Douglas-fir Drought

Asked April 30, 2019, 12:31 PM EDT

We are losing some of our (40 year old) Douglas-fir trees to drought stress. Last year, about one acre on a 30 acre lot. These trees have very shallow roots and are in competition with increasing grass as the tree canopy dies back. I would like to reduce the competing vegetation and mulch around the trees. Is there a recommended herbicide I can use in the month of May around the bases of the 40 year old Douglas-firs? This timber was thinned in 2015, before that there was no grass. When the tree canopy fills in I would expect the competing vegetation to decrease. There are another four acres of shallow rooted trees adjacent to the dead trees. I know these are not great site conditions for Douglas-fir but would like to save them anyway. Most of the property drains better and the trees have much deeper roots.

Marion County Oregon

1 Response

Based on your reference to shallow rooting and poor drainage, controlling the grass may not make much difference in those areas. Douglas-fir does not tolerate wet soils very well and this tends to become more apparent as the trees get bigger and need more rooting space. Douglas-fir will suffer from too wet in spring and then too dry in summer and die out over time in those soil conditions.

Controlling the grass may help somewhat by reducing competition for soil moisture in summer as you say. But trees in areas of shallow soils due to poor drainage will continue to have problems.

Herbicides for killing grass in Forestry applications are listed in the "PNW Weed Management Handbook" https://pnwhandbooks.org/sites/pnwhandbooks/files/weed/contentpdf/pdfs/forestry-effectiveness-table....
where you will see that glyphosate is the main one. Many brand names contain glyphosate and you should always follow the directions on the label. Douglas-fir (and most other tree species) will be most sensitive to damage from glyphosate and other herbicides during the active growth period, usually May-July. With larger trees that do not have foliage near the ground, there should be minimal risk of harm to the tree when applying glyphosate to grass underneath large trees under the appropriate weather conditions (calm winds). You should avoid application in spring-summer if there is a chance of getting chemical on actively growing Douglas-fir foliage.