Roundup & Trees

Smaller Trees

• Trees up to 15 feet tall get foliar treatment in which the plant leaves are sprayed with herbicide, as recommended by the Washington State University Extension's website. Glyphosate is absorbed by the tree foliage and transported through the plant and into the roots. When spraying trees, cover all sides well, but do not saturate the tree or spray to the point of runoff.

Larger Trees

• Cut the larger sized trees down to a stump before treating. Make sure stump surface is smooth and level to keep the chemical from running off. On stumps with a diameter of 3 inches or less, spray the entire surface. On larger stumps, spray the chemical only on the outermost 2 to 3 inches.

Timing

• The best time to use glyphosate is during August and September. Treat stump immediately after cutting as the fresh tree tissue is more absorbent. If the treatment is delayed for any reason, recut the stump surface again to expose fresh tissue before herbicide application. Avoid using the chemical when temperatures are over 85 degrees Fahrenheit. Also do not treat the trees with glyphosate if rain is expected within six hours of treatment.

Use Suggestions

• When buying glyphosate, make sure to get the undiluted, water-soluble formula. Water trees well before application of herbicides as drought stress reduces efficacy of the chemical. Avoid spraying on very windy days to minimize chances of drift to non-target plants. In foliar treatment, spray on trees when plants are in their active stage of growth with all leaves fully grown and elongated. Repeat application of glyphosate are often required for complete control.