Pollen: The Incredible
The availability of pollen provides the opportunity to achieve our hybridizing goals. It allows us to create beautiful rhododendrons with selected characteristics and to produce plants that will serve as parents to carry desirable genes onto future generations. We have the ability to produce hybrids that bloom early or late, with greater hardiness, and with more beautiful flowers and foliage. Our avenues are endless if we have the pollen to work with. Pollen has many twists and turns along the way either to fulfilling its mission to create seed pods or to total failure. We will explore some of those pathways and look at the importance of testing viability of pollen and receptivity to pollen.
There is very little information available about the viability of pollen or about the ability of plants to accept pollen. For this reason, I make 500 to 800 or more crosses a season in order to understand which parents are sterile and which plants have sterile pollen. In late July I stroll through my garden two or three times a week, observing seed pods as they develop. As the summer goes on, I begin to assess what is happening and try to understand what works and what doesn't work. Each season I make a master list of every cross I made that year; then when I clean the seed in the fall, I correlate the "takes" (the crosses that produced seed pods) against this list. This provides a comprehensive view of which pollen is viable and which pollen appears to be sterile. It also tells me which pollen is very potent (the pollen that takes on plants where no other pollen works). Yes, there is such pollen. 'Janet Blair' and 'Mary Belle' are examples of plants with very effective pollen.
Presently, I am working with a number of tetraploids. The take on these plants is very low, perhaps 5 percent, but one must make the crosses in order to know what is happening. I found that many of the hybrids derived from 'Hotei' and 'Nancy Evans' parentage have sterile pollen but that most of these plants will accept pollen. Perhaps an offspring of 'Hotei' and 'Nancy Evans' could have fertile pollen. This year I found an ('Apricot Fantasy' x 'Sunstone') hybrid with viable pollen. ('Sunstone' is a cross of 'Fawcett's Fantasy'* x 'Hill's Low Red'.) This is a major breakthrough since it is a hybrid derived from 'Hotei' parentage and 'Apricot Fantasy' has sterile pollen. I used pollen from three different siblings of that cross on at least five other hybrids proven to be receptive to pollen to confirm this. All three seedlings proved to have viable pollen. Out of twenty seedlings from a cross of ('Nancy Evans' x 'Mrs. Furnivall'), only one had viable pollen. Five of those twenty seedlings would not accept pollen.
I want to know what each of my hybridizing plants is capable of. The only way I will ever know this is by making three or four crosses on each plant every year. I have a 'Mrs. Furnivall' cross named 'Satin Memories' that I have crossed onto for the past five years without one take. This year I achieved a take with 'Noyo Don'* (R. degronianum ssp. yakushimanum x 'Mars'). 'Noyo Don' is one of those plants with very potent pollen. This pollen also worked on other plants where nothing else took. Gathering seed is quite an exciting adventure. That is when you learn which plants are very good plants for setting seed, which plants might be sterile, and which pollen is sterile. It also tells you which plants are borderline in trying to set seed. The results aren't conclusive. Perhaps the pollen just didn't work in that given year. Some crosses keep you guessing. Did the pollen really take? Is there seed in that tiny pod, and if so, will it germinate?
Testing pollen early is advantageous. If it doesn't work, why continue to use it. I use three plants for this testing purpose. All three accept pollen readily and start to set seed pods very quickly, usually within three or four weeks. 'Invitation' ('Anita Dunstan' x 'Lem's Cameo') blooms in mid-April, 'Nancy Evans' blooms in mid- May, and 'Karalee' ('Hotei' x 'Whitney's Late Frilled Yellow'*) in mid-June. If no seed pods develop, there is no reason to continue to use the pollen. 'Nancy Evans' is the most receptive plant I have ever encountered. This plant has qualities that are truly rare. If any pollen is even close to being viable, it will set seed on 'Nancy Evans'. I frequently make sixty or more crosses on 'Nancy Evans' just for that knowledge. I have tried 'Lem's Tangerine' pollen on at least ten different hybrids for the past five years with nothing to show for it. As I was making my rounds this year, looking at tags, I saw three big seed pods on 'Nancy Evans' and was surprised to find that they were from 'Lem's Tangerine' pollen. Previous experience says that this just can't be, or can it? I will try to confirm this next year. The harvest will tell me if it worked anywhere else. Continued persistence year after year is the answer, even if it takes ten years, it just might work.
