Answer: The difficulties with planting beneath established trees are three-fold. Mature trees won't like having their roots disturbed; they will compete for moisture and nutrients; they won't appreciate a change in watering frequency (which your new plants may require). With that in mind, there are a few plants you can plant, providing you keep them 6-8' away from the trunk of your trees: Autumn sage (Salvia greggii) Bushy 3- to 4-foot shrub bears red, salmon, or purple flowers from late spring to summer. California iris (I. douglasiana). Knee-high plant with purple, blue, white, or cream flowers in spring. Related Pacific Coast hybrids also work well; they need a smattering of summer water. Catmint (Nepeta faassenii). Lavender-blue flowers make 2-foot mounds in late spring, early summer. Ceanothus (California lilac) is a native shrub. Low-growing types such as C. griseus horizontalis and C. rigidus 'Snowball' reach 2 to 6 feet tall. Clusters of lavender-blue or white flowers appear in spring. Coral bells (Heuchera); Compact perennials with roundish leaves in shades of dark green to purplish red; dusters of coral pink, red, or white flowers in spring or summer. Daffodil (Narcissus); Flowers from February through April, then dies back and doesn't need water in summer. Red valerian (Centranthus ruber); Perennial 3 feet tall that blooms from spring into summer. Rosemary (Rosmarinus officinalis); Upright (to 6 feet tall) or prostrate (to 2 feet tall), plants have aromatic dark green leaves, plus lavender-blue flowers in late winter or early spring. Santa Barbara daisy (Erigeron karvinskianus); This 20-inch perennial is covered with pink or white daisies spring into fall. Blooms best with extra light. You can plant hydrangea there (although the digging might be difficult) and once established it should be happy and healthy. Hope these suggestions are helpful!
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