Mary Heie, UM Extension Master Gardener in Anoka County, May 10, 2014
The snow has finally melted and the bare areas under the trees are highly visible. Even grass designated for shady areas has a hard time growing there. Since shade is the obstacle here, what else can be grown under trees?
Trees are strong competitors for the nutrients in the soil, water, and light. Approximately 80% of the roots are within the top 6-8 inches of soil. With so many roots near the surface it is easy to see why they absorb the nutrients and water. Also maples often have roots that almost on the level with the ground making mowing or planting nearly impossible. There may be other challenges: limbs hanging low to the ground, very dense shade, or specific soil needs.
Rather than trying to plant grass and mow around a tree, when possible, incorporate the tree into a landscape bed with other shrubs and plants and mulch the area. Depending on soil type and its pH, there are many shrubs that will grow in shade such as boxwoods, yews, rhododendrons, azaleas, some hydrangeas. Ferns work very well in a landscape bed. Consider ostrich fern, maidenhair fern (Adiantgum pendeatum), lady fern (Athyrium filix-femina), or one of theJapanese painted ferns.
A partial list of other shade loving plants include lady’s mantle (Alchemnilla mollis), bergenia (Bergenia cordifolia), astilbe, ligularia, goatsbeard (Aruncus) Brunnera, Helleborus, Tirella, Lungwart (Pulmoneria) Coral Bells or Heuchera. (Note that the newer cultivars of Heuchera may need more sun.) And, ofcourse, hostas. There are hundreds of varieties providing an almost endless array of sizes and leaf colors. A planting of hostas around oak trees can be very pleasing. Spring flowering bulbs such as daffodils, squill (Scilla), snowdrops (Galanthus nivalis) and crocus work particularly well in a bed under deciduous trees. By the time the trees leaf out they will have finished blooming and are going to sleep for the summer.
Ground covers are perfect for planting under trees. They will fill in spaces under trees forming a kind of living mulch. Some winter hardy shade loving ground covers for Anoka County include: Ajuga or bugle weed, Silver or Yellow Archangel (lamiastrum), wild ginger, European ginger, pachysandra or Japanese spurge, lamium (dead nettle), sweet woodruff (fragrant) and periwinkle or vinca minor. Since it is the nature of these plants to spread, it may be necessary to provide some control, like a wall or edging.
Black walnut trees merit additional comments. They release a chemical called juglone into the soil. This chemical is toxic to some plants, but not to humans. Plants that will grow under black walnut trees include Jack-in-the-pulpit, astilbe, crocus, snowdrops, sweet woodruff, coral bells, and pulmonaria.