Greg Grant

Fall is just around the corner, which means that winter and spring weeds can’t be too far behind.

Common annual cool-season weeds that germinate from seeds with the first rains and cooler weather of fall are annual bluegrass, chickweed, clover, dandelions, henbit, ryegrass and rescuegrass. Always remember that the best control against weeds in a lawn is having a thick, healthy turf. If there are bare spots and sunlight can hit the ground, weed seeds will germinate. Otherwise, the only effective control for these annual weeds is using a pre-emergent herbicide.

Pre-emergent herbicides are ones you apply before the weed seeds emerge or start to germinate and sprout. This means you must apply these types of herbicides before you see the weeds. If you see them growing, it’s too late. They only work on annual weeds germinating from seed, not on perennial weeds sprouting from an existing root system or crown. The time to apply pre-emergent herbicides on lawns in East Texas is during the next few weeks before fall weather arrives.

Winter lawn weeds are divided into two categories — grassy weeds, such as annual bluegrass, and broadleaf weeds, such as henbit. Different pre-emergent herbicides work better on one or the other so you’ll need to know what your problem weeds are. Products that work on grassy weeds include Halts (pendimethalin), Dimension (dithiopyr), and Balan (benefin). For broadleaf weeds, Gallery (isoxaben) is the herbicide of choice. Luckily broadleaf weeds can also be controlled later in the fall, winter and early spring by carefully spraying with a post-emergent broadleaf weed killer with 2,4-D, like Trimec.

Pre-emergent herbicides are generally stocked by garden centers, hardware stores, feed stores and agricultural chemical supply stores. They can be liquid or granules, with granules being easier for typical homeowners to apply.

If you decide to apply two different products to control both broadleaf and grassy weeds, you must make two passes over the lawn, one with each type of pre-emergent since the granule size may be different and they are difficult to adequately pre-mix yourself.

Granular pre-emergent herbicides must be watered in to be effective, as they form a chemical herbicide barrier at the soil level that kills the seedlings as they begin to germinate. It’s critical to use only the recommended rate so as not to damage the existing lawn, trees and shrubs. Used at the labeled rate, pre-emergent herbicides do not harm existing trees, shrubs and perennials.

However, do not use a fall pre-emergent herbicide if you intend to over-seed your lawn with winter ryegrass or wildflowers or if your lawn is less than four months old.

Pre-emergent herbicides are for those who can’t sleep at night without a perfect lawn of 100 percent turfrass. Others have learned by necessity to appreciate anything green instead of bare dirt. I fall somewhere in between with a very small zoysia lawn in front of my house that I try and keep weed free while the rest of my drive-through farmscape is composed of weeds, grass and anything else green that wants to join the team.

Greg Grant is the Smith County horticulturist for the Texas A&M AgriLife Extension Service. He is co-author of “Texas Home Landscaping.” You can read his “Greg’s Ramblings” blog at arborgate.com or read his “In Greg’s Garden” in each issue of Texas Gardener magazine (texasgardener.com). More science-based lawn care information from the Texas A&M AgriLife Extension Service can be found at aggieturf.tamu.edu.