Is this a weed? Nearby I planted mirabilis, which has a similar looking leaf, so I've been reluctant to yank it..

It could be a species of Persicaria. Maybe P. odorata.

P. odorata is a weed. But it also can be used for cooking.

Lisa, post a photo that clearly shows a side view of leaf attachment to the stem. Check to see if an ocrea is present - a membranous sheath that surrounds the stem at the base of the leaf - See https://www.google.com/search?...
Not present in Mirabilis. If present, plant is probably a species of Persicaria, and likely a weed.

When we click the second photo we will see the deail. There show ocrea on the right stem. The V shape mark on the leaf also usually refer to Persicaria

Yep, ocrea all over the place. It's a weed. Thanks, gentlemen.