Delaying action against weeds is a mistake you’ll regret. To keep unwanted intruders at bay, the optimal strategy is to apply a pre-emergent treatment prior to the winter weed seeds sprouting. “If you ignore them, warm-season grasses will turn unappealing after a frost, while the weeds stubbornly stay green,” asserts Clint Waltz, PhD, a turfgrass expert from the University of Georgia. “This isn’t the outcome most homeowners desire.”
Winter weeds typically make their appearance in late summer and into autumn; certain varieties persist in germinating through December and even into the spring months. During dry spells, the germination of winter weeds might be postponed until later in the season. “However, for lawns that are watered, all precautions are out the window,” Waltz cautions. Without employing a pre-emergent, your irrigated lawn is almost guaranteed to be overrun by winter weeds.
Winter weeds can be broadly categorized into two groups: broadleaf and grassy. Broadleaf weeds, like henbit and chickweed, feature wide leaves with a characteristic netted venation. In contrast, grassy winter weeds, such as annual bluegrass, have narrower, grass-like structures with parallel veins, explains Waltz.
Identifying any weeds in your lawn is crucial. This is because preemergent treatments are ineffective against established perennial weeds, Waltz advises. If you’re uncertain about the type of weed, reach out to your local university’s cooperative extension agent (find yours here) or utilize apps like Google Lens or the University of Tennessee’s Mobile Weed Manual for assistance.
A pre-emergent treatment is at its peak when applied before weeds sprout. Yet, certain products, including those with dithiopyr, offer a “early reachback” benefit, allowing them to eliminate tiny weeds that have germinated within the past two to three weeks, Waltz explains.
For winter weeds in the Southern region, it’s advisable to apply pre-emergents between late summer and mid-fall. After this window, it’s usually too late for effective pre-emergent treatment. Nonetheless, if winter weeds are already popping up, you might still opt for a dithiopyr-containing product due to its reachback potential and its ability to control ungerminated weed seeds.
Lastly, be mindful that pre-emergents should not be used when planning to sow, reseed, or overseed your lawn. The lingering effects can hinder the growth of all types of seedlings, including your new grass seed, Waltz warns.
