Collecting and Germinating Seed
Rhododendron ABC's #12
For more than 30 years, I have been hybridizing as described in the previous ABC's discussion. Hybridizing involves placing the pollen from the male anthers of one flower on the receptive stigma of another flower. Following the successful fusion of the male gametes (sperm) with the female gametes (ova), new embryonic individuals are produced and stored in seed capsules. What then? Gradually, I have improved my ways of collecting and germinating the seeds in order to grow new plants and try to find the elusive winners of the genetic lotteries.
The first step is to harvest the seed capsules after sufficient time has elapsed after fertilization for the seeds to contain complete embryos that can develop into new plants, given the right cultural conditions. Generally, I have found that at least four months are required and I prefer to allow about six months. As a target date. I aim for a October 15 harvest in my Northwest Garden. It may go without saying, but it is very important to record and tag each capsule with the names of the female and the male parents. By custom, the female (or seed parent) is always listed first.
The capsules are stored at room temperature for from one to two weeks to allow them to dry. During drying, the capsules win generally split open and release the seeds. Some capsules may have to be mechanically split. Plant your rhododendron seeds soon after harvesting as these seeds are small and tend to lose their ability to sprout fairly rapidly, especially if they are kept in very dry and warm conditions. Vireya (tropical) rhododendrons are especially liable to lose their ability to sprout and should be planted as soon after harvest as possible. Seeds should be separated from bits of chaff and inert material, as these latter items tend to carry harmful spores of molds and fungi.
I have found that, in preparing a seed bed for rhododendrons, one seems to get the best results by using two different layers of seed growing materials. I use two plastic bands (4-square inches of growing surface) and loosely pack the bottom half of the bands with a clamp mixture of the following (percentages are by volume): 40% Canadian peat, 40% Agricultural perlite, and 20% sharp sand (mason's sand).
On top of this sub-layer, I add a moist layer of about 1/4-inch thickness of milled sphagnum, Nodamp-Off seed starter. This latter material can be purchased at most garden shops and contains its own antibiotic properties. By its use, I rarely, if ever, have any trouble with fungal diseases and the antibiotic properties obviate the necessity for sterilization of the seed growing mixtures. It will be noted that the bottom layers contain sand. Some growers don't like sand but it is my belief that it gives some weight and solidity that the seedlings like. Perhaps it helps to more nearly simulate natural growing conditions in the wild.
Following the seed bed preparation, the seeds are sprinkled on the moist sphagnum surface. I find that a convenient way to obtain a proper spacing of the seedlings is to add the seed clumps to a sheet of white paper and gently tap the paper to separatethe individual seeds. Then it is possible to estimate their number. After considerable trial and error, I have come to believe that the best spacing is obtained by using about 100 seeds per band (4-square inches). The object is to scatter the seeds as uniformly as possible over the sphagnum surface. After this step, place the bands in a larger plastic box covered with a transparent glass or plastic. The covered bands are held at about 70° F. Although I do not know why, germination of the seeds appears to take place better in the dark. I use a paper or wood barrier to exclude light during the germination period which takes about 12-14 days. By keeping the container covered with glass or plastic, no additional moisture needs to be added. The light barrier is removed after germination. Keep the container covered with transparent material to conserve moisture. In the next ABCs issue, I will discuss the steps of growing germinated seeds to produce new plants.
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