I have a welp summoned above a summoning crystal, which sits on top of a single block. This is located adjacent to my garden. The welp has been given the harvest spell. There is a mana jar deployed next to him, which has mana. The welp is not casting the harvest spell on any fully grown plant.

Growth command not working on tilled soil, while simple-farming mod is installed. Harvest spell works when cast by player, may not work when being cast by whelp.

Try the latest version, some changes were made to whelps and grow as well.