How to Create your Own Conifer Varieties
Tap into the technique of controlled pollination and make your own conifer cultivar.
I am working on my second conifer book and thought it might be a good idea to share some of the information with ACS members. This article will be included in one of the chapters. The title of this article is a bit misleading since hybrids tend to be interspecific, such as Cupressus nootkatensis and Cupressus macrocarpa being cross pollinated to produce Cupressus x leylandii.
I am going to discuss crossing two different cultivars within one species to create new cultivars. Strictly speaking, not true hybridization. A typical conifer will possess both male and female reproductive organs. The male organs (sporangiophyll) will produce the male sex cells (sperm cells) contained within capsules called pollen grains. The female organs, commonly called cones, sometimes mistakenly referred to as flowers, will produce female sex cells (eggs) which, when fertilized, will develop into seed-containing cones.
Controlled Crosspollination in Conifers
The female organs develop high on the plant and the male organs develop lower on the plant so that self-fertilization is less likely to occur. Since conifers are not insect-pollinated and depend upon the wind for transferring the pollen from the male to female parts, there is little likelihood of the pollen just falling onto the female cones of the same tree.
To perform a controlled crosspollination of two conifers, the pollen from one conifer must be transferred to the other by some mechanical means, usually a simple brush. Greg Williams of Vermont has grown a number of interesting conifer seedlings through a process of cross-pollination within a species. Pinus strobus ‘Horsham’ is a dwarf cultivar which produces viable seeds.
By planting a ‘Horsham’ beneath a cultivar of Pinus strobus, it is possible to produce seedlings with characteristics of both plants, since the pollen will fall from the larger plant onto the smaller one. Williams’ selections of dwarf Pinus strobus with twisted needles and branches were developed in this way by crossing Pinus strobus ‘Torulosa’ with Pinus strobus ‘Horsham’ to produce ‘Mini Twists’ and ‘Tiny Kurls’.
Pollinating Picea abies
I was visiting with Karel Maly in the Czech Republic in 2000 when he showed me assorted batches of seedlings he was growing from Abies koreana. I asked him how he did his cross-pollination, and it turned out to be a very simple process. In the spring, as soon as the pollen started to fall from the male strobili of Abies koreana, he would collect pollen in a paper bag and then brush some of that pollen onto the appropriate female strobili.
He did this every day for at least a week. By smothering each female strobilus with such copious amounts of pollen, he did not need to worry about wind-transported pollen. If he were attempting scientifically pure crosses, he would have to cover the cone.
During the spring of 2001, I noticed that one of my Picea abies ‘Gold Drift’ garden plants had a male strobilus about to produce pollen. I thought I might as well put the information that I gleaned from Maly to good use. I collected the pollen in a plastic bag as soon as it was ripe. Then, using a fine brush, I applied some of the pollen to several cones on an older Picea abies ‘Acrocona’ growing in another area of our gardens.
I did not bag the cones on the ‘Acrocona’, but I did do repeated applications of the pollen over a period of about one week. I knew the cones were ready for pollinating because the scales were spread open, and the ‘Acrocona’ was producing pollen.
Seed Collection and Selection
I collected the seeds in the fall of 2001 and stratified them for three months in a refrigerator. I germinated the seeds in the spring of 2002 in a seed flat. I potted 60 of the lightest colored seedlings the following spring with a repotting in 2005. Then, I selected the golden seedlings for planting out into the ground in 2006. There were 28 golden seedlings finally selected out of about 200 germinated seedlings.
I had a specific goal in mind. I wanted to develop a golden weeping spruce which would produce cones at the ends of its branches. If I were successful, I expected the cones would be red when they first appeared, and that the contrast with the golden foliage would be striking.
As part of the selection process, I wanted to compare grafted plants with the original seedlings. At the first opportunity, I grafted scions from each seedling. The plants produced by this grafting were set aside for later evaluation. These grafts were three years old in 2009; so, I planted one of each close to its parent plant. They all appeared to perform in the same manner as the original seedlings. That was an important observation.
Seedling Comparison
In October 2010, I made six selections from the seedlings and gave them permanent names. The other twenty-two original seedlings were given provisional names. The selection process proved to be difficult since all of the seedlings exhibited a variety of growth habits as well as a range of shades of yellow through gold. Five of the plants exhibited very bright yellow foliage and burned in the sun.
These I moved to partially shaded locations, which stopped the burning and slightly reduced the brightness. These all have ‘Lemon’ as part of their names. The others are performing nicely in the full sun. They flush yellow, then they become lime green before turning yellow on the sunlit surfaces; they color up similarly to ‘Gold Drift’. One original selection only shows some color in the winter and is only lime green in the spring. It does not even have a provisional name. They all burn to some extent until established. Some watering during dry spells helps to prevent burning on smaller plants.
