To Spray or Not to Spray Herbicide: That is the Question!

To Spray or Not to Spray Herbicide: That is the Question! - Garden Pest Tip

Invasive Plants

Excerpted From Invasive Plants
Weeds for the Global Culture
From Brooklyn Botanic Garden

"Whether 'tis nobler in the mind to suffer
The slings and arrows of outrageous fortune,
Or to take arms against a sea of troubles,
And by opposing end them?"

You don't have to be a Shakespeare scholar to know that the Prince of Denmark is not contemplating a castle grounds filled with invasive exotics when he utters these lines. However, Hamlet's quandary of whether to suffer or to take up arms is felt by many of us confronted by decisions about a sea of troubling, persistent, aggressive weeds that have the potential to reduce the diversity of landscapes once dominated by native species.

As an ecologist I am particularly sensitive about recommending the use of herbicides to control unwanted plants because there is a tendency to overuse these poisons to create sterile landscapes without much regard to the environmental consequences. I certainly do not advocate the use of herbicides of any sort on the plant community we have come to call the lawn. Frankly, I find a diverse lawn with a mixture of grasses, clovers, ground-ivy and other green, living plants (regardless of their origins) to be both aesthetically pleasing and easily maintained by periodic mowing. The added benefit is that when a dry spell eventually occurs, there is always something green that survives.

Mechanical removal or non-chemical treatments should be the first line of defense against invasive exotics. Sometimes these are as simple as hand weeding, repeated mowings before the plants set seed or smothering the weeds with layers of newspapers or black plastic sheeting for several months. Propane weeders, which kill plants by searing them with a flame, should be used with extreme caution. Once I set the lowest whorl of branches of a white pine on fire when I inadvertently ignited the soil humus layer in addition to the shrubs I was attempting to kill. My wife graciously offered to call the volunteer fire department, but I was spared the embarrassment (and the loss of valuable trees) when I extinguished the smoky blaze with the aid of a garden hose — I was lucky.

These measures may not work with all invasive species. Some aggressive weeds, such as Japanese knotweed (Fallopia japonica), common reed (Phragmites australis, sometimes called P. communis) and Japanese honeysuckle (Lonicera japonica), have very deep or extensive rhizomes that are nearly impossible to dig out unless you use a back hoe or steam shovel. Furthermore, there are real problems with disposing of the soil, which is laden with buried seeds and fragments of live plants that could become reestablished. Under these circumstances the judicious use of the least persistent, most effective and environment-friendly (if that is not an oxymoron) herbicide may be warranted.

I have used glyphosate herbicide (marketed under such names as Roundup, Kleen-up and Rodeo) to eradicate poison ivy (Rhus radicans) along the roadside on my property, having grown over the years terribly allergic to the plant. This non-selective herbicide is taken up through the leaves and translocated to the roots, where it disrupts the plant's metabolism. It generally takes several days before wilting of the plant is visible, and several weeks for the plant to die. The great advantage of glyphosate is that it is rapidly broken down by microorganisms in the soil, so it does not persist in the environment. Another group of relatively environment-friendly herbicides is formulated from soap-based fatty acids (marketed under such names as SharpShooter and Scythe) that kill foliage by entering leaves and disrupting plant membranes. Within several hours the shoots of the plant wither and dry out. Soap-based fatty acids are rapidly dissipated in the soil and affect only plants that are contacted.

One windless Sunday morning in August I sneaked out at 7:00 am to apply herbicide using a low-pressure spray tank. Being careful to administer the poison only to the leaves of the target plants, I carefully worked my way along the edge of my meadow. As luck would have it, the chairman of the local conservation commission drove by on her way home from an early morning bird walk and stopped to check up on what this ecologist was doing with a spray tank. I was discovered! After my blushing subsided we did have an opportunity to discuss the rational use of herbicides.

A few important things to keep in mind before you apply these poisons:

  • Read all of the instructions that come with the packaging before purchasing an herbicide to make sure you buy one that will be effective on the species you want to kill.
  • Use only herbicides that are recommended to kill the species that you wish to control, and mix them according to directions. Stronger herbicide is not better.
  • Dress appropriately with protective clothing to avoid skin contact with herbicides. Long-sleeved shirts, trousers, rubber gloves and boots are recommended.
  • Apply only during the season that the herbicide is effective. Many plants are most susceptible to herbicides during that period of their growth when substances are being translocated out of the leaves and into the roots-usually toward the end of the summer. Some woody plants are best attacked by cutting the shoot, painting herbicide on the cut stump and covering the stump with black plastic sheeting.
  • Do not apply herbicides in hot weather-avoid spraying at midday or in the afternoon in summer as some herbicides are easily volatilized and may move off the target plant as a cloud of vapor only to condense elsewhere. Be careful to treat only leaves of plants you desire to kill. A low-pressure tank with a controlled spray, or applying the herbicide with a 1-2-inch paint brush work well. If you have a large area to treat, you can apply it to the plant surfaces using your hand, protected by a heavy rubber glove over which you put a cotton glove or an old sock. Then mix the herbicide in a small plastic bucket, dip in your protected hand, squeeze out the excess and apply to the target leaves, being careful not to spill or splash.
  • Be prepared to repeat treatments. Species like mile-a-minute (Polygonum perfoliatum) are rarely controlled by a single application.

Finally, when considering whether to spray, or not to spray, you can do no better than to keep the following guidelines of the Connecticut Department of Transportation Herbicide Program in mind: Apply the least amount of the safest chemical to specific species of plants in a specific area at the appropriate time to obtain a desired 90 percent control.

There are a number of tools that a gardener can use to combat slugs. Handpicking, traps, barriers, baits, and predators are just a few techniques. So, rather than shrugging off slug damage as inevitable, choose from the slug control menu and you'll be surprised by the results.

Invasive Plants Invasive Plants:Weeds for the Global Culture

Hundreds of horticultural plants have jumped the garden gate, threatening native species. This ground-breaking book tells you which plant invaders are problems in your area and how to control them. It ought to be in every library, and on every gardener's bookshelf!

About this Author

GardenGuides.com