I just recently had to get 25 farming as a requirement for Turm, and I had never done it before in my life. I bought a bunch of trees (oaks and willows) and bagged plants from that lady in falador who sells garden supplies, then I went to my POH and created a 3x3 garden space and just started planting trees and plants. After it was full, I removed them and planted more. The farming exp was SUPER fast, and the great part is that the trees you can plant all depends on your construction level, not your farming level

So if you are 50+ con, you can expect some pretty fast exp.
It was a little expensive (the trees cost alot) so it might not be the most cost effective way. However, I wouldn't know that as it was my first time farming EVER. If you want a fast way to get it up though with little hassle, thats a great method.