I harvested kelp from the ocean (which is also bugged but covered elsewhere; namely, the plant doesn't break all the way to the top, leaving floating kelp blocks) and tried planting it for my kelp farm. However, planting a single kelp produces a full plant, with random height. Sometimes all the way to the top of my water (about 20 blocks), sometimes just 3 or 4. With the higher ones, my observer of course immediately triggers the piston, breaking the kelp plant above the bottom one, but this immediately regrows the kelp again.

Needless to say, this breaks the kelp farm, but also makes it obsolete. I can just create infinite kelp by planting and breaking. I can fill a chest like this in less than a minute...