Underplanting Mature Trees

Flower bed underneath a holly
(Ilex hybrid), SAS Institute.

 Flower bed during Spring under Japanese maple
(Acer palmatum), SAS Institute campus, Cary, NC.




















Article and photos by Kathleen Guerra  

Durham Co. Master Gardener

Do you long to remake those bare, dusty patches under your trees into a lush mosaic of feathery foliage, spiky sedges and springtime blooms? Underplanting mature trees can transform a garden space, but doing so requires patience and care to ensure the continued health and vitality of the existing trees.

Consider the Tree First

To start, ensure that you do no harm to the tree. Many homeowners share the common misperception that trees are deeply rooted and that outward root growth mirrors the tree’s crown. In truth, most trees have shallow root systems with 90 percent of roots occurring in the top 12-18 inches of the soil and extending far beyond the drip-line of the canopy. Since most tree roots are close to the surface, it is imperative to avoid changing the grade of the soil under mature trees. Adding just a few inches of soil can significantly reduce the oxygen available to feeder roots, effectively smothering them. Moreover, installing a raised bed under existing trees can dramatically increase moisture levels near the trunk, leading to bark decay and the eventual decline and death of the tree. Some trees such as beech, cherries, pines, red oaks, and sugar maples to name just a few, are highly sensitive to having their roots disturbed. Be sure you know the tolerance level of your particular tree. You may give a second thought to underplanting a highly sensitive tree or choose to limit your plantings to perennials rather than annuals to avoid disturbing the soil more than once.

Select Plants for Dry Shade

Next, consider the type of shade that exists under the tree – do you have filtered (dappled) shade, partial shade, open shade or deep shade? You will want to choose  plants that grow well in the specific shade conditions and in dry shade. Competition for water and nutrients from tree roots may create shortages for your plants, so select only those that will thrive in reduced moisture and light. Native plants that grow naturally under similar conditions are good options. As you select plants, keep in mind their mature height and width to avoid crowding the lower tree branches. Lastly, some trees such as black walnut and butternut produce juglone, a natural substance that can stunt growth and cause plant death. For underplanting beneath these trees, be sure to select plants that will tolerate juglone.

Minimize Root Disturbance

How do you plant in a Medusa’s nest of tangled tree roots? Planting under mature trees requires the art of “pocket” planting – that is, the gentle digging and tucking of small plants into open spaces between tree roots. To minimize root disturbance, install several small-sized plants (2 ½ - 4 ½ inch pots) that don’t require large planting holes. While it will take time for the small plants to fill in, the bed will look better and fuller each subsequent season. Never use a tiller or other means of mechanical cultivation under trees to avoid damaging precious root systems. When planting, if you come across a root that is one and a half inches in diameter or greater, move your planting hole off to one side to avoid disturbing it.

You can prepare the planting area under a tree by spreading a top dressing of two to three inches of organic material, such as well-rotted compost or shredded leaves. Shredded leaves can do double duty as both a soil amendment and mulch. Carefully dig individual holes for the plants between the tree roots and add organic material to the planting holes. Position the tallest plants nearest to the trunk, cascading down to the shortest plants in the front, to avoid hiding small plants behind taller ones and to ensure adequate light for all. To minimize soil compaction and disturbance, install the plants closest to the tree first and work your way outwards toward the drip-line. Finally, if you used something other than shredded leaves as your soil amendment, add two to three inches of organic mulch between your plantings to conserve moisture, taking care to keep the mulch away from the tree trunk.

Monitor Soil Moisture

Trees vary in their competitiveness for soil moisture, so plan on actively monitoring moisture levels while your new plantings are becoming established. In the beginning, ensure your new plants and tree receive one deep soaking each week equivalent to one inch of water. Once established, you can reduce the watering frequency to once every two weeks. During periods of inadequate rainfall, you may need to supplement and water more often.

Underplanting mature trees can transform a garden space into a lush, shady oasis. Understanding and meeting the needs of your tree’s root system is the first step to realizing your vision and ensuring the ongoing health of your mature tree.

References:

Brown, Deborah L. (2013). Gardening in the shade. University of Minnesota Extension Service. Retrieved from http://www.extension.umn.edu/distribution/horticulture/dg1428.html

Browning, Sarah. (June 2011). Planting under trees. University of Nebraska-Lincoln Extension Service in Lancaster County. Retrieved from http://lancaster.unl.edu/hort/articles/2011/PlantingUnderTree.shtml

Evans, Erv. (2000). Gardening in the shade. NC State University, North Carolina Cooperative Extension Service. Retrieved from http://www.ces.ncsu.edu/depts/hort/consumer/factsheets/trees-new/text/gardening_shade.html

Meyer, Mary and Sparks, Kelsey. (January 2006). Planting under trees. University of Minnesota Extension Service. Retrieved from http://www.extension.umn.edu/distribution/horticulture/components/8237ppt.pdf

Ripke, Kathy. (2006). Planting under existing trees. University of Minnesota, Department of Horticultural Science. Retrieved from http://www.sustland.umn.edu/implement/planting.html

Roach, Margaret. (June 2008). 10 thoughts on successful underplanting. Retrieved from http://awaytogarden.com/10-thoughts-on-successful-underplanting

Roman, Don and Sellmer, Jim. (March 2003). Landscaping and gardening around walnuts and other juglone producing plants. Penn State College of Agricultural Sciences, Department of Horticulture. Retrieved from http://pubs.cas.psu.edu/FreePubs/pdfs/xj0039.pdf

Taylor, Doris. Planting under a tree. Fine Gardening, 105, 40-43. Retrieved from http://www.finegardening.com/print.aspx?id=87334