I know it's hard to identify plants just from the seedlings but I get these weeds every year throughout my garden in Northern New Jersey and I pull them before they grow.
They start to germinate around early May and continue to popup at other times of the year but I think it's mainly in early spring and late summer/fall.
My raised bed was covered with black plastic. When I removed it to prepare my raised bed a few days ago there were hundreds of them underneath the plastic. Most were starting to die off from lack of light.Never really had that bad of a problem with them in the raised bed but I guess the plastic made conditions ideal for germination.
The seed leaves are round and smooth but the true leaves appear to be serrated.
Here are some photos with their first true leaves. As the plants mature the stems get woody and hard.