Answer: This is the type of situation where an ounce of prevention is worth several pounds of cure. It's easier to keep weeds and grasses out of the garden than to get them out once they've established themselves. The safest treatment is to dig the weeds and grass out. Crabgrass is an annual that begins to sprout about the time the forsythia blooms, so one of the best controls is to either cultivate the soil lightly to disturb those seeds (lightly so you don't bring more seeds up near the surface where they'll germinate, too) or to use a layer of mulch to smother them. Another option is to use corn gluten as an herbicide. The gluten releases an enzyme that kills germinating seeds, and at the same time adds nitrogen to your soil. So, pull out the existing crabgrass, plant your perennials and annuals, then apply a pre emergent herbicide and cover over the bare soil with a thick mulch to exclude light to the surface of the soil and no new crabgrass should sprout. Best wishes with your new perennial bed!
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