The only way you can do that is to use the scatter and replicate tools, which I've never used much anyway because it does it randomly and I like to place my vegetation in specific locations. Create the plant species you want, then (while it is selected) go to edit/scatter and replicate and that will take you to the dialog box you're looking for, just check the box for "with variations" and you should be all set. You'll have to play with the numbers a little bit in order to get the effect you're looking for though.