Unless you model your plants, this probably won't help. When I model a plant in Lightwave, I usually make several versions -- different sizes, rotations, flowers or not, etc. Then I can "clump" them together as a separate object as well as keeping the separate plants. As mentioned, unless you plan on a large area of flat terrain, spreading them too wide may cause "floating" or "buried" plants (building the model with a stem slightly below a Y elevation of zero can sometimes help.)
In the southwest US, there are species of yucca's that tend to clump in a small circle ( the younger plants radiate out and the older ones die off leaving a circle of living plants), so this technique works particularly well in that case. Palm trees with the classic "V" can be done that way too.
[edit] should have written "Palm trees with the classic "X".