A common plaint of gardeners: “What can I plant under my tree?” It depends on the tree, but natives are especially good options under established landscape trees.
Consider four things when deciding whether you can plant under your tree: leaf drop, root competition, watering and shade.
All trees drop leaves, especially evergreens. Most leaves will decay in place, nurturing the tree. At best, plantings under trees are open enough to allow the leaves to fall through or sturdy enough not to be bothered by them. Under deciduous trees, perennials that can be cut back in the winter are good choices.
Our state tree drops lots of prickly leaves, but in moist coastal habitats, plants such as redwood sorrel, wild ginger, evergreen huckleberry, bleeding heart, western azalea and ferns populate the deep-shade to part-shade areas of a redwood forest. (Technically, the state tree is “redwood,” which means both coast redwood and giant sequoia, but I’m referring to coast redwood here.)
One exception is the large thick leaves of non-native evergreen magnolia, which fall year-round and take a long time to decompose. I “planted” mulch under my curbside trees and picked up the leaves by hand when they built up. At the sunniest edge of the canopy, I eventually added hummingbird fuchsia, which can be cut to the ground each winter.
Root competition and watering
Root competition and watering are related problems. Because the area under a tree’s canopy is likely to be networked with roots, it’s best to add plants that don’t require an irrigation system to be installed, to dig smaller holes (preferably planting from 4-inch containers up to 1-gallon maximum) and to avoid disturbing the area closest to the trunk. For native oaks, the “do not dig” radius is at least 6-10 feet.
The water needs of the tree will determine whether to choose drought-tolerant understory plants or plants that need more water to flourish. Until they can grow sufficient root systems, most plants will need to be regularly watered their first summer or two. Hand-watering is an efficient way to help drought-tolerant plants get established.
Shade is the last important variable. Some plants can flourish in full shade (yerba buena, snowberry, cream bush, ferns), and Canyon Snow iris can bloom even in deep shade. The part of the understory that gets afternoon shade allows many more choices, however, than full shade.
Drought-tolerant plants that can bloom in afternoon shade (assuming they get enough morning sun) under oaks or pines include:
• Evergreen shrubs such as Yankee Point ceanothus or coffeeberry
• Deciduous shrubs such as pink-flowering currant
• Sprawling low shrubs such as Catalina perfume or island snapdragon
• Compact perennials or subshrubs such as sticky monkeyflower, Douglas iris or coast buckwheat
• Spreading perennials such as hummingbird sage
• Grasses such as California fescue or California melic
Under a tree that can tolerate some water, try the following plants, which perform better with moderate water and can flourish with morning sun and afternoon shade.
• Taller arching shrubs such as Western mock orange
• Herbaceous perennials such as California aster
• Subshrubs such as canyon sunflower
• Evergreen groundcovers such as beach or woodland strawberry
• Clumping perennials such as western columbine or California coneflower