The foliage of the seedlings also differs from the parents in two ways. First, the needles are shorter and much thinner than with either ‘Acrocona’ or ‘Gold Drift’. Second, some of the new growth will occasionally exhibit foliage with exceptionally pale shades of yellow. I expected cone production to begin any time after about five years.
Seedlings of ‘Acrocona’ started producing cones at about that age. Sure enough, some red cone scales started to appear on a few of the seedlings about when expected. The cone production will never be quite as prolific as ‘Acrocona’ since the chlorophyll content is less, and the energy available for cone production is more limited. None of the “Lemon” series have shown any signs of coning as of 2015.
Seedling Growth
One of the more surprising developments is the wide range of growth rates. Neither of the parent plants is dwarf. Even ‘Gold Drift’ with its golden foliage has a rapid rate of growth. The source of the dwarfing gene is unknown at this time since it is not apparent in either of the parent plants. In addition, the seedlings have shown some interesting intermittent characteristics.
Cone scales, which do not develop fully into cones, are located at the ends of some of the branches. Numerous buds will often form within and below these scales. When the buds push in the spring, tufts of congested branchlets result. On occasion, large numbers of buds form at the ends of the new year’s growth.
The following spring, these buds erupt to create masses of short branchlets all over the plant. I originally named the following six plants. The rest will be featured in my upcoming conifer book.
Picea abies ‘Lemon Drop’
Dense, dwarf and globose, it does show a tendency to burn since it has small, thin needles and bright yellow foliage. It grows 1–2 inches per year and will possibly never bear cones due to its small size and bright yellow foliage. However, as it gets larger, it may be able to produce enough extra food to bear cones. The plant pictured here is growing in indirect light and spends less than one hour in the sun. It is thirteen years old in this picture.
Picea abies ‘Dandylion’
Broad and pendulous, this selection is the one most like ‘Gold Drift’ in growth habit. It does produce upright shoots, but, so far, they do not continue upward. Its growth habit is spreading with horizontal to slightly pendulous side branches. This seedling first produced cones in the spring of 2010. The foliage is bright yellow in the spring, dulling slightly in the summer and then coming back in the fall. The needles are smaller and thinner than the species. It receives about four hours of direct sun per day and is thirteen years old in the picture.
Picea abies ‘Gold Finch’
Dwarf and spreading with pendulous branches, this selection is brighter yellow and considerably slower growing than ‘Dandylion’. ‘Gold Finch’ has not yet started producing cones due to its dwarfness and yellow foliage. It may produce terminal cones as it develops more size. It is a dwarf ‘Gold Drift’ and may be staked or grown as a ground cover. The original plant is to the right and gets limited direct sunlight during the day. It was thirteen years old when this picture was taken. The plant to the left was nine years old and grafted from the original selection. Notice the brighter color in the sunlight.
Picea abies ‘Chub’
I named this plant ‘Chub’ in honor of Justin “Chub” Harper and have donated the mother plant and all but one graft to the Harper Collection at Hidden Lake Gardens in Michigan. They will control any future availability of this selection. Densely branched and broadly conical, it has been producing cones since 2010. During the spring 2009, it had over 30 buds on terminal shoots, and every one pushed, contributing to the dense branch structure shown in the pictures. Apparently, this plant will be a dense, broadly conical, small tree. I no longer have the original plant growing here; so, this picture is from 2010, and the plant was eight years old.
Picea abies ‘Summer Daze’
This selection has developed a very interesting growth habit as it approaches its thirteenth year of growth. It appears to be developing an upright growth habit, but the side branches are all strongly pendulous. The yellow color with its terminal cones and pendulous side branches makes for a plant with unlimited potential for the landscape. The plant was ten years old in this picture and shows good color. Since then, it has become partially shaded, and the color is not as bright.
Picea abies ‘Honey Pot’
A slow-growing plant, Picea abies ‘Honey Pot’ is mounding with nice yellow foliage. The needles are small, and the branching is dense. It is almost globose and is staying quite dwarf. Coning has not occurred yet, but I have expectations that, as the plant ages, the cones will appear. The plant was ten years old when this picture was taken.
Cultivar photographs by Bob Fincham. Thumbnail photograph by Mohammad Emami.
This article was originally published in the Fall 2015 issue of Conifer Quarterly. Join the American Conifer Society to access our extensive library of conifer-related articles and connect to a nationwide group of plant lovers! Become a member for only $40 a year and get discounts with our growing list of participating nurseries in our Nursery Discount Program.