Hilchos Shemittah v’Yoveil 4:19

If wild onions of the sixth year, summer onions or rubia completed their growth before shemittah, they may be uprooted during shemittah with metal axes, which is not considered working the land.

Hilchos Shemittah v’Yoveil 4:20

If rain falls on onions during shemittah, causing them to sprout leaves, the leaves are permitted so long as they’re light green; once they darken, it’s as if the onions were planted in the ground and the leaves are prohibited as aftergrowth. In either case, however, the actual onions remain permitted.