We live under oaks and are surrounded by redwoods. We know the value of trees in the landscape. Trees shade us in the summer. Their showy blossoms herald a much awaited spring and their colorful foliage in the fall quietly marks the end of the growing season. You can hang a hammock between two of them or tie a rope swing for the kids from a large branch. Yes, trees are our companions, but how can you create a garden under one of them?
Planting under a mature tree can be a challenge. Caution is required to avoid damaging their roots and the plants will need to cope with dry soil, shade, root competition and ever-changing moisture and light conditions. You want both your new plants and your tree to thrive.
Meet your tree’s needs first. Some trees are more agreeable than others about giving up some of their ground. You can still plant beneath trees that are sensitive to having their roots disturbed, but you’ll need to make a few concessions. When purchasing plants to grow under trees, think small. Small plants require a smaller planting hole and this will minimize disturbance to the roots. You may have to buy more plants but you’ll have an easier time tucking them among the roots.
Don’t alter the grade of the soil or change the soil pH very much. Even adding a layer of soil that is more than 2" deep can reduce the amount of moisture and oxygen available to the tree and hinder gas exchange to existing roots, causing trees to suffer or even die.
Only the toughest plants have a chance of surviving among the surface roots of shallow rooted trees. Be careful when disturbing sugar maples, elms. cherries and plums, dogwoods, magnolias, pines and oaks. The majority of a trees roots are small woody roots and fine hair roots that grow within the upper 12-18" of soil and extend far beyond the trees drip line. These roots are responsible for absorbing water and nutrients from the soil.
If you encounter a root larger than 1 1/2 – 2" in diameter while digging a hole for a plant, move the planting hole a few inches away to avoid slicing through the root. You will sever mats of small tree roots when digging, but they’ll regenerate fairly quickly.
To avoid wounding the bark, which may cause insect and disease problems, start planting at least 12" away from the trunk. Oaks, remember, shouldn’t have any plantings closer than 6-10 feet from the trunk and those should be drought tolerant. After planting, water to settle the soil and spread 2-3" of mulch to conserve moisture and keep weeds down. Be sure to keep mulch at least 12" away from the base of the tree. Mulch can hold moisture against a tree’s bark and cause rot and disease.
Trees that will tolerate some disturbance to the root zone include Eastern redbuds( both the green-leafed species and the purple- leafed Forest Pansy ) and red maples ( also a good lawn tree. ).
Common trees that are easy going about planting underneath are crabapples, ginkgos, hawthorns, honey locust, poplars, silver maples and willows.
So what plants will transform your bare patch of hard earth and knobby roots into a shady nook? If you’re going for a lush look, consider hostas and ferns, paired with the hardy geranium Biokova. Other good companions are astilbes with their feathery flower plumes and variegated euonymus fortunei with bergenia or digitalis mertonensis.
Trees with branches limbed high look good with small shrubs planted underneath. Red-leaf barberry can brighten up this spot and also provide fall color. Small nandinas like Harbor Dwarf make a good ground cover and their foliage takes on an orange-red color in winter. Fragrant sarcococca grows well in this situation, too.
Low groundcovers make a simple statement under the crown of a tree. Ajuga, pachysandra and sweet woodruff all grow well here. Or you might like the look of the shade tolerant grass-like plant , cares morrowii ‘Evergold’. This stunning sedge makes a beautiful clump 1-2 ft. high and 2-3 ft wide with dark green leaves and a central band of creamy white.
You can have a beautiful garden under a mature tree by following these tips and conquering this challenging site.
Need more late summer perennials to extend your season? Purple coneflowers will continue to bloom until frost then go dormant for the winter. Showy 4" rosy purple daisies are lightly fragrant and make good cut flowers for bouquets. The clumps spread slowly and can be carefully divided after 3 or 4 years. There is also a beautiful white variety called White Swan. If faded flowers are left in place, the bristly seed heads provide food for finches in winter. 
