Invasive Species Management (Part 3): Using Herbicides

Heather Brasell 

It is important that we understand how herbicides fit into our Integrated Pest Management (IPM) toolkit of methods for managing invasive plants and how we can use herbicides responsibly and appropriately as part of good land management practices.

I think we can stipulate that we all want to kill the bad guys (non-native invasive species), save the good guys (native plants) and do no harm to soil, water, and wildlife. The question we face is what is the best way to accomplish these objectives simultaneously. How can we kill the non-native invasive plants without damaging the desirable native plants? IPM includes the use of chemicals (herbicides) as one of the strategies we can use.

As a forest manager involved with environmental education, I hear a broad spectrum of people’s views. On one end of the spectrum, people who reflexively cross their arms and “just say no” to any use of herbicides. On the other end are people who want to apply herbicide to any inconvenient plant in their vicinity. However, most people are somewhere in between these extremes. 

When I posted on FaceBook about a site where I had used herbicide in site preparation for reforestation, I got this comment: “I pity all living plants and creatures and ground water near this spot. Herbicides kill everything.” I replied: My response will be brief but oversimplified. I understand your concerns, but only partly share them. Careful selection and use of herbicides can minimize those concerns. First, herbicides interfere with metabolic pathways that exist in plants, not animals. Second, there are about a dozen types of herbicide that target specific metabolic pathways in plants. That means they can be broadscale (kill lots of species) or selective (kill only a subset of plant species). Third, herbicides differ in whether they are soil active, how quickly they break down (half-life), and how much they move through the soil. For instance, glyphosate is rapidly inactivated by soil. Like me, most herbicide applicators don’t use herbicides that move through the soil and survive in the soil long enough to impact groundwater.

In the last issue of NativeScape, I discussed the relatively uncontroversial use of mechanical methods for managing invasive plants. In practice, mechanical treatments are always time consuming. If this is the only method you use in larger areas, highly invasive plants probably reproduce and spread faster than you can get rid of them. You are often losing the race. In a recent Washington Post article, Dana Milbank reluctantly came to the conclusion that using herbicides is essential to managing invasive species. One of his comments was that it would take 900 hours to handpull invasive plants that can be treated by herbicide in just nine hours.

The real challenge is with perennial plants that resprout readily from deep roots, tubers, etc. You have to kill the roots. Most herbicides are really effective in killing these invasive plants because the chemicals are transported within the plant from above-ground parts to the roots. An adequate dose kills the whole plant, without it resprouting. However, to use herbicides responsibly, you should consider how to minimize possible harm to plants, soil, and groundwater. You need to select carefully the most appropriate herbicide, application method, season, and weather for each species and each site. You also want to minimize the total amount of herbicide used. 

Weed Wrangle volunteers use cut-stump method to apply herbicide to small woody plants.

You can limit the side effects and collateral damage by using the most targeted application method. For woody species, you can cut the stem and apply herbicide to the stump (cut stump method) or you can hack through the bark and squirt herbicide onto the cambium layer under the bark (hack-and-squirt method). These methods use very small quantities of herbicide and they are applied directly to the target plant only. For herbaceous plants and seedlings of woody plants, in sites where hand pulling is not feasible or unlikely to be effective, spot-spraying herbicide on foliage with a backpack will have less collateral damage than spot-spraying from an ATV or broadcast spraying. 

A Weed Wrangle volunteer uses hack-and-squirt method to apply herbicide to larger shrubs and trees.

For projects that involve inexperienced volunteers, it is prudent to use only cut-stump application. You should restrict foliar application to people who have pesticide certification and experience; they need to know what they are doing. If the invasive plants are spreading (such as vines and Japanese climbing fern), an effective strategy is to mow, cut, or burn the site to remove excess foliage of both target and non-target plants. The resprouting foliage covers a smaller area with less overlap between plants, making it much easier to target invasive plants without harming nearby non-target plants. You also use less herbicide. 

Certified pesticide applicator uses foliar spray method to spot-spray herbicide on invasive plants.

Using a selective herbicide can also limit damage. Each herbicide is formulated to interfere with a specific metabolic pathway in plants (not humans or other animals). This allows you to select an herbicide that will kill some plants, but not others. You can kill broadleaf plants without harming grasses, or kill hardwoods but not conifers, etc.  When I prepare sites for reforestation, I use a selective herbicide that kills woody species that compete with the pine trees but it does not kill the grasses that provide good wildlife habitat. 

Timing is critical, particularly for foliar application. Plants absorb herbicides most efficiently when they are actively growing. If you apply herbicide when plants are not actively growing, for instance when they are suffering from water stress, the herbicide is not likely to be taken into the plant; you will have wasted your time and herbicide. For woody species, herbicides are usually more effective when they are applied in late summer-early fall when the plants are translocating starch into the roots for storage during the dormant season. However, winter may be a good time to spray herbicide on the foliage of evergreen plants at a site where non-target native plants are mostly deciduous. For instance, in wet areas, you can kill evergreen privet while minimizing damage to ferns that have lost their fronds for winter.

As with any treatment, the top priority is to prevent existing plants from producing mature seed or from spreading. You should expect to need multiple treatments to eradicate invasive species, so you should plan to revisit sites regularly.

The bottom line is that if we don’t include herbicides in our toolkit, we are likely to be losing the war to non-native invasive plants. We are also likely to be wearing out the goodwill of volunteers, who want to see results for their hard work and dedication. Using herbicides, if carefully planned and implemented, can accomplish the multiple objectives of habitat management to control invasive plants without unacceptable damage to non-target native plants. 

Site prepared for reforestation after applying a selective herbicide that does not kill grasses.