Yes, for a granular postemergent product the blades should be wet when the prills land. A weed and feed damp lawn holds those granules on the weed leaves instead of letting them drop through into the thatch, and morning dew does that job on its own. You do not have to drag a hose out first. Preemergent products that work down in the soil are the other case, and those follow whatever the label says about watering in.
The fescue patch by my back steps sits in shade until close to ten, and the dew there soaks the toes of my boots long after the rest of the yard has gone dry. That strip is where granular weed and feed always works best. Out in the open, the leaves are dry by nine, and a prill sits on a dandelion for a second and then rolls off into the thatch. Granular products want moist, dew covered foliage if you want the weeds to take the dose.
The reason is stickiness. Each prill carries a dose of herbicide, often a mix of 2,4-D, MCPP and dicamba, and that dose has to move out of the granule and into the leaf. Water is the bridge. A wet leaf grabs the granule and holds it there while the coating breaks down, so the plant can pull the active ingredient in through the surface. The University of Georgia ties absorption straight to moisture from dew or rain. A dry leaf gives the prill nothing to hold onto, and a granule sitting in the soil under a weed is not treating that weed.
Two kinds of wet get mixed up here, and they belong at opposite ends of the job. The first is foliage moisture before you spread, which is what makes a product work on weeds that are already up and growing. The second is watering after weed and feed, which matters for the soil-active part of the bag. Those ingredients have to move off the granule and down to the root zone, so give them rainfall or 15 to 20 minutes of irrigation once the product is down. Read your label and see which of the two jobs your bag is doing, because plenty of combination products do both.
Liquids bring their own water. A spray carries the herbicide in solution and wets the leaf as it lands, so dew is not doing much for you there. Use a minimum of 1 gallon (3.8 L) of water per 1,000 sq ft (93 sq m) so the spray actually covers the leaf surface. Granules are the formulation that leans on dew.
Rain in the forecast changes the plan. Skip the application if rain is expected within 24 hours, unless your label says otherwise. A hard rain washes the herbicide off the leaves, and the weed keeps growing because it never took in a full dose. An overnight shower is a different story. Grass that is still damp at sunrise gives you the same wet the dew would have.
Plan the job for early morning while the dew is still sitting on the blades. Walk the lawn first and check that the leaves are beaded, not just cool, since a light dew burns off fast on a breezy morning. Spread when the air is still. Prills that drift into a bed will damage the ornamentals they land on, and that is a hard mistake to walk back. Once the granules are down, leave the sprinkler off and let them stay put where they landed, and check your own label for how long that wait should be.
Read the full article: Weed and Feed: When It Works and When It Fails
