RHA Newsletter 3(4): 8-9 (1972)
GERMINATION OF ROSE SEEDS
The living seed is a very complex form of life. It is able to endure a very great amount of drought, heat and cold to which the growing plant would succumb. A protective mechanism, which is not understood, prevents the seed from germinating until certain conditions of its environment have been met. The length of the dormant period varies with seeds from different rose cultivars and also with seeds from a single cultivar.
Even if conditions of the environment are favourable, certain chemical and physical changes have to take place before a dormant seed is capable of germinating. The time during which these changes take place is called the after-ripening period. Some seeds will germinate only when after-ripened at low temperature, i.e., when stratified. Stratification is a method for after-ripening seeds in a moist medium for certain lengths of time at temperatures between 37 and 41°F. Some seeds germinate more readily if after-ripened at 68°F. and then stratified.
Not all roses have seeds that are difficult to germinate. Depending on the type of rose and on the climatic conditions to which the plant was exposed during seed development, certain seeds will germinate readily after 2 or 3 months' stratification. Other roses have seeds that germinate sporadically even after 5 or 6 months' stratification. It is easy to realize, then, why an amateur gardener might not understand why his seeds did not germinate.
Like the seed of the peach, cherry and plum, the rose seed is enclosed in a hard shell that prevents water absorption and gaseous exchange of the embryo. Germination can be accelerated by removal of the shell. Different means may be used to remove the hard shell, but no matter what the method, the seed must be carefully handled so that the embryo is not injured. The shell can be abraded with a file, or sandpaper, or it can be partially removed by soaking in concentrated sulfuric acid.
I have used a small rotary file to make the abrasion. A single seed, held in a pair of short, needle-nosed pliers, was pressed lightly against the file which rotated at 5000 r.p.m. In spite of careful manipulation, removal of the shell by this method did not promote germination.
Seeds from Ekta, a hardy shrub rose of the R. gallica group, were used to study the effectiveness of different seed treatments. Three treatments, filing with a rotary file, soaking one hour in concentrated sulfuric acid, and immersion for one minute in water of 194°F. were studied alone and in combination with two ways of after-ripening. The two after-ripening treatments were; stratification for 5 months at 40°F.; and storage in moist sand at 68°F. for 5 months and then 5 months' stratification at 40°F.
The most effective treatment combinations were soaking for one hour in concentrated sulfuric acid and stratification for 5 months at 40°F, or storage in moist sand at 68°F for 5 months and then 5 months' stratification at 40°F. Filing and stratification were less effective than stratification alone. Germination of hot-water-treated seeds was promoted only if the seeds were kept for 5 months at 68°F. before stratification. Since water temperature and time of immersion are critical and the reaction of different seeds is not predictable, immersion in hot water is not recommended.
Suitable treatments for rose seeds that are difficult to germinate are either soaking in sulfuric acid and stratification, or a combination of warm and cold treatment. Each treatment has disadvantages. The acid treatment is laborious because the seeds have to be washed several times afterwards to remove the burnt tissues and the free acid. It will produce satisfactory results with most seeds, but it is not suitable for seeds that float in water. The warm-cold treatment has the decisive disadvantage 0that it is difficult to balance the time period at 68°F. with the time period at 40°F. because the temperature requirements for after-ripening varies for different seeds. A prolonged warm period can induce secondary dormancy, which has to be broken like primary dormancy, by stratification. Secondary dormancy refers merely to a dormant condition that has been inflicted in mature seeds. The treatment of secondary dormancy does not differ from that of primary dormancy.
The hardy rugosa hybrids used in our breeding program produce only floating seeds. Satisfactory results were achieved with these seeds by either: stratification for 5 months at 40°F., one month at 68°F. and 5 months at 40°F., or 2 months at 68°F. and 4 months at 40°F. We are still experimenting to find a combination that produces best results with most seeds.