It really sounds like you are the project manager here. I think you should get 10% at the level of participation that you described. You should also get 100% of your design fee on top of that.

But, if you need this guy in order to make design sales and are dead in the water without him, I think you just have to do what you can to make it work.

It would be better if you could team up with a more polished landscaper because unless you have a lot of experience in the field on site in landscape construction (rather than home gardening) it will be the blind leading the blind when it comes to issues beyond pruning and planting. Believe me, these come up in almost any job even if it is small. It would be better if you had someone who has some experience in areas that you do not in order that you can learn more and experience more as you move forward.

However, it is hard to hook up until you have something the other partner wants (I sound like Dr. Ruth), so getting some projects done with this guy might be the launching pad to get hooked up with a higher level contractor.

The key to your success as a designer is to have clients (not contractors) coming to you when they want something done.If you become a source for work you are valued by contractors. If you need the contractor as a source for work, you are not so valued. It is not an easy road.