Nothing's as good as having a good predator. But if your predator is poor and veggies outgrow, then you need to limit amount of energy that veggies are getting.
So, solutions are:
1. Give them less energy per cycle.
2. Add day/night cycles to cut their energy supply in half.
3. Make them do something to waste energy - run, spin, shoot, whatever.
Remember that you can always make veggies divide less often, that would cut down on their numbers as well.
Hope that helps.