Within the Northern District of California there have been a number of designated Weed and Seed sites over the past 15 years. When To Put Seedlings Under MH or HPS. Post-emergent herbicides, however, must be applied while the weeds are actively growing because for the chemical to work, the herbicide must be absorbed into the plant.  Source: <a href=https://poematrix.com/autores/francis-hermiston/poemas/how-choose-best-seeds>https://poematrix.com/autores/francis-hermiston/poemas/how-choose-best-seeds</a>