Many people have concerns about planting around oak trees and for good reason. Oaks do not tolerate summer irrigation, and those that insist on planting water thirsty plants like lawns near their oaks risk killing their trees by encouraging sudden oak death syndrome or other diseases. So that is why it’s especially important to select natives that are acclimated to this particular hydrozone. Once established, you can reduce or stop irrigation. As a tip, if you install your oak understory in the fall before the rains, you’ll have one season of “free irrigation” under your belt.
Coral Bells or Heuchera are an excellent choice for dry shade groundcover and will take your breath away when planted in large masses. There are many wonderful hybrids of Heuchera to try. For an easy growing choice, use Heuchera ‘Wendy’ with its green leaves, purple veining and tall flower spikes. This evergreen perennial can also be enjoyed in window boxes and will attract hummingbirds that you can watch from indoors.
If you want to entice hummingbirds into the garden, Hummingbird sage is another winner under oaks. Salvia spathacea spreads on its own to create a groundcover with bright pink flowers that bloom typically March through May.
Our Coastal Wood Fern, Dryoperteris arguta, is one of the few ferns that is both drought tolerant and evergreen. Since it can withstand root competition, it’s suitable for under trees like oak, bay and pine. To keep its lush, green appearance, simply remove older fronds as they brown.
Again since it can survive with little supplemental water, Pacific Coast Iris work wonderfully as an understory plant. Iris douglasiana with purple flowers or I. douglasiana ‘Canyon Snow’ with white flowers are sure to brighten any oak woodland.
These native plants work especially well with oak communities in the following cities: Portola Valley, Menlo Park, Atherton, Los Altos, Woodside and Hillsborough.