A common question and a golden rule of lawn care revolves around the question of how soon a herbicide can be safely applied to a lawn after mowing. Getting it wrong can mean major damage or even resulting in killing a lawn.
All lawn herbicides should be applied at half the lawn mowing service cycle. For example, if the lawn was being mowed every two weeks, then herbicide application should be after one week, and if lawn mowing was occurring every four weeks then herbicide should be applied 2 weeks after the last lawn mowing.
Correct lawn mowing service frequencies are matched to the growth rate of the lawn, and the growth rate is also matched to the repair rate for the lawn to recover from mowing.
After mowing, the lawn needs time to repair and seal it's cut leaves, and in some cases, stolons (runners) may have been cut depending on thatch levels or unevenness of the lawn.
If a herbicide were applied directly after lawn mowing, the lawns freshly cut wounds would take the herbicide in, killing the lawn.
Due to the increased growth rate of lawns in Summer, the lawn will be repaired after a week. Applying the herbicide at this time, also allows the weeds 1 week to uptake the herbicide before the leaf material containing the herbicide is removed at the next mowing.
Lets have a look at the standard Summer mowing frequency which is to mow every 2 weeks.
If you mow every week in Summer, use the same method as outlined above, while skipping one week of mowing.
The same principle applies to 3 weekly and 4 weekly service cycles. Due to the lawns decreased growth during the periods of Spring, Winter and Autumn, it will need greater time to heal the wounds made by lawn mowing, before it will be safe to spray herbicides, without risking damage to the lawn.