Management and control methods for weeds varies with the time of season. For example, certain weeds will be better controlled by herbicides in the fall than they are in early summer. Still other weeds are controlled better in very early spring than any other time of year. So what to do about the common weeds you find in late summer?
Late summer is a good time to apply 2,4-D to dandelions. If infestations are not too heavy, spot application rather than broadcast spraying will limit the amount that is placed in the environment, possibly leaching into groundwater. Spot treatment will also minimize potential damage to ornamental and tree roots and foliage.
For quackgrass and bindweed, apply a glyphosate herbicide such as Roundup, it will be more effective now than it is in midsummer. Spot treatment with glyphosate rather than broadcast application is important as this herbicide will kill grass, trees, and ornamentals, and can travel through the root systems.
Mallow and purslane both produce very large numbers of seeds, so hoe them or pull them before the seeds form. If you give up on these weeds now, you will have huge numbers of seeds that find their way into the seedbank to plague you next year and for many years to come. (See the fact sheet in this series "The Soil Seedbank".) Purslane that has been hoed or pulled can put down new roots as it lays on the surface of the ground, so turn it upside down or lay it on something that prevents contact with the soil.
Canada thistle will usually succumb to spot applications of a glyphosate herbicide. If this persistent weed is hoed religiously, cutting all new shoots when they are less than 2 inches in diameter, it can eventually be controlled without chemicals. However, so much time and patience are required for this method, up to 13 years in some studies, that is usually more feasible to apply a glyphosate, 2,4-D, or dicamba herbicide instead.
If spotted knapweed is a problem on your property, a one-time mowing about mid-August can be effective. Mowing is not recommended as a regular method of knapweed control because the plant simply assumes a prostrate form, producing flowers and seeds too close to the ground for mower blades to reach. However, if it is mowed only once, it will not adapt to the prostrate form, and seed production will be reduced. Collect and destroy the mowed plants because seeds can continue to form from blossoms on the cut plants.
Asters can also be mowed at this time. Regular mowing of purple aster can cause it to grow in a prostrate form, producing flowers and seeds beneath the mower blades, requiring spot application of a systemic herbicide.
The above-ground portions of some weeds, such plantain, burdock, and others can be killed with the non-toxic herbicidal soaps, such as Sharpshooter or Superfast. With regular applications to regrowth, this herbicide will effectively control many weeds safely. They work best on young annuals. They do not kill below-ground portions of the plants so do not work well on perennial weeds that reproduce from the roots, such as Canada thistle, mouseear chickweed and field bindweed. Weeds that produce shoots from the roots will continue to do so. Like the glyphosate herbicides such as Roundup, herbicidal soaps will kill both grasses and desirable plants, so allow it to come in contact only with the plants you wish to kill. The herbicidal soaps, which are nearly non-toxic, also work well for weed control in sidewalk cracks, driveways, and in mulches. For more information on herbicidal soap, see the fact sheet in this series.
Glufosinate-ammonium (e.g. Finale) is a least-toxic partially systemic herbicide that kills both roots and above-ground portions of weeds. Like glyphosate and herbicidal soap, it will kill both desirable plants (grasses and ornamentals) as well as weeds, so must be allowed to touch only plants you wish to kill. Glufosinate-ammonium is a synthetic version of a naturally-occurring compound produced by soil bacteria. It is of low mammalian toxicity and is rapidly biodegraded.
Read and follow label directions when using any pesticide. For assistance with weed identification, contact your county Extension agent.
References:
Farm Chemicals Handbook. 1997. Meister Pub. Co. Willoughby, OH 44094-5992. 537 pp.
Written by Sherry Lajeunesse, Extension Urban Pest Management Specialist. Sept., 1997