How do you get 400% overgrow? Mine is only 20%. I'll try the overgrow gene though. Also I haven't seen the plantation upgrade yet. I waited 2 days on carrots and got to 348% and gave up.
The upgradable part of overgrowth gene gives you a flat 15 % bonus per level - which is added to your base ovegrowth and then multiplied by 2.5 modifier of the gene itself. So it doesn't really matter if your base overgrowth is small, the gene is gonna do most of the heavy lifting (provided you upgrade it)