· by Michael Blair, Nashville's Weed and Feed
In Middle Tennessee, September and October carry two jobs that pull against each other. Fescue lawns get aerated and overseeded now, because the soil is still warm and the nights are cooling. And perennial broadleaf weeds, dandelion, clover, plantain and wild violet among them, are at their most vulnerable, because they are storing food in their roots for winter and carry whatever is sprayed on them down with it. A weed control product that kills those weeds can also damage grass seedlings that are only a few weeks old. The answer is in the order and the labels.
Why fall spraying works better than spring
In spring, a perennial weed is pushing energy up from its roots into new leaves and flowers. Spray it then and the top often dies while the root, still full, sends up new growth a few weeks later. In fall the flow runs the other way. The plant is moving sugars down to the root for the winter, and a systemic herbicide applied to the leaves travels with them.
That is why a dandelion that shrugged off a spring treatment will often die out completely from one sprayed in October. It also means one well-timed fall application can do what two or three spring ones did not, which is less product on the lawn overall.
The conflict with new seed
Most broadleaf herbicides for lawns carry a label instruction about newly seeded turf, and it usually says to wait until the new grass has been mowed a certain number of times, or for a set number of weeks, before applying. Young fescue has not built the tolerance that established grass has, and it can yellow, stunt or die. The exact wait varies by product, so read that line on the label rather than going by what worked last year.
Pre-emergent products are a separate problem. Anything designed to stop seeds from sprouting will stop fescue seed too, so a fall pre-emergent for winter annuals like henbit and chickweed cannot go down on an area that is being overseeded. Areas that are not being seeded can still get one.
Two ways to sequence it
If the weeds are the bigger problem, treat them first. Spray broadleaf weeds in early September, wait out the interval on the label before seeding, then aerate and overseed. The weeds are dying while the seedbed is prepared, and the new grass comes up into cleaner ground.
If thin turf is the bigger problem, seed first and let the weeds wait. Overseed in mid to late September, keep it watered, and mow it as it grows. Once it has been mowed the number of times the label asks, usually by late October or into November, spot-treat what is left while the weeds are still green. They are still moving food to the roots then, and a treatment before the first hard frost still works.
- Weeds first: spray early September, wait the label interval, then seed
- Seed first: overseed, mow the new grass as the label requires, spray in late fall
- No pre-emergent on any area being overseeded
- Spot-treat rather than blanket-spray around young grass
Bermuda and zoysia lawns in the fall
Warm-season lawns are the opposite case. They are slowing down and heading for dormancy, and there is no fall overseeding in a bermuda or zoysia program unless it is being overseeded with ryegrass for winter color. Broadleaf control on them can go down in the fall without that conflict, and a pre-emergent aimed at the winter annuals that show up in dormant bermuda, the henbit and poa annua that look so obvious on a brown lawn in February, fits here too.
When we set up a lawn program, the grass type decides this before anything else. A fescue lawn with a strip of bermuda along the drive gets treated as two lawns in the fall, because what is right for one this month is wrong for the other.