The flower stem, which one usually cuts off to save the strength of the plant towards next year"™s blooming, is allowed to bloom and set seeds. The rhubarb is cross-fertilizing and the pollen is scattered by both insects and wind. It is necessary to have a security distance of several hundred metres between sorts to get absolutely pure seeds of each one.
Cut off the seed-stem when the seeds are ripe, dry and brown, just before they are about to let go and fall to the ground. Allow the seeds to continue drying indoors and then, carefully, rub the seeds away from the stems. The seeds are usually sufficiently clean but if there is some rubbish left, remove it or do it by sieving and winnowing.
The fertility of rhubarb seeds diminishes quickly after 2-3 years.