There is no set number of days, and the published record does not give you one. Extension services list repeat intervals instead of a countdown, so your weed and feed results land on a schedule the weed sets rather than the bag. Some broadleaf weeds fade after a single pass. Others need a second one weeks later, and a few never respond at all because the active ingredient in your bag does not cover them. Herbicides are an aid, not a cure for lawn weeds.
The clover was gone by the middle of June, after the second pass. From the kitchen window it had been a pale green haze sitting across my front lawn, and the first bag barely dented it. I checked it again at five weeks and went over the yard a second time. Clover and other legumes can take two passes 4 to 6 weeks apart, which is close to the gap I ended up with.
A single number does not exist because four things move at once. The species decides whether the active in your bag touches that weed at all. Age matters too, since herbicides work best on young weeds. The weed also has to be actively growing when it takes the product in, and a drought-stressed lawn should not be treated in the first place. A bag aimed at dandelion may do nothing to nutsedge. That is why weed and feed results swing so much from one lawn to the next.
The two intervals you will find are both repeat herbicide application windows, and neither one is an onset figure. Rutgers Cooperative Extension says clovers and other legumes may need two applications on a 4 to 6 week interval. Delaware extension guidance says nutsedge herbicides may need a repeat 6 to 10 weeks after the first one. Those numbers tell you when to think about going again. They say nothing about when your first pass starts to show.
First pass
Treat young weeds that are actively growing. Skip the application if your lawn is dry and stressed.
The wait
No published onset figure exists for a bagged product. Identify the weed and read the label before you judge the result.
Weeks 4 to 6
The published repeat window for clovers and other legumes, if your label allows a second pass.
Weeks 6 to 10
The published repeat window for nutsedge herbicides, which are a separate product class from most bags.
Granular bags are typically weaker than sprays, and clover is the weed that shows you that gap first. So treat the clover two applications pattern as normal, not as a failed bag. Iron works the same way. In trials, repeat passes of chelated iron killed white clover. Dandelion grew back from the taproot.
Do not reach for the bag again just because nothing looks different yet. Pull one weed and identify it first. Read the label and check whether that species is even listed on it, then see what your local extension service says about timing where you live. Reapplying on a hunch costs you money, puts more product on your lawn, and still will not touch a weed the active ingredient never covered.
Conditions at treatment time set your pace more than patience does. A mature weed in a lawn already stressed by dry weather is the worst case you can pick, and no amount of waiting fixes it. Water first, let the lawn and the weeds grow, then treat. Reading weed and feed results honestly takes a few weeks, one weed pulled up for a proper look, and a careful read of what your bag claims to control.
Read the full article: Weed and Feed: When It Works and When It Fails
