What are these weed seedlings with round cotyledons?

Question

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.

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Here are some photos with their first true leaves. As the plants mature the stems get woody and hard.

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Leave one as a specimen and it should be come clear. Not one I recognize at that stage. I have gotten good at ID-ing certain "weeds" that are really "self-seeding plants I want in some places, but not everywhere they sow themselves" in my own gardens, as well as some of my actual weeds. — Ecnerwal, May 15 '15 at 16:35
I'm going to try to remember to find a pot to grow it in and see what happens but this year I'm having some issues with weeds in the backyard do to issues with a couple of neighbors letting their weeds grow unchecked. — OrganicLawnDIY, May 15 '15 at 22:27
I really feel like I should recognize this plant. If you can get a picture of a fully opened true leaf I might recognize it. — GardenerJ, Jun 03 '15 at 02:52
@GardenerJ pinging you too now that I added pictures with the true leaves. — OrganicLawnDIY, Jun 03 '15 at 17:18
It's possibly something in the mint family. Creeping Charlie tends to have similarly round seed leaves. However, this isn't creeping Charlie, horehound, or catnip by the look of it. When you rub the leaves, do they smell at all? — Brōtsyorfuzthrāx, Jun 03 '15 at 19:09
@Shule they smell like generic green foliage. I removed one of the photos because I wasn't sure if it's the same plant. — OrganicLawnDIY, Jun 03 '15 at 21:04

OrganicLawnDIY · Accepted Answer · 2015-06-09T15:19:04.170

After feeling the stems get woody I started looking at possible trees it could be and I believe these seedlings are hibiscus syriacus L. aka rose of Sharon, althea, althea rose, shrub althea, shrubby Althea, Syrian rose. They're also in the area where I see the most of these seedlings. They are in the mallow family

They're a popular ornamental tree/shrub in my area but they are self seeders. They are fairly easy to control. The seedlings pull up easily and can be controlled with herbicides (distilled vinegar on a hot sunny day also does the trick) but boy do they love the Mel's Mix in my square foot garden bed. Especially when the mix was kept moist with plastic mulch over it.

Some pictures:

Early seedlings:

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