Hydroseeding machines vary from large trucks to towed trailers, but they all have a water tank with an agitator and are capable of mixing grass seed, fertilizer, and paper mulch with water to be sprayed on a planting area. Getting the seed wet right from the start speeds seed germination since seed begins to absorb moisture in a process called 'imbibition.'
The mixture is typically sprayed onto a new lawn area or difficult to reach steep bank that's been finish graded. The paper fiber mulch usually has a green coloration to help the applicator be able to tell visually where he has already sprayed the mixture. Interstate highway banks in the northeastern United States are commonly hydroseeded with crown vetch due to its aggressiveness and low maintenance requirements.
Larger truck mounted hydroseeding units have a catwalk on top where an applicator can shoot the mix from a water cannon while an assistant drives and re-positions the truck as needed. Smaller towed trailer hydroseed machines have a long hose used by an applicator to 'spray paint' the mixture on new lawns and banks.