By Brad Withrow-Robinson, OSU Forestry & Natural Resources Extension – Benton, Linn & Polk Counties

In my previous installment of this series on the basics of herbicide use in weed control, I distinguished between foliar and soil active herbicides. In this post I begin discussing foliar herbicides in more detail. Note: The attention given to herbicides in this series does not indicate an advocacy for their use but an acknowledgement that using herbicides presents some unique risks, and that landowners and managers need to know enough about them to make informed decisions on their use.

Foliar herbicides are applied to the leaves or stems of plants to be absorbed and carried throughout the plant to affect control. They are common and widely used to control annual and perennial herbs and also woody shrubs.

Glyphosate is probably the most familiar foliar herbicide, and one of the most-used chemicals in forestry and agriculture. Created and exclusively marketed under patent for many years by Monsanto with the trade name Round Up, glyphosate is now produced by many companies, in many formulations and sold under dozens of trade names. The individual labels allow a range of specific uses and applications. Many, but not all, are labeled forestry use. Be sure that whatever product you have is labeled for the specific use you have in mind (be it forestry, farm or restoration). More on that another time.

Glyphosate is an amino acid derivative in a class of herbicides called amino acid synthesis inhibitors. Once absorbed by the plant, it binds to an enzyme involved in the production of certain amino acids which then disrupts the flow of carbohydrates in the plant– this is its “mode of action”. Symptoms often appear slowly (10-14 days) as amino acids are depleted and membranes weaken, causing the foliage to turn yellow then brown.

Glyphosate symtpoms showing on quackgrass, foreground
Glyphosate symtpoms showing on quackgrass, foreground

Glyphosate is a broad-spectrum (non-selective) herbicide that will damage or kill most vegetation it contacts, if absorbed. Waxy surfaces of some plants may limit absorption. It is short lived, rapidly deactivated after contact with soil and not very mobile in the environment. Glyphosate is relatively nontoxic to mammals, and to fish and other aquatic organisms. This all makes it a popular choice for reforestation and restoration professionals alike.

To be effective in controlling established perennial weeds of any kind, foliar herbicides such as glyphosate must be absorbed by leaves or stems and carried to the roots along with the carbohydrates produced in the leaves. For herbaceous perennials, this generally means that the optimal timing for treatment is when flower buds are developing in summer, or shortly after. The best time to treat woody species such as poison oak or blackberry with glyphosate is the early fall when these plants are transporting reserves to their roots, but still active, and before the leaves start to color in the fall. For many sites, that means the window to treat poison oak has closed by mid-August, but remains open for blackberries, often into October. But you need to observe how different plants grow on your site. It is the plants’ physiology, not the calendar that counts!

Annual weeds and seedling perennials (as in a new plantation) are easily controlled with glyphosate, any time they are actively growing and still small, if possible. The problem here is that this is often when young tree seedlings are also actively growing, and at their most susceptible to spray damage.

Knowing something about how an herbicide works helps you choose the right one(s) to use, and time your application to be effective (meaning killing the weeds but NOT killing your seedlings).  Read the second part of the series, Understanding foliar herbicides-part 2.

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