Purdue News

September 20, 1996

Control lawn weeds in October, not May, expert says

The explanation for the timing is one part plant physiology and two parts gardening sense, according to Zac Reicher, turfgrass specialist for the Purdue Cooperative Extension Service.

• During the summer, plants move nutrients from their roots to the leaves to spur growth. In very late summer and early fall the plants prepare for winter by reversing this movement from leaves to the roots. By applying herbicide in the fall, the weeds are less likely to return because the entire plant is killed, not just the leaves.

• Killing weeds in the summer or spring leaves a bare spot of ground that allows a space for opportunistic weeds such as crabgrass to move in.

• Herbicides can cause damage to vegetable and flower gardens, which isn't much of a concern in October when the gardens are finished for the year. There is also less chance of damage to fruit trees and ornamental plants, because they aren't blossoming.

  Reicher is quick to point out that homeowners shouldn't come to rely on herbicides to control lawn weeds. "The best way to control weeds is do have healthy turfgrass," he said. "Follow the three steps to good lawn management, and you won't have so many problems with weeds: Mow high (at least two inches), apply fertilizer in September, and irrigate your lawn when there isn't enough rainfall.

  "Herbicides aren't the answer to a good lawn. They are just short-term remedies for a lawn that needs help," he said.

  If herbicides are called for, Reicher cautioned that the homeowner first should make sure that the lawn is green and growing. "This is especially true this year, because many areas in the Midwest haven't had enough rainfall in August and September," he said. "It sounds funny, but weeds have to be healthy before you can kill them. If they are dry, they don't absorb the herbicide as well. Also, you might damage the turfgrass by applying herbicides when the lawn is dry."

  If there are brown spots in certain areas of the lawn, don't apply the herbicide to those areas -- apply it only to those areas that are green and have weeds. Even a hand sprayer can be an effective way to treat a lawn, Reicher said. "I'm a big advocate of spot spraying, but a little of this goes a long way. Don't spray until you see liquid dripping off of the plant leaves."

  More difficult weeds such as violets or creeping charley may need more than one application. "Apply the herbicide in early October and again about three weeks later," Reicher said. "But these weeds are very difficult to control, and you'll still probably not be able to eradicate them, but just beat them back."

  The best herbicides to use to kill weeds in the lawn are called three-way products, because they are a mixture of the herbicides 2,4-D; dicamba, which is also known as baurvel; and MCPP, or mecoprop. "A lawn herbicide with just two of these is OK to use, but ideally you want all three," Reicher said. "This will give you the widest window of control."

  The procedure for applying liquid and granular herbicides is completely different, Reicher noted. "Liquid herbicides should be applied when the lawn is dry and rain is not expected for a few days following the application. Granular herbicides have to be applied when the grass is wet," he said.

  "Homeowners should worry more about applying herbicides effectively than about which type to use. If applied correctly, a lot of different products work. You just have to read the label and follow the directions.

  "You still have to apply herbicides accurately and think about what you are doing. You never want to do it on windy days or use more than you need. It's just not environmentally wise."

  Other lawn and turf information, including weekly updates by Purdue turf experts, can be found on the Internet at https://www.aes.purdue.edu/agronomy/turf/turftips.htm

Source: Zac Reicher, (317) 494-9737; e-mail, zreicher@dept.agry.purdue.edu

Writer: Steve Tally, (317) 494-9809; home, (317) 463-4355; e-mail, tally@ecn.purdue.edu

To the Purdue News and Photos Page