Planting Under Oak Trees

9 Feb

After my last blog on planting under redwoods, I received several questions about whether the same information applies to gardening under oak trees. The answer is no. Oaks need a low moisture environment in order to thrive, and while there are many plants that grow naturally under oaks the introduction of  other species is not usually successful and can result in killing the new plants, the tree or both.

Here are a few simple yet critical rules for keeping your oak tree healthy:

  1. NO SUMMER WATER: over-watering is a common cause of oak death in urban environments. Irrigation should :never be installed under oak trees. If you have a lawn under your oak, it should be removed.
  2. REMOVE  UNHELPFUL WEEDS: weeds under urban oak trees are often a mix of native and non-native grasses. Non-native varieties should be removed.
  3. NATURAL MULCH: oak trees produce their own mulch in the form of  leaf litter, which supports microorganisms critical to the health of the tree. Do not remove leaf litter from under your tree.
  4. NO AMENDMENTS: do not fertilize under oak trees.

The best strategy for gardening under oaks is to install associated plants. This means plants that are either drought tolerant or summer dormant. Here are some suggestions for plants that grow naturally under oaks:

  • Currants and gooseberries (Ribes): this family of plants grows naturally in oak forests. Pink-flowering currant (Ribes sanguineum) is particularly attractive with lovely pink flowers and attractive foliage.

Ceanothus – California Mountain Lilac: likes part shade, so plant in a part sun location at the edge of the tree canopy.
  • Manzanita: manzanitas are another natural ally of oak trees and generally grow well under oaks. Locate in a sunnier spot at the edge of the tree canopy.
  • Nightshade (Solanum) is a 2-3 ft. perennial with attractive purple or blue flowers. Likes leaf litter, and the deer don’t usually touch it.

  • California Monkey Flower (Diplacus or Mimulus) also like a sun spot under oaks. Lovely orange blossoms add color. Be sure to select the drought tolerant type…. Some Mimulus need a lot of moisture.

  • Hummingbird Sage (Salvia spathacea) is a great plant to put under the oak tree. Will grow in shade or sun.

  • Alum root (Heuchera) do well under oaks, and their small bell-shaped flowers and heart shaped foliage combine beautifully with the other plants mentioned above.

  • Douglas Iris (Iris douglasiana) grows under oaks, in a part sun location. Attractive upright foliage and purple flowers.

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