September is a time for the lawn. In my last article, I discussed overseeding and fertilizing the lawn, but weed control is another key to a healthy lawn. Some weeds in the lawn are often tolerated, but when the weed population begins to outweigh the turf population, management should be incorporated.
The battle with weeds
Plants are considered weeds because they are adaptive, aggressive and opportunistic. Weeds come into a lawn that is thin or bare. They are often found on the edges of our lawn or places where the grass doesn’t grow as well. Eventually they will work their way into the rest of our lawn. So overseeding a lawn may be the answer to reducing the weed population.
Growing turfgrass in the shade is not always possible. Even the shade mixes are not made to grow in high shade. Dense shade is not the growing condition for turfgrass and it often leads to weeds. In some cases, trees may be pruned to improve the sunlight getting to the turf, but be careful not to ruin the shape or health of a tree or shrub just to get more sunlight to the turf. If the shade is too dense, a good alternative might be a shade tolerant groundcover or apply mulch where grass won’t grow.
Crabgrass has been found in high populations this year, even in locations where crabgrass preventer was used. The University of Nebraska-Lincoln Turf department says the weather is to blame. Crabgrass germinates when soil temperatures reach 57-64 degrees at a one-inch depth, with the highest germination when soil temperatures increase to 73 degrees. Crabgrass will continue to germinate through the summer until soil temperatures reach 95 degrees. With the unusual weather we had this spring and summer, the largest amount of crabgrass germinated later than normal when the concentration of our original spring crabgrass pre-emergence herbicide was declining. This is why we saw so much more crabgrass later this summer than we usually do.
To help prevent this problem with later emerging crabgrass in the future, switch to longer lasting chemicals, such as prodiamine. You can also look to a second application of crabgrass preventer in mid-June to stop the germination of the late flush of crabgrass. There are also post-emergent herbicides to use for crabgrass, but I don’t recommend them this late in the season. Remember, crabgrass you are seeing now will die when the first frost hits. Also, it is difficult to kill a large crabgrass plant with a post-emergent herbicide.
Broadleaf weed control
As for when to treat for broadleaf weeds that do come into our lawns, fall is the best time to control them. In fall, perennial weeds are moving carbohydrates from the leaves into the roots for winter storage to help get them going again next spring. If you spray them in the fall, the herbicide will also be moved into the roots which makes the herbicide more effective. Also, the weather is more suitable for herbicide use than in the spring when Dicamba and 2,4-D have a potential to drift to non-target plants. Fall is also a great time to apply herbicides to kill the winter annual weeds such as Henbit either with a pre-emergent herbicide such as prodiamine (Barricade) or dithiopyr (Dimension) or with a post emergent after the henbit has germinated.
The best time to apply herbicides in the fall is mid-late September and again in mid-late October. The second application helps to get better control on perennial weeds that may have been missed or were not fully killed. The second application also helps to ensure that winter annuals have germinated to help get control of those with a post-emergent herbicide. Products containing 2,4-D, carfentrazone, sulfentrazone, quinclorac, or triclopyr are all good for controlling perennial weeds in the lawn. Use caution around trees, shrubs, and landscape beds as these products can damage broadleaf plants but they will not harm our turf.