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There are organic herbicide options that will kill grass but not flowers, including white vinegar or a dish soap solution. Commercial selective herbicides can also target grass over other plants, but read the instructions carefully to avoid damaging your garden. Herbicides that kill grass but not flowers usually include ingredients like clethodim and sethoxydim. Otherwise, your best bet will be to weed by hand.

The best time to fight crabgrass with pre-emergent herbicide is when forsythias tend to bloom. In the mid-Atlantic region, this is usually between April and May. Even if you're late to apply the herbicide, it’s effective as long as the crabgrass plant has fewer than five leaves.

It usually takes between two weeks and a month for weeds to fully die off after you spray them with herbicide, but they will start to wilt within the first week. Different weed sprays produce different results, with some working more quickly and effectively than others. Similarly, some weeds are killed more easily than others. Consult your weed removal pro for an exact timeline based on their weed removal method.

Vinegar cannot tell the difference between weeds and desired grass types; it kills everything it touches. Only use a vinegar or vinegar-based solution if you are relatively confident that you can navigate around the plants and grasses you want to keep. Also, don’t spray vinegar on a windy day, or it will blow onto other plants.