@DeadTomorrow - Are you rendering or just keeping plants in your model? The challenge is that with SketchUp, you have to balance high-quality (ie realistic) plants, textures, and assets with performance. For example, one tree from VizPark will come in at over a million polygons. So 2, 3, 20 trees in your model and it’s going to be a problem.
With rendering extensions such as V-Ray, Thea, Enscape (PC only), etc., the high quality plant geometry and textures are stored outside of SketchUp and call upon only at render time…thus reducing the geometry load on the model and keeping things running fast and of course, with great looking results.
Also, you didn’t mention budget. Do you need free textures or are you willing to invest some money to grow your library? Keep in mind with 3D Warehouse, you get what you pay for most of the time. For me personally, I’ve enjoyed the quality and ease of use of Laubwerk plant kits. They obviously come at a cost but keep in mind that each plant model is actually 36 models since they come with variants for age (x3), season (x4), and form (x3).
Also check out something like Megascans as well. They have photo-scanned PBR textures, atlases, full 3D plants and more. Keep in mind that most high-quality assets you buy or find online are likely to be in a non-SketchUp format…meaning that you’ll need an additional extension to import
See quick rendering made by combining Lauberk trees with Megascans surface and entourage assets:
Let me know if you have any specific questions about either of these options.