Pollinating and Collecting Pollen
H. Roland Schroeder and Forrest E. Bump
As the spring comes to a close and the last Rhododendron auriculatum, discolor, maximum and their hybrids have bloomed, hybridizing for the year has come to an end. Only the very late R. prunifolium, serrulatum and their hybrids are yet to bloom.
During the past six months of research and planning for the hybridizing program for the next spring, it seemed pertinent to write about some of the things necessary and convenient to have ready when "pollen dabbing time" is here again.
The first essential is a "pollen pool" of all the different rhododendrons that you have decided on using a year or so in advance. The pollen may be obtained from friends in different parts of the country or world, from the ARS pollen bank, (we should all contribute more to the bank), or collecting what you can personally.
The technique of collecting and storing pollen needs careful attention for fear of contamination. Pollen, if at all possible, should be obtained from flower buds that are about to open. If pollen isn't collected in this manner, bees and other insects crawling over the anthers are most likely to cause contamination of the pollen.
With pointed tweezers, prick the anthers off the filaments and place them in gelatin capsules or tape filaments on cardboard and place in small manila envelope as discussed later in this paper. Then cleanse tweezers with alcohol. Fill small bottle one third full with calcium chloride, silicate crystals, or gel and pack cotton over the desiccate, then lay pollen containers on the cotton and close bottle tightly. Place the bottle in the refrigerator above freezing for three to seven days. Then put the prepared pollen in the freezing compartment permanently. If the pollen is prepared in this manner it will stay viable for two years and possibly longer. The reason to refrigerate the pollen first above freezing is to keep the pollen from deteriorating until it is desiccated. After desiccation and freezing, the pollen can be taken out and used as needed and then replaced in freezer as many times as necessary.
By using small strips of cellophane tape you can attach a label to the gelatin capsules or write on the cardboard holders. Always use #1 lead pencil for writing names, dates and other information since the very soft lead gives a deeper impression and is not as likely to be rubbed off.
It is very expeditious to have all your equipment in a box when you start gathering pollen or hybridizing. It seems there is always something else needed to keep a body running back and forth to the house. A wooden or plastic box 16" x 8" is about the right size, such as a fishing tackle box. On the outside of the box lid attach a leather strip about 2" x 4" x ¼" so you can print your tags on it. The leather has enough resiliency so you can imprint aluminum or other soft metals.
The box should contain the following items:
1. Small pointed scissors
2. Waxed string
3. Jar of gelatin capsules — size 0 to 000
4. #1 lead pencil — two with erasers
5. Laundry tags with metal clips — colored
6. "Colored price tags" with strings attached
7. Pointed tweezers — 4" to 6" long
8. Cotton (you can get individually sealed alcohol-cotton squares — 2" x 2")
10. Cellophane tape
11. Small tablet of writing paper
12. Small pencil sharpener
13. Rolled sheet aluminum tubes — ½" to 1½" long contained in small flat jar
15. Inch wide thin cardboard strips (heavy card file paper or book match folders)
16. Small magnifying glass — 10 to 30 x
17. Small manila envelopes
18. Round toothpicks
The sharp pointed scissors serve a dual purpose to cut around the base of the flower petals and also to cut string, cellophane tape, etc. Waxed string will not rot during a season so you will not lose the identifying tag. If waxed string is not available you can melt paraffin and soak the string in the paraffin.
The laundry tags are preferred if the branch will accommodate the metal clip. Always use colored tags because white tags tend to draw fungus and can cause trouble in your seed capsules and also affect germination of seeds. Never tag a leaf or flower pedicel because they can be eaten off or blown away or the leaf might drop just due to abscission.
The number 000 gelatin capsules are preferred when collecting pollen because they are not only larger bore but also thicker, and at this time of year, the heat and humidity cause melting of the gelatin when held in your fingers. The gelatin capsules are also nice because when you want to pollinate a pistil you can shake or flip the capsule before opening it. The pollen will collect on the inside surface and, generally, you can insert the capsule over the pistil to pollinate it.
An alternate method to collect pollen is to use strips of thin cardboard about 1" wide and 3" to 4" long. Lay the cardboard down flat and take off a flower from shrub. Remove petals and lay the stamens and pistil on the cardboard close to one end so the anthers protrude just above the cardboard. Place your thumb on the stamens to secure and then bend the pistil and ovary back and snap off. Anchor the stamens to the cardboard with cellophane tape. Bend the other end of cardboard over the cellophane taped stamens and secure the ends together with the cellophane tape. The cardboard with pollen laden anthers can be placed in small manila envelope. This keeps the pollen quite nicely and when you wish to pollinate you hold the cardboard and hit the chosen pistil with the pollen laden anthers that are protruding from the edge of the cardboard.1
It must be remembered to clean your tweezers before going to work on another rhododendron. The alcohol removes any unwanted pollen and other debris.
The little tubes of aluminum foil are made by rolling sheet aluminum foil around a pencil two or three times and then cut in lengths of one to two inches and crimp one end. These little tubes can be placed over the pistil to keep it protected until the pistil is acceptable to the pollen. Sometimes the pistil will not accept the pollen at the time you make ready the pistil, or you might want to "hit the pistil" again in a day or two to insure "a take".
In my experience R. maximum and some maximum hybrids are examples of difficult ones to pollinate since the pistil will generally not accept foreign pollen when flower first opens and certainly not before the flower opens, when you should be preparing flower for pollination. The pollen from R. maximum seems to be able to pollinate the pistil about 24 hours before it opens, whereas it will reject foreign pollen at this time. Great care must be taken when removing the petals and stamens from this type of plant for you will contaminate more than one flower before you succeed if you are not careful. It is wise to use your magnifying glass when working with these plants, and it will become obvious if you have been successful in not causing self-pollination.
Many times the stigma of the rhododendron you are working on is not "sticky", so the pollen doesn't want to stick to the stigma. If you take a toothpick and gather some nectar from around the base of the ovary and apply it to the stigma, your pollen will stick. Generally there is more nectar in the early morning than later in the day. You can also use honey, but this isn't as good as the flower's own nectar and your percentage of "takes" are not as high.
In conclusion, it should be re-emphasized you can't be too careful about gathering pollen or in pollinating. You may raise seedlings for many years only to find out in the end the cross was not what you thought you made but something altogether different and unsatisfactory.
1 Personal communication — Dr. Ray Minor, Forest Grove, Oregon