You can cut grass with the knife to get Biomass!
On the greenhouse towers there are these fields of grass, of which each tuft can be cut for 1 Biomass. 5 tufts on the small fields and somewhere like 15 - 20 on the big ones (less if there is a tree on it)!

Between the grass and the infinite (or close enough to count) dirty water that you can purify from their central pools, this turns each and every greenhouse tower into a Biofuel depot!
I was so annoyed at myself when I cut grass for the first time on accident, I hope this helps someone not go through this (I visited about 5 G towers previous to learning this). This trick will save you so much synthetics which would otherwise go to synthetic fuel which you can now put into more worthy pursuits, like making polymer to make more lures! Which is another thing Greenhouse towers are good for since they have the raw materials for both the polymer and the best lures so far.

Happy flying, Pilots!