Put it down when the soil has warmed and the grass is growing on its own again. Weed and feed timing runs on soil temperature and active growth, not on a month you circled back in February. A warm afternoon tells you nothing about the ground 4 inches (10 cm) down, and that is where the roots live. Feed too early and you pay for nitrogen the lawn cannot use. The weed killer in the same bag runs on a separate clock, and the two rarely land on the same weekend.
I had the spreader loaded and halfway down the drive when I checked the thermometer in the strip of lawn by the mailbox. It read 58°F (14°C), though the air felt like the middle of May, so I pulled the whole rig back into the garage. Nine days later the same probe hit 65°F (18°C), the mark University of Georgia Extension sets before you put down anything carrying nitrogen. That Tuesday I opened the bag.
Two clocks run inside one bag. The fertilizer half waits for growth, and a lawn soil temperature of 65°F at 4 inches is one way to read it. UF/IFAS Extension gives a version that needs no thermometer at all. Wait until you have had to mow twice before the first feeding of the season. Grass that demands cutting twice has come out of dormancy and can use what you give it.
Going early costs you on both halves. Grass still asleep is not taking up much of anything, so the nitrogen sits there waiting on a lawn that is not ready for it. Weed killers have the same problem in reverse. They move in through leaves that are actively growing, so the air temperature matters as much as the date on the bag.
The herbicide half splits in two. For broadleaf weeds already up, Maryland Extension puts the working range at daily air temperatures of 65 to 85°F (18 to 29°C). Skip the job if rain is due inside 24 hours. Preemergents run on lead time instead, and they go down 10 to 14 days before crabgrass germinates. Delaware Extension ties that moment to the top inch (2.5 cm) of soil holding 55 to 58°F (13 to 14°C) at daybreak for four or five days.
That gap is the real trouble with a combination product. One date has to serve two jobs, and the day that suits the fertilizer can sit weeks away from the day that suits the herbicide. When the two are far apart, I split the work. Spray the weeds while they are in range, then feed the lawn once the ground says it is ready. You give up the convenience and you get both jobs done right.
So when to apply weed and feed? Every state answers with a different month. Rutgers Cooperative Extension calls late September through October the best broadleaf window in New Jersey, with May running second. Georgia preemergent dates sit earlier and tighter, March 1 to 20 in the north of the state and February 15 to March 15 in the south. Maryland and Rutgers both make autumn the primary window. The bags on store shelves push spring, because spring is when people buy them.
Buy a soil thermometer and take the reading at 4 inches in the morning, out in the open lawn rather than a shaded bed. Check the forecast before you load the spreader, and hold the job a few days when rain is coming. New lawns wait longer than most people expect. Seeded turf needs three mowings before it gets any broadleaf product, which lands about 6 to 8 weeks after the seedlings come up. Preemergents belong only on turf established at least a year. Your county extension office will hand you local dates for both halves of the bag, and those dates beat anything printed on a national label.
Read the full article: Weed and Feed: When It Works and When It Fails
