Due to how slowly the plants attack it's really hard to make use of them. Late game they can do around 200+ damage but because of how slow they are it's difficult to actually make them useful past the laning phase as people can simply just walk out of it's range and it could focus onto a minion or something.
Her plants are pretty good poke and yes I've obviously seen a number of people die to them (I feel like when you play Zyra, half of your kills you don't even know how you got until you realize your plant over their dead bodies.) but it's mainly because Zyra's Q-E combo is pretty bursty and your auto attacks/plants are great for finishing them off when they're low. It's not solely because of the plants they died.
I think the plants attack speed should scale to 1.5 late game with 2.0 in her ultimate, or something similar because right now they're just nothing too noticable late game which sucks because Zyra is an awesome minion mancer.
If not I think they should totally change her useless passive and make it incorporate something to do with her plants and the spells she uses. Like depending on how many seeds are hit by one spell it increases the plants range, damage, attack speed, and the slow on the melee plants. Her passive is good maybe once in a game or two to kill a really low enemy who forgets you died, but it's just so bad compared to people like Yasuo, Kalista, Ahri. They all get either mobility, sustain, or damage that helps them a lot. She just gets 440 true damage (which isn't much at level 18 and there's no reason to not dodge it.) whenever she dies after 2 seconds.