This idea is pretty...whacked.

Doesn't mean it wouldn't do anything. It might. But even if it did, I bet the effect would be near-negligible unless you had a whole lot of loads to dump on your plants. However, the image of a guy standing and whacking off onto some pot plants, if made into a video and set to some appropriate music (what's appropriate music for blowing your load on some plants?), would undoubtedly garner lots of views on YouTube. Try it, you'll be famous!