Check out Sketch3D from DynaSCAPE - https://sketch3d.dynascape.com/ great resource for SketchUp plants as well as other garden goodies.
Take care with 3d high poly trees, even at start look fancy, sometimes aint better option cause make your scene really heavy.
Try Dynascape Sketch 3D. They have a good collection of a variety of plants and furniture etc..
Land F/X also has great plants, trees, etc. for your scene.