Regional Gardening Reports :: National Gardening Association

In the Garden:
New England
May, 2010
Regional Report

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Solomon's seal, bleeding heart, periwinkle, ajuga and woodland phlox bloom in spring under my oak tree.

From Sun to Shade: Planting a Garden under a Mature Tree

When we built our house 21 years ago, one of the first things I did after we moved in was start planning my landscape design. The backyard was partly wooded and the rest was given over to sandbox, swingset and food gardens. The front yard was my blank canvas, without a tree or shrub, just a few inches of builder-supplied topsoil over very sandy subsoil. So this is where I began my ornamental gardening efforts, starting first with a lot of soil improvement.

When I started actually planting, I began with trees first, as the cornerstone of my design. Knowing that a shade tree would need to be able to thrive in my acid, sandy soil, I chose a northern red oak (Quercus rubra) for the southern corner of my front yard. This relatively fast-growing native tree develops a tall, straight trunk and a rounded, upright crown and is adapted to well-drained, acidic soils. Since this long-lived tree will eventually become at least a 60' tall and 50' wide specimen, I made sure it was sited 25' out from the house. Although it was hard to imagine at the time, I knew that the 10' tall tree I was starting with would one day provide welcome summer shade for me and future owners of my home, and I wanted to make sure my oak had room to develop its full stature. In other spots in the yard, I planted smaller trees, including Japanese tree lilac (Syringa reticulata), fringetree (Chionanthus virginicus) and stewartia (Stewartia koreana).

With my trees in place, I began planting flower gardens. And, since my trees were all so young, my flower choices needed to be ones that would thrive in full sun. So I had a great time putting in peonies, phlox, daylilies, globe thistles, asters and many others. But, like most gardeners, I'm greedy. So while I delighted in the many sun lovers I could choose from, I also wished I had a spot for all those lovely shade plants- I wanted hostas, astilbes and ferns as well.

Well, I now have my wish! That spindly little oak has grown into a 35' tall tree whose broad crown shades much of the front yard in summer. Several years ago I began to notice that the grass under the canopy was struggling, as were many of the flowers in a nearby perennial border. It was time for a shade garden! I began to move the sun-lovers to a new spot and replace them with hostas, ferns, and astilbe. But what to do about that struggling grass?

I wanted to put in a garden extending out from the trunk of the oak and to do this I needed to get rid of the grass there and put in perennials. But I knew that many trees resent disturbance of their feeder roots, the ones that absorb water and nutrients in the top 12 inches of soil beneath their canopy. (Or even beyond- tree roots can extend up to two to three times the extent of the crown.) Trees also don't take kindly to raising the grade over their root zone. So I knew I needed to proceed with caution.

I began in spring by spreading several layers of newspaper over the grass and covering them with a 2 inch thick layer of compost. I hand weeded any stray grass shoots that came up over the summer. The following summer I again spread another 2 inches of compost and let that work itself into the soil. Finally, the next spring, I was ready to begin planting.

To minimize disturbance to the oak's roots, I chose shallow-rooted herbaceous perennials, ground covers and bulbs, and I also started with small specimens of these, in no larger than 4" pots. This way I could dig the smallest holes and disturb the fewest roots when I set the plants out. I was fortunate to be planting under an oak, which has larger primary horizontal roots around which small plants can be tucked. Some trees, like maples, have such a dense mat of fibrous roots that it is almost impossible to plant under them.

I also tried to choose plants that will gradually spread on their own with out my having to disturb the soil again, such as European ginger (Asarum europaeum), creeping woodland phlox (Phlox stolonifera) and variegated Solomon's seal (Polygonatum odoratum 'Variegatum'), or plants that don't need frequent division to thrive, such as hostas, fringed bleeding heart (Dicenta formosa 'Luxuriant'), lady's mantle (Alchemilla mollis) and dwarf goatsbeard (Aruncus aethusifolius).

Because many of the shade-adapted perennials are spring bloomers, I think annuals such as impatiens are a nice addition for summer color. But I plant them in containers rather than in the ground to avoid having to disturb tree roots each year putting them in. My newest addition to the garden? Two Adirondack chairs so I can sit under the spreading branches of my oak and enjoy its cool shade and my shade garden on a hot summer's day.

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