Update on longtime herbicide's newest active ingredient

Backyard Q&A

In the Garden

March 28, 2004|By Dennis Bishop | Dennis Bishop,Special to the Sun

I recently purchased a bottle of Roundup Plus herbicide and noticed that it contained a new chemical called diquat. What is diquat? Is it safe?

Diquat is not new; it is just new to Roundup. Roundup herbicide has been on the market for at least 25 years, and I have never known it to contain any other active ingredient besides glyphosate. Glyphosate is the chemical that actually kills the plant ("root and all"). It is blended with inert ingredients to make what is called a formulation. I have seen many different Roundup formulations, but this is the first time I've seen a second active ingredient.

A friend recently went to a local home center and found that the only concentrated formulation they were selling was Roundup ConcentratePlus containing diquat. If he had not read the label carefully, he would not have known he was spraying glyphosate and diquat. Diquat is a contact herbicide that desiccates (dries out) plants, but it is not taken into and moved through the plant like a systemic herbicide.

Why does Monsanto put diquat into its product when it works so well without it? The answer is that diquat works fast. When sprayed on plant leaves, it kills them very quickly and gives the illusion that the plant has died. In the meantime, the glyphosate is being moved to the roots to kill them.

Is it safe? According to the Environmental Protection Agency, it is, but you need to make that determination yourself and I would be a little cautious. My friend purchased the Roundup Plus herbicide to prepare an area for a vegetable garden, but this formulation with diquat has not been deemed safe (labeled) for use in vegetable gardens. This fact is not mentioned anywhere on the label. It should not be sprayed around food crops.

When is the best time to spray a herbicide like Roundup?

Roundup herbicide is taken into plants through the leaves and then moves into the roots. Herbicides like this are called systemic. They work only when plants are actively growing. In most cases, that would be between spring and fall; however, there are a few winter weeds that might be killed during a warm spell. There should be little or no wind and the temperature must be above 60 degrees. Cloudy days are better than sunny ones because they allow the herbicide more time to enter the plant before it dries on the leaf surface.

Also, be sure to read the label to see how far in advance the herbicide should be sprayed before it rains. Be very careful with the wind. Herbicides can drift a long way and damage other plants.

Dennis Bishop is an urban horticulture educator for the Baltimore office of the Maryland Cooperative Extension Services. If you have a gardening or pest problem, you can call the Home and Garden Information Center hot line (Monday through Friday, 8 a.m. to 1 p.m.) at 800-342-2507. You can also e-mail questions, order publications and diagnose plant problems by visiting the Web site www.hgic.umd.edu.


1. Divide and replant overgrown perennials. Discard the weak center portion of plant crowns.

2. Work rotted manure or compost into vegetable and flowerbeds. Never add fresh manure to vegetable beds in the spring.

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