A kelp frond is a kind of comestible. It is a vegetable, preserving both vegan and vegetarian conduct, and can be fed to herbivorous pets, though it counts only as food and not as a treat like apples. It is only generated under water: 1/10 chance of being generated in each pool square and 1/30 chance of being generated in each moat square.
Perhaps the easiest way to harvest kelp frond is to cast detect treasure or detect food, levitate over the water and apply a bullwhip down. Juiblex's and Medusa's lairs are good places to collect kelp, but watch out for the eels. By the time a player has access to lots of kelp, food is usually not an issue anymore.
In Slash'EM Extended kelp fronds have another use: throwing one to a monster flagged as "petty" will tame it. Keeping a petty monster tame can be done by using any vegetarian food, but another kelp frond is required to re-tame a petty monster gone feral.
I noticed that all the plants were attached to the soil by
an almost imperceptible bond. Devoid of roots, they seemed
not to require any nourishment from sand, soil, or pebble.
All they required was a point of support -- nothing else.
These plants are self-propagated, and their existence depends
entirely on the water that supports and nourishes them.
Most of them do not sprout leaves, but sprout blades of
various whimsical shapes, and their colors are limited to
pink, carmine, green, olive, fawn, and brown. I had the
opportunity to observe once more -- not the dried specimens
I had studied on the _Nautilus_ -- but the fresh, living
specimens in their native setting.
[ 20,000 Leagues Under the Sea, by Jules Verne ]