomon’s Seal (Polygonatum biflorum)