(News article for January 23, 2021)
I was browsing the herbicides at a local home improvement store recently and was struck by how many new products were available for home landscape or garden use.
Some of the new products have similar trade names to each other but contain active ingredients that have very different effects. It’s as important as ever to read the label of any herbicide or other pesticide that you are considering buying to make sure that it’s appropriate for your situation.
For example, if you are planning to use an herbicide to kill weeds in your lawn, make sure it’s labeled for use on lawns and for the type of turfgrass you have, specifically.
I want to talk a bit about an active ingredient called imazapyr. This has been found in herbicides for professional forestry use and other specific purposes for some time. It’s now available in some consumer-oriented products, generally in combination with another active ingredient.
Imazapyr is a broad-spectrum, systemic herbicide. This means that it kills a wide range of types of plants and can move within a plant rather than just affecting the part to which it’s applied. Additionally, an important characteristic of imazapyr is that it remains active when applied to soil instead of being bound to soil particles and losing its efficacy.
This soil activity makes it useful for sites on which you want to keep almost all types of vegetation from growing back for approximately one year. (The actual length of effectiveness depends on factors such as soil texture and the amount of rainfall received.) However, if it’s not your intention to keep the ground clear of vegetation, avoid using products with imazapyr on that site.
Another consequence of imazapyr’s activity in soil is that it can kill plants – including trees and shrubs – if applied over soil in which their roots are growing. Labels of the consumer-oriented imazapyr products generally state that they should not be used within an area twice the distance between the trunk and the edge of a plant’s canopy (drip line).
An example of a use for an imazapyr-containing herbicide would be to kill weeds in sidewalk cracks and keep them from coming back. If that sidewalk is 15 feet from the trunk of a shrub or small tree that has an 8-foot-wide canopy (approximately 4 feet from trunk to drip line on each side), this may be a good option, since the sidewalk is more than 8 feet from the trunk. However, if the sidewalk is 15 feet from the trunk of a tree with a 20-foot-wide canopy (10 feet from trunk to drip line), the imazapyr product should not be used in that location.
Like all herbicide ingredients, imazapyr is a useful tool in specific situations but can have unintended consequences if label instructions are not followed.
Let me know if you have questions.
Contact Mary Helen Ferguson.
The LSU AgCenter and the LSU College of Agriculture