How to Grow Grass Under Oak Trees in the Pacific Northwest

The Challenge

• All of the large oaks grown in the landscapes of the Pacific Northwest cast dense shade that most common lawn grasses cannot tolerate. Plus, the topsoil around oak trees is filled with a dense mat of roots that sucks up all the available water and nutrients, effectively starving the lawn. On the other side of the equation, the fertilizer and water applied to lawns can upset the soil conditions preferred by oaks. Oregon white oak (Quercus garryana), the primary native species in the Pacific Northwest, is adapted to several months of dry weather in summer -- just the time when lawns need the most water. This oak grows in USDA zones 4 to 9.

Oaks and Lawns

• Fescue (Festuca arundinaceae) is the only type of lawn grass adapted to growing conditions in the Pacific Northwest with sufficient shade tolerance to grow under large oaks. The are several varieties of fescue that grow in USDA zones 3 to 7. Fescue requires ample irrigation to stay green in summer, so it is best to plant it under oaks from the eastern United States where summer rain is common. Red oak (Quercus rubra) is one example of an eastern oak commonly planted in the Pacific Northwest and can be grown in USDA zones 4 to 8.

Oaks and Native Grasses

• Another option is to plant native "bunch" grasses under oak trees, that have minimal water requirements. These do not give the same effect as a mowed lawn, but they offer excellent ornamental qualities and the ability to grow naturally under native oaks in the Pacific Northwest. Most native bunch grasses grow in neat, tidy clumps and have decorative seed stalks and fine-textured foliage. Idaho fescue (Festuca idahoensis) is an excellent example, growing in USDA zones 6 to 9. The cultivar "Siskiyou Blue" (Festuca idahoensis "Siskiyou Blue"), a bluish-stemmed variety that grows in 12 inch clumps, looks particularly good around large oak trees.

Management Tips

• Whether attempting to grow shade-tolerant lawn grass or native bunch grasses under oak trees, it is helpful to have the lower limbs of the oak removed to let more light in. No grass grows well in full shade, but most oaks tolerate removal of limbs from the lower one-third of the tree, which concentrates the canopy in the top two-thirds, allowing ample light to reach the lawn. Another thing to keep in mind when planting grass around oaks -- avoid changing the grade or disturbing the soil. This can damage the roots and lead to the tree's slow demise. Instead, work with the soil as it is, adding compost to the surface and using fertilizers sparingly.