I have been asked, "How do you know if this plant is sterile or if its pollen is sterile?" I think the only way is to experiment or ask someone who might have that knowledge. It takes many years to understand how pollen really works and how hybrids accept pollen. Your own experimentation is the best answer, but do not give up too soon. I used 'Capistrano' and 'Hachmann's Charmant' pollen for a number of years successfully. This year I made about twenty crosses with the new pollen from each of these plants. Not one cross is taking. The pollen is not viable this year and I do not know why. This is very strange. A new hybridizer just starting out might conclude that the pollen was sterile and not use it again. I'm sure the pollen from both of these plants will work again sometime in the future. Don't draw conclusions for at least three years or more.
This year I learned the value of collecting and saving seed. When the pollen of 'Capistrano' and 'Hachmann's Charmant' failed, I went back to the seed box in the refrigerator, which contains perhaps 300 crosses mostly from the past three years. There I found crosses of both varieties that I had never planted. You make the crosses when you have the pollen. You may never have that pollen again. On my collected seed, I get good germination during the first four years. It deteriorates little by little beyond that time period. Ten-year-old seed had about a 5 percent germination rate.
Through the process of years of experimentation, I've refined my methods of collecting, storing and applying pollen. Before giving the details, I'd like to review the terminology of the parts of the plant we will be working with. The corolla is the fleshy part of the flower. The centerpiece of the flower is the pistil which has a long tube with the stigma at the tip and the seed chamber where it attaches to the flower. Around the pistil are the stamens. The stamens are composed of filaments and anthers. The anthers, the tips of the filaments, contain the pollen. The stigma is where the pollen is placed. It may be red, black, or green, depending on the parentage of the plant.
Just before the first plants of the season begin to bloom, I review the pollen list from the previous season. I have already done some preliminary planning and have a general idea of the crosses I want to make. I select the capsules I want to use in the near future from the pollen container stored in the freezer. The chosen capsules are compartmentalized in a plastic container according to color, hardiness, or rarity and are stored in the refrigerator. When I am ready to hybridize, I place the capsules I want to use that day in a bowl and return the rest to the refrigerator. As the season progresses, I obtain additional pollen from the freezer. I let it thaw for at least one-half hour before use. When I am through hybridizing for the day, I return all the pollen to the refrigerator.
When possible, I prefer to use fresh pollen from plants that are currently in bloom. I take the filaments with the anthers attached from one truss and pollinate the stigmas of another. If you cannot see any visible pollen (as in 'Hachmann's Charmant'), place the anthers from several flowers in a capsule and allow them to dry. The combined anthers may yield pollen when you thought that none was available.
Before the flowers open, prepare the truss by removing all the corollas. Be very careful when you take the corolla away from the pistil. If pollen is dripping out of the anthers, you may accidentally self pollinate the plant. If this is even close to a possibility, discard that pistil and move on to another one. Trusses vary in size. Some may contain just six pistils, some can have twenty to thirty. I generally pollinate three stigmas with one type of pollen. You may lose one or two to a chewing insect. I may put up to five different crosses on one truss. If the cross is intended for the Seed Exchange, I pollinate at least six to eight stigmas to ensure sufficient seed.
My preferred method of applying pollen is to use a rounded, pointed toothpick. Pollen sticks to the wood and the excess can be scraped back into the capsule. Discard the toothpick when you are through and use a new one for the next pollination. Another method is to use tweezers with a rounded point to easily reach into the pollen capsule. Pollen adheres to the metal very well. Wipe the tweezers over the stigma to complete the pollination and then scrape the excess pollen back into the capsule. Clean the tweezers on a shirtsleeve.
