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

Weed control is a top of mind topic now for many landowners. Following this spring’s strong log market, a lot of folks will be reforesting a harvest unit. Others may be planting a field of Christmas trees, or a swath of trees and shrubs as a restoration project to improve habitat conditions. And it seems everyone is struggling to control one invasive weed or another on the property.

While a number of approaches and strategies (including mowing, pulling and mulching) can and are used in managing weeds, many people will use herbicides as at least part of their approach. This is no surprise given their demonstrated effectiveness and efficiency. But not all users are well-versed in vegetation management, or the science behind it, so some review of herbicides seems to be in order. The attention given to herbicides in this and later articles 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.

Now, it is important to realize that one need not be a crop scientist to use herbicides. The label gives instructions that ensure the safe and allowable use of an herbicide, so the label needs to be read and followed. But responsible and effective use of herbicides requires some additional understanding about herbicides and how they work, as well as knowledge about the life cycle and other characteristics of both crop and target plant species they will be used with. Let’s begin talking about some basics.

One of the first distinctions to be made is between foliar active herbicides and soil active herbicides.

Doug-fir seedling enjoying weed-free start
Doug-fir seedling enjoying weed-free start

Foliar herbicides are applied to the leaves or stems of plants to be absorbed and (in the case of those used in forestry) translocated throughout the plant to affect control. They vary widely in their selectivity. Common foliar herbicides include glyphosate (Accord, Rodeo) and tryclopyr (Garlon, Element) and are widely used to control annual and herbaceous perennials (both grasses and broadleaved plants, often called “forbes”) as well as woody shrubs such as blackberries and broom. Foliar herbicides vary in their selectivity or effectiveness on types of plants controlled. Foliar herbicides are easy to use in broadcast and spot treatments, and can be applied in a number of ways including back pack sprayers, ground based sprayers or aerial applications.
Soil active herbicides are applied to the soil to be picked up by the roots and shoots of emerging plants. Often called pre-emergent herbicides, they are used to keep an area free of weeds by preventing seeds from emerging and establishing following other weed control actions such as tillage or foliar herbicide application. Pre emergent herbicides are generally quite selective, with effectiveness varying by groups (grasses v broad leaf) or particular families of plants. Common soil active herbicides registered for forestry include sulfometuron (Oust) and hexazinone (Velpar). Soil active herbicides are more demanding to use than foliar herbicides since their activity and selectivity is based on their concentration and location in the soil. They need rainfall after application to be activated. Calibration of spray equipment is critical to allow application of the correct amount to 1) ensure effective control and 2) avoid injury to crop plants.


A weeded planting strip in a riparian planting
A weeded planting strip in a riparian planting

Any effective planting plan involves removing perennial weeds before planting and maintaining weed control following planting to improve survival and growth of planted seedlings. Since many involve the use either foliar, soil active herbicides or some combination of the two, look for more information about them in future postings.

Read next post in the series, Understanding foliar herbicides – part 1.

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