Planting Under Trees

Mary Anne Michael, KC Master Gardener 1994

A few years ago I decided to put a bench under a large, mature saucer magnolia (Magnolia x soulangiana ) planted by one corner of our house. The tree cast a wide circle of cool shade and had been high limbed by previous owners, so it seemed like a good place to sit during Charleston's hot and humid summers. A small stone base was laid, giving a level and dry surface underfoot, and the nook was indeed welcoming and cool.

But soon I noticed that the heavily shaded ground under the tree was either bare early in the season or full of weeds as summer progressed. (I know they are really native wildflowers, but truly, they did not seem interesting.) Suddenly, the spot offered a challenge: could it be filled with plants that would be attractive, that would grow in the dry shade environment, and that
would not need too much care? After a lot of research, and some trial and error, I can report that the answer is yes---if attention is paid both to planting methods and to choosing plants.

There is a problem specific to planting under trees, and it involves the roots. As we learned in the Master Gardener class, a good root system is vital for a tree's health. There are two kinds of roots, remember: anchoring roots, which are large and go deep and/or wide; and feeder roots,
which are finer and closer to the soil surface. Although a tree grows best when both kinds of roots are undisturbed, it's possible to alter the feeder roots somewhat if care is taken. But large roots should not be cut, and partly exposed ones shouldn't be smothered with soil or mulch.

There were no big roots to contend with under the magnolia, but the feeder roots made a tight, dry surface. Digging some exploratory holes with a trowel, I found only a few inches of this poor soil over our typical clay. Rather than dig big holes for big plants, it was easier, and seemed less
invasive, to dig small holes and tuck in small plants. I soaked each plant in water for several minutes just before planting, used composted manure under and around each one, and added a two-inch layer of finely shredded hardwood as mulch. Afterwards, each was watered well again through the mulch. I paid special attention to watering the first two years, and only a few plants died. Although established plants in my beds don't get watered often, this area gets watered during the summer whenever there's no rain for longer than two weeks. I'm not sure if this extra care is necessary, since the plants don't ever look wilted, but it¹s small and I'm willing to pamper it.

After the successful magnolia mini garden, I decided two years ago to plant a few things under another tree, a sweetgum (Liquidambar styraciflua). Although fairly young, the tree cast a circle of medium heavy shade, and there were four or five partly exposed roots. Some experts say that small shrubs can be planted between large, exposed roots if the space between the roots is partially filled with loose soil, with the shrubs planted on a slight mound. But even with this method, the shrubs should be kept away from the trunk. I decided not to try this.

Violets had already established themselves under the tree, so I let them stay and added other plants to extend the show. For early spring, species crocus seemed a good choice, since they were tiny and didn¹t need to be planted deeply. The self seeding annual larkspur (Delphinium x elatum) was to add color in late spring before the tree leaves were fully grown. Hostas would add interest during the rest of the growing season. The crocus did well until the deer came through, and the larkspur have done beautifully. But transplanting hostas last fall was a bit harder, since there wasn¹t much soil depth between the roots. I couldn¹t seem to find a space deeper than about four inches no matter where I dug. So I settled on using very small divisions of hostas, snuggling them in with compost and mulching lightly. Since this is their first season, I don't know how they'll do in the future, but right now they are alive and looking perky. As they grow, I¹m hoping their roots will insinuate themselves into whatever spaces are available. Of course, the violets are thriving.

Probably the most important aspect of planting under trees is choosing plants that are adaptable to dry shade. The list is relatively small, but it¹s long enough to make a varied and interesting bed possible. The plants I¹ve had good success with are Helleborus orientalis (Lenten rose), Epimedium x versicolor ‘Sulphureum’ (barrenwort), Tiarella wherryi (foamflower), various hostas, Liriope spicata (green and ‘Silvery Sunproof’), Pachysandra procumbens (after taking 2 years to settle in, Allegheny spurge formed nice, lush clumps, although it hasn¹t spread rapidly), and believe it or not, Japanese painted fern (Athyrium nipponicum ‘Pictum’. Some of those original weeds are now welcome as wildflowers and are, of course, doing beautifully, including violets and wild strawberries. Other plants that haven¹t been great but are satisfactory are Epimedium x rubrum, Tiarella cordifolia, and Pulmonaria saccharata ‘Mrs. Moon.’

1. Interfere as little as possible with the tree¹s root system. 2. Choose plants adaptable to dry shade. 3. Use young, small plants or seeds. 4. Create pockets of enriched medium. 5. Mulch lightly to conserve moisture. 6. Water often when establishing and regularly thereafter.

Sources for further reference

Books: Taylor¹s Guide to Shade Gardening

Right Plant, Right Place

Online: www.hgtv.com (search: dry shade plants)

www.berkeleyhort.com/plants/p_shade_dry.html

www.ccwater.com/conserve/c-dryshade.asp

www.landscaping.about.com (search: plants for dry shady areas)