If all you have is a capsule of anthers with no visible pollen, try the following. Create a handle on the capsule using self-adhesive labels or masking tape. I use 2¼" x ⅜" (5.6 x 0.9 cm) strips around the top of the capsule to serve as both a handle and a label. Hold the handle tightly in your right hand and hit the side of your left hand. This usually will jar the pollen loose. If it still doesn't dislodge, hold the handle and flick the capsule with your forefinger gently but firmly. It is amazing how much pollen can appear in the capsule.
The stigmas are most receptive to pollen when they are glistening. This generally happens in the afternoons on warm sunny days. Stigmas can be receptive to pollen for six to ten days. For plants that can be moved, pollinating in a warm or hot hoop house seems to work the best for me. Cold or cool wet days can make pollinating a real challenge. It may be necessary to pollinate between rain showers. If I want to make a cross, I cover the unpollinated truss with a plastic bag to keep it dry. When the opportunity presents itself, I do my pollinating and recover the truss if rain is pending. On sunny days, I take off the plastic bags. To protect the stigmas from contamination, some hybridizers cover them with aluminum foil. John Weagle in Nova Scotia gets excellent results, rain or shine, using Micropore Tape - Blenderm variety by 3M.
The pollen should be accepted within twenty-four to forty-eight hours if the stigma was in the receptive stage. Some stigmas are ready to be pollinated immediately upon removing the corolla. Others may need a few days to become receptive. I found that Rhododendron brachycarpum 'Sado Island'* stigmas were very waxy and pollen slid right off the stigma. It took five days before I could apply pollen. Timing of course is very important. I sometimes find that all the crosses on a truss have not taken, even though the pollen was viable. The conclusion is that I was either too late or too early in applying pollen. You may want to apply pollen two or three separate times in order to ensure proper timing. When the pollen has taken, the stigma may start to turn black while all the non-pollinated stigmas will still be moist and colorful, still waiting to be pollinated.
Collecting and Storing the Pollen
Collecting pollen from most plants is a simple process. Usually each flower contains considerable pollen. One misconception is that a lot of pollen ensures its viability. 'Nancy Evans' has plenty of pollen but it is sterile. Stringy pollen like little white shafts is usually sterile, but it can change within a week to normal granular pollen which may be viable. Over the years you learn which plants are good parents. These are the ones you want to collect pollen from. If you want to use an early bloomer as the seed parent and a late bloomer as a pollen parent, the pollen must be collected and frozen until the following year.
I harvest pollen early in the morning from flowers that have not yet opened, hoping to beat the bumble bee that wants to get into tiny openings. I clean off the unopened corolla and pick off the filaments with the anthers attached. I may obtain filaments from six to ten flowers and place them on a plastic lid (such as from a cottage cheese container) with the parentage noted. I use a separate lid for each plant and bring them indoors to begin the drying process. That evening I process the pollen by cutting off all the anthers over a 4" x 6" sheet of white paper. I use tweezers to pick out any excess filaments, label the pollen and place it out of the way to dry for twenty-four hours.
The next evening, I continue processing the drying pollen by placing it in size 00 gelatin capsules. (The best place to obtain capsules is a health food store.) Make a slight crease in the middle of the paper and slide the pollen down into the capsule, filling it no more than one-half full. I use permanent type white self adhesive strip 2¼" x ⅜" (5.6 x 0.9 cm) to label the capsule. The label also serves as a little handle. Strips of masking tape will also work well. (Some hybridizers write the parentage directly on the capsule.) I then place them in a plastic container with about ¼" (0.6 cm) of silica gel in the bottom covered with a layer of cotton. Silica gel, used for drying flowers, can be obtained at a craft store. The capsules continue to dry in this container for five to seven days. The pollen that won't be used during the present season is placed in a container with an inch of silica gel on the bottom with a layer of cotton and put in the freezer. I have been asked how long pollen will remain viable. I believe it is too variable for a simple answer. I have had pollen lose its viability within the first year but I have also had pollen remain viable for up to three years.
