If you think in plane view, then any drawing package that has a library of landscape objects will work. If this is a tool to sell customers on what it will look like, consider Sketch-Up. It allows you to work in 3d, re-render with near realistic results, and produce fly-throughs for your customers. Warning: You need to play with it for a couple of months to get good at it, but with practice, and a good library of plants, you can modify the design while the customer is looking over your shoulder.
There is a free version and a pro version. The free version has all sorts of limitations. The pro version can generate materials lists, put multiple drawings on a single page...
Lots of 3rd party add-ons for it.
It's memory intensive. You will want the fastest single processor you can afford (It doesn't take advantage of multiple cores) and max out the memory. 16-32 GB of memory is NOT too much.