Any kind of cover crop needs to be added before plants. 1- putting cover crops under a canopy will hinder cover crop growth and development due to inadequate lighting, growing indoors. The soil hardly gets any light under a full canopy growing indoors. 2 will cover crop sprout, live, die and decompose within a soon enough time to be effective?. Adding a cover crop near flower that's highly unlikely. That's Imo if someone has had successful results planting cover crops near flower instead of before transplant that's awesome, I wouldn't have thought it.