I had that problem when I used to run a lot of .esp's (200+). The moment I got rid of some of them and ended up under 140 everything was back to normal. Try to merge your esp's together (weapon mods for example, or armors, etc). It will help.
If you mean the whole models of plants, trees, etc there's not much you can do about it except having view distance on max (and some other ini tweaks). If you mean the plant's actual textures popping in it's probably a LOD issue. I think there are some custom LOD files on the mod's page under Optional. Use the one that fits your other mods (NMC, ojo, etc),
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