If my sim has other sims do work on her garden (like weeding) can she then not submit plants to the museum?
While I have no particular authority, my personal feeling would be that it depends on the answer to"Did she do most of the work herself?"
For most of the other items, it is rather clear cut in that adding X number of items takes X number of actions - want 8 paintings, then paint 8 things. For gardening on the other hand, getting to the point where you have them takes rather more actions and time - Plant, repeated waterings/weedings, possible fertilizing, then harvesting. Though you could make the counter argument that you can just plant a single Lily and then get all 8-10 components in one harvest a few days later.