When collecting pollen, you have to understand your plants. 'Hachmann's Charmant' is reluctant to release pollen. Just before the flower opens, the pollen, which is minimal, is poised to release immediately. You must capture this pollen before the flower opens; otherwise most of it is lost. Some flowers will have no pollen and others will have a lot. Some varieties such as 'Lem's Cameo' or 'Point Defiance' may have many flowers without a trace of pollen, but the next flower may have pollen dripping from the anthers.
If you have been given permission to collect from another hybridizer's garden, be prepared for adverse weather. I have been collecting on a rainy day with my capsules and scissors and have had the capsules collapse from just a few drops of rain. Now I use small glassine envelopes from a coin store and process the pollen as soon as possible after collecting it.
The Pollen Exchange
The ARS Pollen Bank is a very valuable asset to hybridizers. It is a unique resource that the world can draw from. Ron Rabideau deserves credit for his significant contribution in administering the exchange. He gives us complete and extended descriptions of as many pollen varieties as he can. The contributors who have kept this worthwhile project going also deserve credit.
Species pollen is the most sought after, particularly species with indumentum. Many hybridizers want to stay close to primary crosses. This is particularly true for those of us who have limited space. The outcome from primary crosses is far more predictable than from crosses with multiple parents on each side.
One advantage of the Pollen Bank is it acquires pollen from the cutting edge. Information about parentage is important in selecting pollen. If we are not familiar with the names of the parents, we might not recognize some that would be of interest. For example, if I send in pollen from 'Recital', a registered name, I could include that it is a cross of ('Mindy's Love' x 'Jessie's Song'), again both registered names. But if you are not familiar with those names and cannot find them in any book, you might not select the pollen. If I send it in as [('Nancy Evans' x 'Lionel's Triumph') x ('Nancy Evans' x 'Golden Anniversary')], you have recognizable names and therefore may be more inclined to want the pollen. Parentage and photographs of all three of the above registered names are on Homer Salley's Rhododendron Hybrids (Third Edition) on CD ROM. This helps in understanding names that aren't recognized, but not all hybridizers own a computer. Specificity in stating names and parentage for the Pollen Exchange is useful. Also, registering names of plants that are used extensively for hybridizing is of value.
'Recital' ('Mindy's Love' x 'Jessies Song'), a cross by Jim Barlup.
Photo by Jim Barlup
If just a few more hybridizers would send some capsules of valued varieties, we could all benefit. Shipping can be done in a bubble pack using a film canister as a container. With a touch of desiccant and cotton on both ends, it's ready to ship. Pollen contributors have first choice of all pollen before it is released to other members. Collecting and processing pollen is a time consuming job, but it is our most precious resource. Pollen from other hybridizers as well as my own has given me the opportunity to make unbelievable crosses. My work is a collection from all over the world. Gathering full seed pods is your dream come true. When December comes and I'm ready to start planting the seed, I am grateful for all the pollen that came my way. I evaluate the logic of each cross based on the appearances of past seedlings, on the value of the cross. Is this what I really wanted to accomplish. Does the parentage of new crosses meet my ever changing criteria? I find it easy to discard 25 percent of this year's crosses because of the knowledge I have gained during the year. Without pollen, we accomplish nothing. Understanding our pollen and the parents of the plants we wish to use gives us insights into what we are capable of achieving. I am truly thankful for the availability and the sharing of pollen, the incredible resource.
* Name is not registered.
Jim Barlup, a member of the Cascade Chapter, authored the journal series "Tips for Beginners: Hybridizing Notes" appearing in issues from spring 1996 to fall 1997.