I wouldn't even think about it (using the RoundUp on the suckers). If the plant shares common roots, it would likely kill the whole tree. Not worth the risk, IMHO.

I agree with J.D., even if it didn't kill it, it would probably damage it.
Jim

No No! I quite idiotically did that and it stunted my crepes! I am still fighting to get them to live. Their leaves are stunted and about a quarter inch wide and shriveled.
RoniJ

Ok, thanks. It's such a huge tree it's hard to imagine, but I am glad you have the experience to save me!

What would you suggest - this particular clump of suckers has all come up together and is about a foot wide. It's in the alleyway, so whatever I do to it doesn't need to be attractive, or even quick, really. Just don't want a tree back there!

Some crape myrtles sucker more than others. My Natches tends to send up some shoots up to a foot or 2 away from the trunk, but they are planted in grass so the babies just get mowed down every week. My pink one in a garden bed suckers badly, and I'm just resigned to got after them every 2-3 weeks and cut them down. I'm seriously considering replacing that tree and planting something that doesn't require so much maintenance! And is native.

Thanks, deb. The suckers I'm having trouble with are probably 6 feet from the trunk. I'm not sure everyone can picture what a huge crape it is. It's as tall as my 2 story house, and extremely full and wide. I don't mind pulling up the little ones around, but these came up in a big clump. They've been mowed several times but keep coming back stronger.

arrg.

Cut them flush with the ground, then cover with cardboard or carpet pieces. Cover that with mulch.