Annual bluegrass (Poa annua) is a cool-season annual that forms dense clumps that are lighter green than cultivated turf grasses. Unlike perennial bluegrass, the tips of annual bluegrass leaves show a slight curve, like the bow of a boat. Seedlings that sprout in fall or early spring produce seeds in early summer, and then often die in summer's heat. A corn gluten herbicide applied in early fall and again in early spring will reduce seed germination. Reduce reseeding by mowing often or collecting grass clippings when the plants begin producing seeds in May and June.
Corn gluten herbicides. Powdered herbicides made from corn gluten keep crabgrass and other weed seeds from germinating and growing. They are typically spread on established lawns, but they also can be used in gardens where no seeds will be planted, such as in perennial beds. As the corn gluten degrades, it provides a small amount of nitrogen to the soil. Crabgrass begins to germinate at about the time that azaleas, dogwoods, and forsythias bloom, so spread corn gluten at that time for best results. Application procedures vary with the particular product; be sure to read and follow the directions on the label. Do not use corn gluten in newly seeded lawns, or in garden beds where you plan to sow seeds.
Reducing reseeding. Most weeds reproduce primarily from seeds, and the seeds of some weeds can remain viable when buried in the soil for decades. So it's essential to keep weeds from shedding seeds in the garden. Garden weeds that are neglected until they reach seed-bearing age can be lopped off near the soil line with pruning shears, a stout knife, or a string trimmer with a blade attachment. Cutting back perennial weeds again and again not only reduces reseeding, it also forces the plants to use up food reserves stored in their roots. In a garden that has gone hopelessly weedy, mowing it down promptly, raking out the seed-bearing debris, and starting over next year is a big step in the right direction. Mowing regularly helps keep weeds under control in lawns. When mowing lawns where seed-bearing weeds are present, collect the clippings in a bagger and dispose of them in a shady place.Photograph courtesy of Jenna Antonino DiMare, National Gardening Association