What Herbicide to Use in the Fall for Weed Spraying

Weed control in the lawn and landscape can be a year-round chore, but waging war on unwanted vegetation doesn’t have to be a full-time job. Treat the lawn or garden for weeds in the fall to get a jump on the battle before new weeds get a chance to rear their ugly heads in spring. Spraying for weeds in the fall is effective on perennial weeds such as dandelions and ivy and annuals such as henbit and chickweed. Herbicides keep new weeds from germinating or kill them outright, reducing the number of weeds that plague the garden in spring.

Pre-emergent herbicides are designed to keep weeds from germinating. Post-emergent herbicides are designed to kill weeds that have already sprouted and are actively growing. Herbicides may be liquid or granular. Liquid herbicides are applied with handheld sprayers, pump sprayers, backpack sprayers or in premeasured amounts in spray bottles that attach to a garden hose. The spray pattern on most units can be adjusted from a broad, light mist to a concentrated, thinner stream, depending on whether you are spraying a single weed at a time or a large patch of weeds. Avoid spraying herbicides on windy days, when the chemical can drift to desirable plants.

Herbicides are formulated to control specific types of weeds and may be limited to use on lawns or be listed as appropriate for both lawn and garden. Herbicides are generally not appropriate to use in actively growing vegetable gardens. Pre-emergent herbicides are best applied after existing weeds are removed. Spraying in the fall controls winter annual weeds. Delaying or spraying again in late winter controls perennial and summer annual weeds. Pre-emergent herbicides appropriate for landscapes include products containing dithiopyr, isoxaben, napropamide, metolachlor, oxadiazon and trifluralin. These products control most weedy annual grasses and broadleaf weeds such as dandelions. The label should provide application rates and timing and list which weeds are controlled by the product.

Spraying post-emergent herbicides on weeds still alive in the fall prevents them from spreading or setting seeds that can sprout in the spring. Although no herbicides are recommended for established weeds in actively growing vegetable gardens, herbicides may be used in lawns and landscapes. Herbicides suitable for controlling growing weeds include dithiopyr, fluazifop, glyphosate, clove oil, triclopyr, pelargonic and glufosinate. Many post-emergent herbicides such as glyphosate kill systemically as the chemical is absorbed through the leaves and stem and makes its way to the roots. Weedy grasses may be sprayed in early fall, but they are more difficult to control.

Using the same weed control product every time you spray the lawn or garden can sometimes result in weeds adapting or becoming resistant to the chemical. According to the University of California at Davis Statewide Integrated Pest Management Program website, rigid ryegrass, hairy fleabane and horseweed have become resistant to glyphosate in some vineyards and orchards. Since other types of weeds may also develop resistance, you should alternate weed control products or use products formulated with a combination of chemicals such as isoxaben and trifluralin or oxyfluorfen and pendimethalin.