Simplified Seedling Production
Fred J. Nisbet, Ph.D., West Virginia University
There are many growers who would like to raise large numbers of rhododendron from seed. Plant breeders dealing with this genus have no choice; they must use this method. In either case, the problem has always been one of constant attention to details. Watering, shading and maintenance of humidity were necessary day by day, during the many weeks required for the slowly developing seedlings to gain stature - and a good root system.
Preliminary results of a new method promise freedom from much of the tedious care formerly necessary for the production of sturdy seedlings. Basically the method is this. Seed is sown in glass or plastic containers fitted with tight covers. The medium used varies, but Michigan Peat lightly covered with sand looks most promising. Because the plants are essentially sealed in the containers, watering is necessary only at rather long intervals. Well spaced seed produces seedlings which need not be shifted for some months after germination.
Under such a method the time needed for the production of flowering plants is not lessened, but the necessity of being present every day during the germination and young-seedling stage is removed. Also, the number of hours necessary for the production of seedlings is radically reduced. Furthermore, the necessary equipment is inexpensive, readily obtainable and portable. Greenhouse facilities are not required.
Fig. 30: Left to right, rectangular plastic dish, top
removed. Rhododendron Exbury hybrids on
Sphagnum Moss over soil; square glass dish, R.
Dexter hybrids, F2 on Michigan Peat and square
plastic dish suitable for very small lots only.
Fig. 31 The rectangular refrigerator dishes are
preferable for all but tiny lots of seed. Spray
bulb is used for all watering.
The containers used are "refrigerator dishes" of clear plastic or glass. In (Fig. 30) the rectangular plastic dish (49¢) has been quite satisfactory for small lots of seed. The larger, square, glass dish (59¢) works well and is less expensive for a given area, but has an ill-fitting cover. Their depth of three to four inches seems adequate. Larger containers may be available but generally their cost per square inch is much greater than the sizes shown. Many containers are of colored materials but they should not be used under any circumstances. Color filters and reduces the efficiency of the light which results in poor seedlings.
It is probable that a wide range of media might be used in these containers; final word as to the relative merits of these must wait for further work. To date, however, quite satisfactory results have been obtained from straight Michigan Peat covered lightly with a sharp, clean sand. The peat should be moistened before use, squeezed firmly to remove excess water and pressed into place gently. Hard packing can form a crust which tiny roots penetrate with difficulty.
Another medium is shredded sphagnum moss, either dried or live. This should be put through a 1/4-inch mesh screen, moistened, wrung out and then lightly packed into the container. A 1" layer of sterilized, acid (pH 5.2) potting soil, high in humus, seems to work well also.
Seed should be sown rather lightly if the seedlings are to stay in situ for extended periods. Those sown on the Michigan Peat may be covered lightly with sand. Those sown on sphagnum are generally covered with a very light sprinkling of the same material. Water lightly, in either case, to settle the seed firmly, then put the cover tightly in place.
The location of the seedling containers should pose no great problem. Adequate light, with no direct sunlight, is desired. In our laboratory, a table placed four feet from north-facing windows, holds the containers. A light wooden frame supports a single thickness of cheesecloth across the top and sides of the area.
In preliminary trials of this propagation method there were high losses of seedlings. The sole difficulty proved to be that we were watering and fussing with the seedlings too much. If the medium is properly moist at seeding time and the covers are sufficiently tight, no additional watering should be necessary before the seedlings appear. After germination, three weeks with many species and. varieties, water should be given only when the medium seems firm, not spongy. It should be borne in mind that since there is no bottom drainage and essentially no evaporation (this varies with the tightness of the covers used) water loss is slight. Too much moisture is fully as bad as, if not worse than, too little.
A .simple bulb type sprinkler has been found excellent for watering. The spray is fine and with only light hand pressure, there is little tendency to beat the seedlings.
Fig. 32: Part of the experimental set up with
cheesecloth shade laid back. Uncovered
seedlings in 1st row are Lilium amabile .
Fig. 33: Cheesecloth shade in usual position.
In the past, all directions for raising plants in sealed containers, such as wardian cases, have emphasized the need for removing the condensation from the sides and top of the container daily. Further checks will be made but to date fine results have been obtained under the given conditions, without this tedious operation.
Cheesecloth has worked well for shade. As many layers as necessary may be used to obtain the required cut in illumination. Under our conditioning of 6.5 to 25, on a Weston Photographic Light Meter, represents the usual light intensity. Very late in the afternoon during the summer, for brief periods, readings of 50 are encountered. The seedlings grow well and have good color under these conditions but no studies have been made to determine what optimum conditions would be. Tobacco cloth could well be substituted and would undoubtedly be cheaper.
When the seedlings have made sufficient growth to warrant transplanting, they must be hardened gradually. This operation requires considerable care as the closed atmosphere of the containers produces growth which is quite soft. The cover should be raised slightly for a few days, preferably during cloudy weather. As the plants harden somewhat, the opening can be gradually increased. Entire removal of the cover for several hours can then follow. When the young plants show no tendency to wilt, the cover may be removed entirely. During this period, of course, increased watering is necessary, but care should be taken not to make the medium soggy. As long as the seedlings can make good growth without excessive crowding, so long may they be allowed to stay in these containers. This is desirable as increased attention is necessary once they are removed from the closed atmosphere. Only when growth starts to suffer should they be transferred to larger quarters for more usual methods of growing.
As set up, this method of raising rhododendron seedlings is essentially a small-scale operation suitable for plant breeding, small experimental projects and amateur plantings. There seem to be no inherent obstacles, however, that would bar adaptation to larger programs. For any of these situations, the saving of time and attention should make this method appeal to those who wish to grow rhododendron from seed.