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

In previous installments of this series on the basics of herbicide use in weed control, I distinguished between foliar and soil active herbicides, describing foliar herbicides as those applied to the leaves or stems of plants to be absorbed and carried throughout the plant to affect control. In the previous post I began discussing foliar herbicides in more detail with an overview of glyphosate.

In this entry I will look at a group of herbicides called “growth regulators” that include some important foliar herbicides and popular weed and brush killers commonly used in forestry, agriculture and habitat restoration.

Note: The attention given to herbicides in this series does not indicate an advocacy for their use. Please see Forestry Pesticide Disclaimers.

Growth regulators are selective herbicides, highly effective against broadleaved plants. Most have foliar activity, can be picked up readily by leaves and stems, and are actively translocated in the phloem. Some have foliar activity, meaning they can be picked up by the roots of emerging or established plants. The growth regulators’ mode of action is to disrupt the usual balance of hormones (including auxins, or gibberellins) interrupting normal division and growth of cells. This causes characteristic symptoms of twisted stems, cupped or crinkled leaves which generally appear rapidly after treatment on susceptible broadleaved plants. Some also have soil activity, meaning they can be picked up by roots and shoots of emerging plants, and sometimes established plants. Conifers are generally rather tolerant to growth regulators when dormant, and so some are used for conifer release as well as for site preparation. But there are significant differences within this group.

The growth regulators present some particular challenges. Some are relatively volatile at warm temperatures, meaning that in warm weather they can turn to vapor even after the spray has dried, and can then potentially drift in the air and damage susceptible plants in the vicinity. Grapes are particularly sensitive to growth regulators in the spring, deserving extra thought and caution to prevent injury. Some herbicides are produced in two forms, an ester and an amine salt. Each has some advantages, but an important distinction is that ester form is relatively more volatile than the amine.

So what are some herbicides in this group?

Triclopyr (Garlon) is a familiar and versatile herbicide used to control a variety of woody species. It is can be used as a foliar spray in summer, as basal spray on woody stems in the winter, and also applied to cut stumps. It has little soil activity. Triclopyr comes in both an ester and an amine form. The ester is oil soluble and better able to penetrate bark and waxy leaves. Ponderosa pine is extremely sensitive to triclopyr.

2,4-D is another familiar growth regulator, with many trade names, widely used in agriculture and forestry. In forestry it is used to control certain herbaceous broadleaf plants, as well as certain woody trees and shrubs. It has little soil activity, is relatively volatile and comes in both an ester and an amine form.

Note, Crossbow herbicide is a very popular brush killer which is a mix of 2,4-D and triclopyr. It is registered for non crop areas such as fence rows, but IS NOT labeled for forestry!

Clopyralid (Transline) is especially effective against thistles and many other plants in the aster family, and is also very hard on elderberries. It is produced as an amine salt. Conifers are generally very tolerant. Clopyralid will persist in the soil for several months and has some soil activity. Also in the growth regulator group are some less familiar herbicides including aminopyralid and picloram (a restricted use herbicide). Both have strong residual soil activity. Both are amine salt formulations.

Next time I give an illustration of how this applies in the woods in Using foliar herbicides: Getting the bang for your buck