Biomass is a sturdy, grotesque organic substance added with the Harken Scythe mod.
You must first acquire a Biomass Seed, made by combining fermented spider eye with nether wart. After that, pay some blood in Altar of Blood to "germinate" the "seed". After that's done, use a Creep Ball on some soul sand to create a Creep Block. Hollow it out by right-clicking with a Glaive and plant the germinated seed in the creep block. You may then pour blood on it, (Right click the hollowed Creep Block), from an Essence Keeper or Essence Vessel to speed up the growth process for one stage, after which you will need to renew your tithe.
Growth happens in multiple stages. The biomass will grow no larger than a block in height. First a small stem will grow from the Creep Block, then the stem will begin to grow and branch out smaller stems with pale pink buds. The biomass can then be harvested when it "flowers".
Biomass may be harvested by hand but will only grant one biomass. If it is harvested with a scythe it will grant 2 biomass and a Biomass Seed which can then be regerminated and planted again.