BJR,
I, like you, went to school for horticulture. I only took one design course while in school and my presentation skills have evolved over time, but are nowhere near where they could be. I haven't hit a wall with my drawings not selling the job, but would like to be able to draw a decent plan as I search for that elusive upscale, or almost upscale market.
With some help from agla I have been working on my plan graphics and found a great book about drawing. It should be called plan graphics for idiots but it does a great job of explaining all the stuff that most designers take for granted. I bought it at amazon.com but I found it here. I highly recommend you check this book out:
Plan graphics