Excellent idea.

Hi! This is definitely a hard question to answer as 'Google-ing' it just doesn't seem to produce any results.
What I can say is that just like having client pictures on a website or brochure, you need to ask permission.
Thousands of companies seem to just post images of their clients without even a thought that their customers might object, so it is good that you are making the effort to actually do something about it!
For instance, 'see article attached' posting "before and after" photos of your work, I would object if my garden was a mess before until you fixed it! lol! and it was all over Facebook! Although, just posting a simple picture of a completed project isn't harmful, maybe you will get someone one day that objects without logical reason. We've all encountered someone like that before haven't we?!
So really to sum up I think you should always ask and show someone the photographs before you post them on Facebook to avoid any trouble. This will be to your benefit as well, because they will probably 'like' you on Facebook & tag themselves in your photo for all their friends to see. Hope all goes well.

Here is a link that might be useful: landscape gardener social media

Hello ginger_nh my name is Garrett and I was surfing the web looking on where to buy a Linnea Flower (Twin Flower) and I had seen a comment of yours directing people to check out this nursery: Van Berkum's Nursery in Deerfield, NH. I was having trouble finding the flower on there site. You had said to email you if someone needed some help getting a Linnea Flower but i was unable to find your email so i decided the best way to contact you was by commenting on one of your post hoping it will send you a notification. Please feel free to email me at hoffmanng25@yahoo.com any help would be greatly appreciated!

http://forums.gardenweb.com/discussions/2222912/linnea-borealis