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

In recent entries in this series on the basics of herbicide use in weed control, I have reviewed how some foliar herbicides work, and the relationship between the plant’s physiology and the herbicides’ behavior. Now I want to illustrate how that that information translates to what gets done in the woods, looking at controlling blackberries, a frequent target of foliar herbicides.

Blackberries are a problem because they are widely dispersed by birds and start readily from seed and once established, rapidly spread vegetatively by tip rooting, quickly forming daunting patches seemingly too tall and wide to tackle. We’ve all seen these conditions in old pastures, riparian areas and struggling plantations.

Confronted by this situation (and assuming for this example it is a pre-plant, site preparation situation), many people begin by spraying the vigorously growing berries in the spring. Or by mowing them down in the summer, then starting to spray them when they reemerge later that year with glyphosate or triclopyr. Yes, it sure improves the looks of the place, and often seems to solve the problem for a while…but it is not generally a highly effective approach. The roots are not killed and the plants soon come charging back. Both of these approaches often lead to a cycle of repeated, frequent and arguably unnecessary spraying.

Why do these treatments not work very well? There are two issues involved here: timing and coverage.

Let’s begin with timing. As discussed previously, foliar herbicides need to be carried down to the roots to kill the plant. In the spring and after a disturbance like mowing, the plant is pushing reserves up to support growth rather than stockpiling them. Spraying a translocated herbicide (such as glyphosate or triclopyr) on the plant at that time is like pushing a string up hill. In each case the treatment tends to kill the tops, but not the roots.

Now let’s talk about the target. To be effective, you need to get enough herbicide absorbed by the plant to kill it. Mowing has some clear advantages, but reduces the target. The plant has a much smaller canopy, fewer leaves, so less surface area to catch and absorb the herbicide. But the roots are still about the same size, so you get a much smaller relative dose carried to the roots. Spring spraying generally burns the top without killing the plant. Think of it as “chemical mowing”. Like mechanical mowing this makes subsequent sprays later in the season less effective than they might be.

So what are the alternatives? Generally, your first chance is your best chance. Make it count. Take advantage of timing and coverage. Avoid mowing berries before you spray and spray when the blackberry plant will do some of the work for you and carry the herbicide to the roots. For triclopyr that is probably from flowering to the first frost. For glyphosate, there is a narrower window in the late summer and fall.

“But my blackberries are much too big for that!” you say. Probably not. A blackberry patch is often really many small patches, with some paths and alleyways. And besides, a surprisingly large patch can be treated from the edges with a wand and a small ATV-mounted sprayer, or even a back pack sprayer and an adjustable nozzle. Stand up on an ATV or truck, or lay a couple 2×12 down in the patch to help you reach farther in. This way you can take advantage of the surface area there, and get good spray coverage on the leaves when it can be most effective.

You can mow dead canes the following spring to give better access, but allow a full summer for re-growth before treating again, to allow them to build target and begin translocating downward. You can also treat plants at the center of the patch you could not reach the first time. This way you are using both the plant’s physiology and the herbicide’s activity to your best advantage, improving the effectiveness of the spray and also reducing the time, effort and amount of chemical used.

I hope this gives you some useful insights to the effective use of herbicides, should you choose to use them. The attention given to herbicides in this series does not indicate an advocacy for their use. Please see Forestry Pesticide Disclaimers.

I’ll continue this series later in the year. Many thanks to Bruce Kelpsas and Ed Peachy for their help and review of this series.