called popweed. genus name is Cardamine.

Thank you Jean for pointing my nose in the right direction. I have followed up, and found that the popweed is generally called bittercress (Cardamine hirsuta), and needs to be controlled by pre-emergence treatments, such as those for crabgrass. Especially effective are Snapshot and Dimension, both made by Dow AgroSciences.

Here is a link that might be useful: Bittercress description by Oregon State University

Easy to pull, if you have a reasonable amount.

At the stage you show, the seeds will still mature in spite of any herbicide you apply. This is hand-to-weed combat!

In your area, this weed probably grows mostly in fall and overwinters to then set seed in spring. If you want to use a broadleaf weed killer, the best time is in fall to kill the ones that have sprouted and other weeds too. My friend who is our county extension agent told me this and that pre/post emergents are not that effective in fall.

Proper lawn care can help greatly with weed control. Mowing high, fertilizing in May and September (for my PA area, zone 6b), and proper PH will help the lawn become stronger and outcompete weeds.

The natural environment of this Bittercress seems to be in flood plains and along rivers and streams indicating it may well need a fairly moist soil to grow in, although it has adapted to grow in dryer soils.
I see this growing in lawns that are watered every day but seldom find it in lawns that are watered once a week, which helps point to the flood plain, river, stream areas described by some. You find the same thing with Yellow Nutsedge, it grows in lawns watered daily but seldom in lawns watered once a week. The attached link may provide some help with control.

Here is a link that might be useful: About Bittercress

In your area, this weed probably grows mostly in fall and overwinters to then set seed in spring. If you want to use a broadleaf weed killer, the best time is in fall to kill the ones that have sprouted and other weeds too. My friend who is our county extension agent told me this and that pre/post emergents are not that effective in fall.

Proper lawn care can help greatly with weed control. Mowing high, fertilizing in May and September (for my PA area, zone 6b), and proper PH will help the lawn become stronger and outcompete weeds.

Oops, I posted a second time in error.

I never water my lawn and have lots of Bittercress. We advise homeowners not to water their lawns as rainfall is usually sufficient. I do see the Nutsedge in the low parts of the bed next to my property where my neighbor waters too much!