Hi Richard,
As a fellow Landscape/Hardscape designer who uses sketchup to present my landscape designs, I found the best way to do it is by using faceme components and clipmaps instead of true 3D models. You lose a little in overall shadow and lighting quality, but you make up for it in variety and quicker turnaround times. There are several nice collections of landscape plants for sale online. I use http://www.imagecels.com quite a bit, but the best plants I have are ones I made myself.
I have running relationships with my local nurseries and growers, and they allow me to come in and photograph plants at my leisure. I'll usually take someone with me who can hold up the white background that I rigged up out of some 2x2's and a sheet. They hold this behind the plant or tree and I photograph it. Then I take them into photoshop and use Vertus Fluid Mask to remove the white background. Then I take them into sketchup and save them as faceme components. Come spring and summer, I have plans with a local grower to photograph and catalog over 300 perennials to add to my collection, or possibly turn around for commercial sale.
I think this is the only way that makes sense in our business, as one would need a vast pool of plants in order to properly design, and this would take years and years to try and model. Not to mention trying to put a few hundred plant models into a design, even at low poly, would grind sketchup to a halt. Programs like Vue and Podium will recognize the transparency of your plant components and render them transparent. This makes it fast and easy to render a high quality design and still maintain quick turnaround.
Another option, which I use quite a bit for quick designs, is to import my Sketchup model/hardscape layout into Realtime Landscape Architect (http://www.ideaspectrum.com/) and quick drag and drop from their catalog of 5000 landscape plants. The program will also put out a decent render and animation walk through, with lights, waterfalls and even birds and butterflies. It's a neat program for less demanding customers.
Anyway, below is a quick render I just whipped up in Vue, showing a few of the several hundred plants I've made, to give you an idea.
