I would say all are giant ragweed.

https://kansasnativeplants.com/guide/plant_detail.php?plnt_id=149

Thank you. I was going to guess ragweed, based on the large lobed leaves, but many leaves are entirely unlobed. Is that normal? Interestingly, your link does show both lobed and unlobed leaves pictured. How weird.

I was going to suggest the same, although giant ragweed is not common to my area. And both lobed and unlobed leaves are common, unlobed especially with newer growth or on flowering stems.

Ragweed it is. I've got loads down by the creek, but I've only noticed lobed leaves on those. I'm just not familiar with plants that have such dramatically different leaf shapes. All of these plants in this mini-forest are the same age, and none are close to flowering. They just pop out different leaf shapes randomly.

Ragweed's actually a general name for any plant in the genus Ambrosia, so there's tons of species with loads of leaf shapes. It's "fun" to note that I'm allergic to almost all of them...

This one was identified specifically as Giant Ragweed, Ambrosia trifida.

I can understand that there are Ragweeds with many leaf shapes. I just don't expect to see those many leaf shapes on one plant!

Wild, is it not, how genes express themselves even within a single plant?

Well, that is true, but one only expects to see those differences expressed only occasionally. Fully half of the leaves on these plants are lobed, and half are unlobed. These plants can't seem to quite decide what they want to be.

Don't harsh the poor thing's ontogeny, man. Just don't. A real quick search showed a philodendron doing equally unusual things. I'm fairly certain that if we spent a few more minutes, it's probably not all that unusual.

Interesting to know. I have a lot of phils, and I've never seen that, though.

Look at Mulberry saplings.

Hmmm. I've got several small (6-8 foot) mulberries, and all the leaves look exactly the same.

Mulberries are well known for having extremely variable leaf shapes, often all on the same plant. Morus alba is perhaps the most variable; Morus rubra less so.

Mine all look like the one at lower left. Every single leaf. Well, they just told the nearby Ragweeds to do all the variability for them! I do vaguely remember coming upon one with a range of leaf shapes, though.