Don’t forget knotweed germinates before crabgrass. So if you see germinating seedlings, it’s probably knotweed. Remember that knotweed is a summer annual broad-leaved weed that forms dense patches. It germinates fairly early in the spring, earlier than crabgrass.

At this time of year, people are concerned about missing the opportunity to get their preemergent herbicide down because they see all these young seedlings, assuming they’re crabgrass.Most of the time, they are knotweed.

Look closely at the leaves. 

When it germinates, knotweed has two small leaves like grass leaves, but shortly after germination, the leaves begin to get wider and take on that normal broad-leaved weed shape.

A preemergent herbicide is not normally recommended for controlling knot- weed.

It’s best to try and alleviate compaction, reduce traffic, and increase the density of turf by seeding and fertilization, as well as use a postemergent broad-leaved chemical to control it after it is done germinating.

Nonetheless, crabgrass itself will be germinating very soon.

(Source: Kansas State University Turf