Plant suggestions for using under native oak trees
If you are lucky enough to have a mature oak in your landscape, you will want to take good care of it. One of the best ways to do this is to minimize the water you apply to the root area of the tree. Frequent summer water increases the risk of root disease, which can cause decline and death in oak trees. Luckily, there are many plants that are adapted to the low water that is best for most native oaks. Some of these are common in nurseries, others are a little harder to find.
All of these plants will need summer irrigation to become established (usually after a year or so), then can live on less frequent irrigation (every 2-4 weeks). When you do water, be sure to water the plants thoroughly (drip systems need to run for at least 60 minutes at a time). After the plants are established, you should try to water them as infrequently as possible to preserve the health of your tree. I water most of my oaks and nearby plants only once a month during the summer. This gives the soil time to dry between waterings and reduces the growth of disease causing fungus.
If you oak is getting summer water now, reduce the frequency while making sure you wet the soil deeply when you do water. This change should be made gradually over several years so the tree can adapt to the new schedule.
Another thing to keep in mind is to keep the water and plants as far from the trunk of the tree as possible. Keep plantings at least 10’ from the trunk to protect the important buttress root area from disease. A woody mulch will protect the roots and allow your eye to look at more interesting plants farther away from the tree. It’s OK to plant some native annuals or bulbs in clumps closer to the trunk, but do not water them. Spreading ground covers, such as ‘Emerald Carpet’ Manzanita, planted at 10-12’ from the trunk will grow toward the trunk. If you need further decoration in this zone, add a non-living element such as a statue, a bench or a few rocks. Just don’t compact the soil while you are doing it.
These general recommendations are not a substitute for advice from an experienced arborist who can give you more specific advice after inspecting your oak tree.