One bag holds two products, and they run on two different clocks. Delaware Cooperative Extension puts preemergent reapplication within 60 to 90 days where you want control that holds all season. Weed and feed duration on the fertilizer side has no such published figure, and the bag in your hands is the only place that period is written down. So the honest answer is two answers, and one of them you have to go read off the label.
That 60 to 90 day range is a window, not a promise. It moves with the product, the timing, the location and the rainfall you get. Two neighbors can spread the same bag on the same Saturday and lose weed control weeks apart. Nobody can hand you a single number for your yard. Treat the short end of that range as the point where you start watching the lawn instead of trusting it.
The two halves also quit in different ways, and that tells you which clock ran out. A preemergent works on crabgrass seedlings before they break the surface, so you notice it stopped when new grassy weeds show up along the thin edge of the drive. Fertilizer goes the other direction. Growth slows, the color flattens out, and mowing gets easy again. Watching the lawn beats counting days, though the days give you the rough shape of it.
Rate is where most of the lost time comes from. Season long crabgrass control with prodiamine takes 0.75 lb (0.34 kg) of active ingredient per acre (0.4 ha). A 0.43 granular prodiamine product reaches that only at 4 lb (1.8 kg) of material per 1,000 sq ft (93 sq m). Spread it lighter and the label's timeline was never written for what you put down. A half rate is a half rate, whatever the front of the bag promises.
Check the spreader before you trust that number. The math holds up only if your dial drops 4 lb per 1,000 sq ft on the ground, and settings printed for one bag do not carry over to the next one. Weigh the bag, run a test strip, and see what comes out. It takes ten minutes once a year. Before blaming the chemistry for a short season, look hard at the spreader setting.
The feeding half answers to soil temperature, not the calendar. A weed-and-feed that contains nitrogen should wait until the soil 4 inches (10 cm) down sits at 65°F (18°C) and stays there. A probe thermometer settles that in about a minute, and it costs less than the bag does. Guessing from the air temperature on a warm March afternoon is how people spread three weeks early.
The second bag is where lawns get hurt. Weed-and-feed use can push a yard into overfertilizing, so a repeat pass thrown down out of habit feeds grass that never asked for more. If crabgrass pressure is real and another preemergent is needed inside that window, buy a straight preemergent and leave the nitrogen out of it. That keeps the two decisions separate instead of tying both of them to one purchase.
Write the date you spread it right on the bag with a marker before it goes back in the shed. Then check that product's own reapplication interval when the 60 day mark comes around, and look at the lawn before you buy anything. Fresh crabgrass in the thin spots says the weed side is finished. Deep green growth you are already mowing twice a week says the feed side is not.
Read the full article: Weed and Feed: When It Works and When It Fails
