I was close to winning level 3, but then all my plants died out. For some reason I still managed to win that level. I had no issue with waiting there as I could speed up the time. (Which didn't work as well when there were still plants, as my PC couldn't handle the computational load).
Level 4 and 5 where more challenging, as indeed evolution is out of control. I like the idea. Though might prefer a little more feedback on my actions. When the target crop is the most common, you have no information on what the rest is doing so making corrections becomes impossible. One way to provide more feedback is to have histograms about the length of the plants and the roots.
My strategy was to kill of most plants, then set the parameters to what I believe is best for the target crop and hope for the best.