Dry Shade Gardening

Dry shade gardening under the canopy of trees or in containers in recessed corners of a patio can be a challenging venture. Often the soil, root-choked and bone-dry and the low level of light do not offer new plantings much to live on. To be successful, your understory plants and those in pots have light and moisture requirements, which must be met.

Tree canopies have a lot to do with how much light and moisture enter your garden. If your trees have broad, spreading canopies, they will cast more shade than upright trees, unless these upright trees are planted close together. If these trees are deciduous, the shade they cast will be seasonal and what you can plant beneath them may depend on when they leaf out and how dense their canopies become. 

The most challenging shade trees to plant under are those whose foliage emerges early and whose canopy is dense. The best shade trees give plants beneath them good early season light and cast lighter shade during summer. While retaining the overall shape of trees, thin them in late winter when the branch structure is completely visible. For the impenetrable canopies of some evergreens, remove limbs within 8 feet of the ground to allow direct sunlight to penetrate the area at the base of the tree. 

More critical to reducing shade is increasing moisture levels beneath trees. Tree roots extract every drop of moisture from the ground and leave nothing for other plants. A recommended approach to increasing moisture content in the soil is to build a well-draining raised bed and fill it with humus-rich, water retentive soil. Do not place your raised bed against or close to a tree trunk as this would interfere with oxygen availability to tree roots. Install an automatic irrigation system using soaker hoses or drip irrigation rather than a sprinkler system. The use of mulch materials such as bark, wood chips, straw, shredded leaves, even rocks, will help prevent water in the soil from evaporating too quickly.

Many trees are sensitive to root disturbances. California oaks, for example, have grown with dry summers and cool, wet winters. Poor drainage or summer watering can put these trees at risk of root disease. For that reason, native California plants, which have evolved alongside our oaks and require minimal water, are best suited under these trees.

To keep soil disturbances to a minimum, start with small understory plants. Begin planting at least 12 inches away from the trunk and plant in an outwardly direction. Spacing plants too closely reduces air circulation and encourages fungal diseases like powdery mildew. Avoid nicking tree roots, as you prepare the planting holes. Water the plantings thoroughly. Annually, apply 2 to 3 inches of topdressing such as compost, shredded leaves or well-rotted manure. This organic material replicates nature without damaging tree roots and provides many benefits. 

There are hundreds of varieties of groundcovers, bulbs, perennials, ferns, vines and shrubs available to dry shade gardeners. While these plants tend to store water in roots, rhizomes, tubers and stems, they lose less water through their leaves than most plants and can cope with low light levels. Many can be grown in containers.