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

Boyce Thompson Institute for Plant Research

In speaking of the germination of hybrid seeds today, I want to emphasize mainly the germination of hybrid seeds of Rosaceae of the colder north temperate zone. This family of plants is of especial interest to the members of this Conference, for it includes so many of our fruit and ornamental plants. Of the many inquiries we receive concerning the germination of seeds, those regarding the rose family are most common.

The seeds of three sub-families (Pomeae, Roseae and Pruneae) have been studied in some detail. The seeds of every genus studied to date need a period in germinative conditions at a low temperature to prepare them for germination, or to after-ripen them. The old practice of stratifying certain seeds at a low temperature in moist condition preparatory to planting, furnished this condition in a general way. This practice often failed, in part, because it was not realized that the process of after-ripening in these seeds had a rather definite temperature optimum. Our work has established beyond a doubt the existence of such temperature optima and that the optima may vary somewhat with different species, although it is more generally in the region of 5° C. In some cases the optima are very distinct or involve a narrow range of temperature. In other cases the optima are not so distinct or represent a rather wide range of temperatures. Our work has also established that the various species have a distinct after-ripening time at their respective temperature optima, that a good oxygen and water supply is important in the stratification beds and that it is important to regulate the acidity of the stratification medium in some cases. In our experiments we mix the seeds up with the stratification medium instead of using successive layers of seeds and stratification medium.

The accompanying figure on seeds of Rosa rubiginosa shows the existence of the optimum. The after-ripening occurs at 0° C. and less readily at 10° C., but the process requires a much longer time at either temperature than at 5° C. At temperatures much above 10° C. the seeds tend to go back into the dormant condition so a period at higher temperatures prolongs the necessary time for after-ripening. Periods at temperatures below freezing have no effect on after-ripening provided they are not low enough to give freezing injuries. Contrary to the general view, freezing any of the rosaceous seeds of the three sub-families mentioned does not aid after-ripening in any way so far as our studies indicate. It does not even assist in cracking the stone coats (7). The stone coats of the plum, for instance, crack quickest at 10° C. or a little above, although the after-ripening occurs quickest at lower temperatures. There is no doubt that some seeds are much aided in germination by freezing or repeated freezing, but so far as now certainly known, these are not seeds that demand an after-ripening of the embryos.

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.

See Crocker, Seeds: Tricks and Traits (1925)