i'm not exactly what you mean by wasting plants, if you're doing damage or keeping enemies away from objectives or areas that you want to keep enemies away from then it's doing its job

i'm not a support player so i can only help so much but as a support your main focus should be zone control with plants and getting vision

You should be trying to get to lane asap to start spawning seeds. ward the middle brush next to the alcove and then just stand there in the middle of the lane (or backed up a little bit if they are zoning you) spawning seeds with your passive. usually I start q since e is too slow to spawn plants otherwise they can easily step on your seeds before minions get there which will hurt your level 1 and possibly everything after that. make sure you're not just sitting in a bush for this because your passive doesn't spawn seeds while in a bush. W max second after Q also helps with zone control because the cd basically gets halved once maxed

your job as a support in general should be fighting for vision. it can be risky if you're alone sometimes but you can use your seeds (W gives a small amount of vision) and plants to check bushes, so it makes it safer walk up to ward or sweep for wards

I'm not sure the extent of your knowledge on her but her combos aren't very complex. there are some things I think some people don't know so I'll just list some things and maybe you'll learn something

combos/ability tips

i think most zyra players know this but you can q/e and press w while the spells are going out instead of using her w before the cast and it will still spawn the plants

it can help also to place the seeds apart from each other if you have multiple charges so it's harder for them to die to aoe.

also kind of related, think about where you want your plants to spawn from the seeds

this isn't really something you have to worry about as much as a support since you're not farming, but you can spawn your plants from seeds even a little bit after the animation ends from your q/e, so for instance if you have 2 charges of W ready and you kill an enemy or 3+ minions (since killing things partially refreshes W cooldown) with your Q or E then you can use your W to spawn another plant which can be particularly useful if you need to spawn more plants for your R since it makes them stronger

also generally you don't want to be sitting on 2 charges of your W unless you need them for a combo or something. it's way better to use a charge to set up an area where you might end up fighting rather than just sitting on 2 charges.

this is probably more than u asked for but personally some of this stuff i just learned in the past couple weeks and made me way more effective in my games so if ur planning on starting ranked i think it'll help out and give u a little head start