Q: Can I grow grass, flowering plants, or shrubs under trees?
A: Homeowners often dislike bare spots in a landscape and bare spots often occur under trees. However, growing grass or other plants directly under trees is challenging for a number of reasons:
• The amount of sun and shade will vary greatly throughout the day.
• This changing quality and quantity of light will affect photosynthesis which is necessary for healthy plant growth.
• Most lawn grasses prefer sun to shade.
• Tree roots compete for water and nutrients.
• Leaf litter can inhibit plants growing under trees. Rake and remove leaf litter on a regular basis.
• The extra shade under trees may create additional moisture which can lead to an increase in disease in some plants.
• Lawns benefit from yearly aeration, but aerating grass under a tree can damage tree roots.
A tree’s root system should always be considered when planting under trees. People often believe that a tree’s roots are deep and extend no further than the canopy drip line. In reality, roots are close to the surface, often within a foot or less of ground level and they extend quite a long way beyond the drip line. Sometimes roots can be seen above the surface. When preparing a planting area, rototilling or deep digging around the base of a tree should be avoided so as not to damage tree roots. Use a small trowel to dig planting holes rather than a large shovel.
Adding soil to the base of a tree to make a deeper planting area is also a bad idea. Neither soil nor mulch should be piled up on tree trunks. Added soil can reduce water and oxygen supplies to existing roots, which will lead to the slow decline of the tree. Place a three to four inch layer of mulch around under-tree plantings, but keep mulch two to four inches away from the trunk. Use wood chips, chunk bark, straw, pine needles or shredded leaves. Don’t use plastic sheets beneath the mulch; water or air can’t penetrate plastic. Fabric-type weed-barriers are preferable.
Placing new trees in a lawn area is also not wise. Young trees grown without turf beneath their canopies will have a more vigorous root system and will grow larger and faster above ground, as well. Keep grass from the planting area for at least one year. If you mulch around trees, instead of planting grass, you also prevent possible trunk damage by lawn mowers or string trimmers.
Here are some other considerations for planting under trees:
• Plant perennials, rather than annuals. A one-time planting of perennials will minimize root damage while annuals require replanting every year.
• Start with small container plants to minimize the size of the planting hole.
• Space perennials allowing for their mature size. Place the tallest plants near the trunk, stepping down to the shortest plants in the front.
• Consider using very shade-tolerant plants.
Grass grows best in sun, so a dense area of turf under trees may not be possible. Use shade-tolerant grass mixes. Varieties of chewing fescue and creeping red fescue tolerate light shade quite well. Tall fescue has moderate shade tolerance and some varieties of Kentucky bluegrass have fair shade tolerance.
Consider these shade-tolerant plants instead of turf grass. Some ornamental grasses do well in shade. Try Chasmanthium latifolium: Northern sea oats, which are noted for showy, drooping flowers and light green, upright, bamboo-like foliage and flowers that start green, and then turn copper. Or consider Carex muskingumensis: Palm sedge, which has slowly spreading palm-like foliage and requires shade and moist conditions. Vinca minor: Periwinkle is an excellent shade plant ground cover with semi-evergreen, dark glossy green leaves and purple flowers. Hosta species with large green or variegated green leaves grow well in shade. Dicentra spectabilis: Bleeding heart which has fern-like leaves and pink and white blooms also likes shade.
Check the Colorado State Extension website at http://www.ext.colostate.edu for more information on shade tolerant plants.
Contributed by Deb Ross, Colorado Master Gardener. Photo by Carey Harrington, Colorado Master Gardener. For answers to your horticultural questions, contact the Master Gardener Help Desk at 636.8921 or CSUmg2@elpasoco.com. Access fact sheets and seasonal information on the El Paso County Horticulture website http://elpasoco.colostate.edu/horticulture/.