Freeze R. yakushimanum Seed?
John J. McGuire
University of Rhode Island
Kingston, Rhode Island
Exposure to freezing temperatures appears to be unwarranted as a stimulant for R. yakushimanum seed. When I received my selections of seed packets from the ARS seed exchange program the seeds were accompanied with a note suggesting that seeds for species R. yakushimanum should be subjected to freezing prior to germination. Those seeds were divided into two equal lots. Half were exposed to freezing while the other half were not (this involved several packets of seed). In all cases the seeds which were sown directly germinated very well while those which were frozen either failed to germinate or germinated poorly.
It is interesting that in the same year I acquired Delphinium seed from two commercial sources and in each case they were accompanied with a similar recommendation to subject the seed to freezing prior to sowing. In each case unfrozen seed germinated faster and with more vigor than frozen seed.
Perhaps our method of germinating Rhododendron seed and Delphinium seed under intermittent mist is sufficient to remove any inhibitors that may be present. All Rhododendron seed we have tested germinate very well by this method. They are sown on screened sphagnum peat moss in plastic trays that are 3-5 inches deep. Seedlings remain under mist until they are large enough to prick off into flats. All other conditions are similar to those used by most horticulturists. Perhaps other members of the society have different experiences with R. yakushimanum seed but I would not recommend freezing them nor would I freeze Delphinium seed again.
John McGuire is associated with the Rhode Island Agricultural Experiment Station.