Rose Seeds and Stratification
Stratification is a period of first warm storage, then a period
of cold storage which allows the seeds to mature and is intended to improve the
germination rate (click here for a summary of a scientific
paper on this subject). The cold period also improves the survivability of the
seedlings - scientists have been able to extract the embryos from the seed and study
how the embryos develop. They report that although some embryos will germinate without a
cold period, a higher percentage of these seedlings will be weak or deformed relative to
batches of the same seed that have received a cold treatment.
Stratification can be accomplished by placing the cleaned seeds on moistened (I recommend a solution made up of 5 ml of 3 % drugstore type hydrogen peroxide to every 95 ml of (preferably) distilled water) children's play sand in a zip lock type polyethylene plastic bag, and storing it at room temperature for about 2 months (if you did not clean the hips until late December or early January and if the hips had been kept in an unheated place like a garage, you can probably skip the 2 month warm stratification period), and then storing for 2 months in the refrigerator (not the freezer). If the refrigerator is being used only for rose seeds, set the thermostat at about 41 degrees Fahrenheit. (Note that the seeds should be sitting on the moist sand, not completely surrounded by the moist sand. This is necessary for the " exposure to light step" described below). Label the bag, indicating the parent variety and the date. If there is water separation (next to the sand) in the bag, the mixture is too wet. This condition can easily be rectified by adding small amounts of dry sand until no separate water is observed. Normally, if I see a bag that does not have some water condensation on the inside of the plastic above the sand, I assume that the bag is too dry and add a little additional water-H2O2 mix.
Check the seeds at least every week. When you notice a little white root poking out of a seed (or if two months have past), remove that packet from the refrigerator and place the packet with the seed side upward under "cool white" bulb fluorescent lights in a cooler area of the house such as the basement (rose seeds germinate best in temperatures between 45 and 60 degrees-F; 50 to 55 degrees-F is ideal. Germination slows significantly when the temperature rises much above 70 degrees-F. Some hybridizers believe that fluctuating temperatures are more helpful than a constant temperature.) The reason that the seeds are put under "cool white" bulb fluorescent lights is because the red part of light has been shown to enhance the germination of rose seeds. You can either use the "cool white" bulb florescent light "as is" or cover the seeds with transparent red paper to further enhance the red light effect. After about a month at the warmer temperature, if no germination has occurred or if germination has definitely stopped, place the packet back in the refrigerator for another month of cold exposure. The times given are approximate - a three week cycle after the initial 2 month warm and two month cold cycles probably will also be effective.
The original scientific paper concerning the red light effect was by Y. Yambe, K. Takeno, and T. Saito ( J. Amer. Soc. Hort Sci., volume 120, pages 953-955, (1995)). They tested white light provided by cool white fluorescent tubes, red light provided by a red cellophane sheet over the cool white fluorescent tubes, and blue light provided by a blue cellophane sheet. The red filtered light provided the highest percentage germination (around 38 to 58%), white light provided a germination percentage of around 15 to 28 %. It appears that blue light is absorbed by both forms of the phytochrome and is to be avoided. This is the reason for the red filter - to absorb the blue light. I assume that the authors selected cool white fluorescent lights instead of warm white fluorescent lights because the cool white bulbs have a lower emission in the far-red.
Comment:
The reasons for using moist sand instead of moist paper are: 1) that the sand changes color when drying out, 2) the root of the sprouted seed is not as easily damaged when removed from sand as when removed from paper, 3) the sand does not decompose and 4) the seeds are easily separated from the sand by using a kitchen wire strainer with a mesh large enough to pass the sand particles but not the seeds. The last is important as some seeds leach out inhibitor chemicals. A "freshen up" every once in a while gets rid of the leached out inhibitor. The leaching away of inhibitor is one of the possible reasons given for an explosion of seed germination outside after a heavy rain.