The product in the jug is not usually the problem. Weather, weed age and weed type explain most cases of a weed killer not working, and none of those get fixed by buying a stronger bottle. Put the same product on the same plants under better conditions and it tends to finish the job.
The crabgrass in my back lawn yellowed at the tips, then greened back up and kept spreading. That was August on the tall fescue yard I took over eight seasons ago, and the plants were tough and going to seed after three weeks with no rain. Herbicides do their best work on young weeds that are still growing. A drought-stressed lawn should not be treated at all, and mine was bone dry.
Weed age costs you more results than anything printed on the label. A young plant still putting on growth carries the herbicide down into its roots. An old plant that has already set seed moves much less of it, so the top browns and the crown lives. Same rate, same sprayer, very different outcome.
Air temperature is the next thing to check. University of Maryland Extension puts the window for post-emergent broadleaf products between 65°F and 85°F (18°C and 29°C). Spray when it is hotter and more of the product turns to vapor, so less of it stays on the leaf and the drift can burn plants you wanted to keep. Rain works against you the same way. Skip the spray if precipitation is expected within 24 hours, unless the label says that formulation is rainfast sooner.
Two more misses cost people a whole afternoon. More than 70% of herbicides call for one or more adjuvants in the tank, and that line on the label is not a suggestion. The adjuvant is what makes the spray stick and spread across the leaf. The other miss is granular products. Soil-applied material is mostly pre-emergent, and it has to be watered in 15 to 20 minutes after you spread it. Left dry on the surface, it never reaches the seed zone.
Most herbicide failure causes turn up on a short checklist, and you can run it before you mix anything.
- Check the forecast: You want a daily air temperature between 65°F and 85°F (18°C and 29°C) and no rain due in the next 24 hours.
- Look at the weeds: Young plants in active growth take a herbicide well. Old, seedy plants and anything under drought stress do not.
- Read the adjuvant line: If the label asks for a surfactant or a crop oil, add it at the rate printed there.
- Water the granules in: Soil-applied products need water 15 to 20 minutes after you spread them.
- Name the weed before you spray it, because a few of them will never die selectively.
Some weeds will not die no matter how well you spray them. Perennial grass weeds are the ones to know here. Four names cover most of it: bentgrass, nimblewill, dallisgrass and bermudagrass, the grass also known as wiregrass. None of them can be pulled out of a lawn selectively. Your only real option there is a non-selective herbicide on the patch, then reseeding once the ground is clear. People burn through bottle after bottle of broadleaf product on nimblewill and get nothing for it.
Resistance is real, and it belongs at the bottom of the list rather than the top. About 180 weed species and more than 300 biotypes worldwide now survive roughly ten herbicide families. That kind of shift can build in 5 to 20 years on ground that gets the same chemistry every season. Rotating modes of action is what keeps a working product working. Check the group number on the label and pick a different one next time. The main article covers resistance in more depth.
Fix the conditions first. Spray inside the temperature window, on weeds that are young and not drought-stressed. Most lawns then come around on the same product you already own.
Read the full article: How to Choose a Weed Killer That Actually Works
