How to identify a plant to eradicate.
I suspect this is a type of Persicaria. There are many species and cultivars, and this could be Persicaria polymorpha.
A non-selective herbicide or brush killer should kill it. Always follow all precautions when using chemicals. Shield your desirable plants with cardboard, or paint the chemical on with a disposable paint brush.
If you would allow one section to develop a few weeks more , perhaps showing flower buds, I may be able to confirm the identification.