The natural growth habit of this plant (Polygonum orientale, also named Persicaria orientalis) is naturally fairly tall, growing to several feet (enough to dangle flowers over a typical garden gate, you could say) by the end of the summer. Therefore, pruning it before flowering late this summer may interfere with blooming (likely minimizing it) or stunt the plant overall. It prospers in full to partial sun, though we can't tell what light level the plants pictured are receiving. Too much shade, aside from also hampering flowering, can cause tall plants to flop and arch over.

This is an annual, and while it can self-seed (somewhat invasively in states to our south), the original plant(s) will die by winter. If you want to move the young plants now to a more suitable location for their eventual stature, that should be fine, as long as they can be monitored for watering needs. It typically does not need pruning as routine maintenance.

Miri