Memoirs of the Horticultural Society of New York vol. 3 (1927)
Dormancy in Hybrid seeds
Crocker

Rosa multiflora requires only a little over two months at 5° C. for after-ripening while Rosa canina shows considerable germination under this condition only after nearly a year of favorable stratification at this temperature. The study of these and many other species of roses of the cold temperate zone indicate that the various species demand various periods of cold stratification for after-ripening but in general the optimum is near 5° C.

FIG. 1. Rosa rubiginosa seeds: Check stored dry; the others in moist sand for six months at the temperatures designated and then planted in a flat in the greenhouse. The picture shows the effective temperature for stratification, that is 5° C.

We have also studied the after-ripening of about two hundred and fifty different crosses of roses. The indications from these studies are that hybrids containing largely cold temperate blood respond to low temperature stratification as do the seeds of the species with about 5° C. the optimum, and with a great variation in the time required. Apparently with hybrids of warm climate forms, the low temperature stratification is not so important, if at all necessary. It is probable that the low temperature stratification is merely an imitation of Nature's method for temperate zone forms. One can improve on Nature, however, for his purpose by giving the most favorable conditions as to temperature, time and stratification medium. Nature's purpose is achieved if a few of each year's crop of seeds germinate at a time over a period of years, even with a low total yield. The hybridizer wants a big yield as promptly as possible, hence he must improve on Nature's method.