My perennial garden with several colors of tall phlox seems over run with weeds that actually look identical to the phlox. I have been pulling for years now. It does not bloom. Stem and leaves look like the phlox. I'm now wondering if it is the phlox in a non blooming state. Is this possible? Every year more of the weed, less Phlox.
If you haven't been clipping the flower stems after the flowers fade, they could be dropping seeds. The seedlings generally won't look like the parent plant if the parent plant is a hybrid (which most garden varieties are). Here's an article that may help you: https://www.gardeningknowhow.com/ornamental/flowers/phlox/growing-garden-phlox.htm