Attempting to grow plants under a tree’s canopy presents certain challenges. Tree roots concentrate in the top 6 inches of the soil and can spread far beyond the leaf canopy. To help perennials and bulbs cope, I recommend the following steps.
- Start with fresh soil. If you are starting a new garden bed under trees, spread 3 to 6 inches of a 50:50 topsoil-compost blend over the area, being sure to stay 3 feet away from tree trunks. Tree roots will eventually grow up into the new soil, but this gives the perennials and bulbs a chance to get established first.
Mulch in spring. Apply an annual 2-inch-thick layer of mulch in spring. Partially composted leaves are ideal, but composted softwood bark works well too (top photo). Keep mulch 12 inches away from tree trunks.
Feed your plants early. Broadcast a granular organic fertilizer lightly just as bulbs and perennials begin to grow. Because they typically begin growing earlier than trees, they can take up the nutrients before tree roots become active.
Water in summer. Deeply water the garden once a week in summer if it hasn’t rained. Trees require a great deal of water, and supplementing natural rainfall will benefit both the trees and the flowers beneath.
William Cullina is the executive director of the Morris Arboretum in Philadelphia.
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