A good sized portion of my backyard had to be reseeded 2 years ago after the septic drainage was replaced. The new grass has now come back in but there are large areas (10'x20') of crabgrass in it. What's the best way to go about getting rid of this crabgrass? I live in USDA zone 4, or 5.
Crabgrass becomes quite obvious at this time of year - for most of the summer it remains sort of green filling in patches but then turns a kind of purplish red colour going into the fall. It's an annual grass coming back from seed every year. The way we get it under control is to exploit its fondness for open space. In my yard it thrives in the open veg patch and sandy spots, but anywhere the quack grass and other perennial grasses can thrive it does not appear at all.
When septic beds are installed the soil/fill used is generally a loose well draining material that does not support grasses very well. Crabgrass produces seed prolifically and will colonize a bare or thin area particularly before the other grasses get established. If your septic bed repair was topdressed with some reasonable soil then it could be that just patience will see other grasses move in and crowd out the crabgrass. If not, then consider improving the soil conditions in these patches to encourage the better grasses to move in: richer soil, greater depth of good soil, irrigation and so on.