Sky kelp is one of the main forms of plant life on Thresher that does not live on the upper side of plates.  Despite its name, most varieties of sky kelp does not resemble what most seafaring folk would call "kelp".

All varieties of sky kelp are mostly hollow, their sealed insides carrying a substance called lifting gas, which provides buoyancy in the air and allows them to float.


Orange Kelp
Orange Kelp probably resembles ocean kelp the most of all sky kelp.  It is generally a large dull-orange sphere up to 20 feet in diameter.  The surface is sticky and rubbery, and long vines hang off the sphere.  The vines are used to entrap small animals and krill, providing much needed nutrients to the kelp.  Orange Kelp spheres are filled with a starchy center, and are often harvested for food, while the rubbery tendrils and outer covering can be cut up and dried into a fibrous matter useful in making ropes and clothing, though it loses some of its color in the process.  The rubbery covering also makes effective roofing when stretched into sheets and it is not uncommon to see homes with dull-orange roofs on plates.


Grey Kelp
Grey Kelp appears up close to be a membrane of small air-filled sacs.  Grey Kelp does not have tendrils like Orange Kelp, but instead continues growing indefinitely.  Grey Kelp grows by adding new layers of sacs onto itself.  The inner layers die due to lack of moisture and light, and become stiff and hardened, but still keep their lifting gas trapped inside.  At small sizes it tends toward irregular shapes, but as Grey Kelp prefers certain altitudes, its growth in the vertical dimensions is limited as the climate becomes unfavorable.  However horizontal growth is not so limited, and Grey Kelp patches can grow to truly enormous sizes, often calls "plates", of dozens or hundreds of miles in each direction, although it does not often get more than a mile thick.