I got one seed pretty easily, then a long dry spell. I used some of the advice here and finished the rest of them for both my legacy and city-living games.
To get the rest of them, I went to the parks and waited around until plantsims came by. Then, I manipulated moods as necessary before asking for the seed. Uncomfortable was definitely the hardest on my second game (it spawned first in my legacy game). I managed it by having a pile of trash (as suggested in an earlier post). As soon as the plantsim showed up, I built a wall around them to keep them near the trash and keep others from cleaning it up. When my sim needed a bathroom or food, I walled her off alone and knocked out a wall to deal with her needs.
One caution: Don't do this with a hot-headed sim. She only got angry, very angry. It took all day and a fight before she finally became uncomfortable. And I think that was mostly because she had to use the toilet.