The Origin of Conifer Cultivars

Text and Photography Bob Fincham

The Pinetum at Devizes in the UK once belonged to Humphrey Welch, its creator. He wrote an authoritative book on dwarf conifers. In this picture, taken at the pinetum, is a most interesting specimen. Just above the golden Lawson cypress to the left is a Picea abies ‘Humilis’ (Humilis Norway spruce) with three different kinds of foliage: an exceptionally dense ball at the peak of the Lawson, another slightly less dense ball to its left, and, more typically, ‘Humilis’ foliage just above it. These are all growth sports or reversions on the same plant. If cuttings were taken and propagated from each of these sports, new cultivars might be the result.

Picea pungens ‘St. Mary’ (St. Mary Colorado spruce) is a most attractive, low-mounding form of Colorado spruce that originated as a witch’s broom. Pinus strobus ‘Horsford’ (Horsford eastern white pine) is a dense bun discovered as a seedling growing in Vermont. At the same time, Pinus strobus ‘Sea Urchin’ (Sea Urchin eastern white pine) is a dense, bluish bun grown from a witch’s broom seedling. Picea glauca ‘Blue Teardrop’ (Blue Teardrop white spruce) developed as a fast-growing branch on Picea glauca ‘Echiniformis’ (Echiniformis white spruce).

Obviously, all known cultivars had to originate in some manner. The ones listed in this article are a few examples used to explain the various origins of these plants. All the plants are cultivars.

Cultivars are selected variants of the typical species with garden merit. They can be propagated asexually to produce duplicates of themselves. A cultivar cannot be grown from seed and can be traced back to a single mother plant. The name is written inside single quotes, and Latin forms have not been recognized since 1958.

Several plants are incorrectly named cultivars. Let me mention two groups. The first group would be most plants that are called simply ‘Pendula’. For example, the very first Picea abies ‘Pendula’ (weeping Norway spruce) may exist somewhere, but no one can be certain. Since seedlings are commonly produced from weeping forms of Picea abies, they have been propagated, grown, and sold under this name. The same is also true for Pinus strobus ‘Pendula’ (weeping eastern white pine). It comes true from seed as well, and the “mother cultivar” cannot be proven to exist. They need to be given a designation of forma pendula (f. pendula) since that is how they grow. Picea abies f. pendula (pendulous form Norway spruce) and Pinus strobus f. pendula (pendulous form eastern white pine) would be their correct names.

The second group would be plants that are artificially induced to grow in the desired manner by propagating selected material. They are considered cultivariants, a term coined by Humphrey Welch. An excellent example of a cultivariant is Abies procera ‘Glauca Prostrata’ (blue prostrate Noble fir), described as a flat-growing plant that invariably produces an upright leader and eventually becomes a large, conical tree. The grafting of a side branch of Abies (spruce) will generally make a cultivariant exhibiting this kind of behavior.

The mechanisms that produce cultivars are not very well understood. Still, there are some excellent observations and exciting theories about the various processes at work. Cultivars tend to remain stable, and propagations grow like the parent plant. However, reversions back to species normal do sometimes occur and serve to confuse the issue. I described Picea glauca ‘Blue Teardrop’ (Blue Teardrop white spruce) as originating from a fast-growing branch on Picea glauca ‘Echiniformis’ (Echiniformis white spruce), itself a slow-growing cultivar. This type of activity is quite common in many species. Mutations occur in nature and are often induced by the background radiation present all around us.

When cell divisions are taking place in growing tissues, they are most susceptible to this radiation damage. If such damage occurs at the right time and place, a mutation may result. Since a typical plant of Picea glauca ‘Echiniformis’ has many growing tips, it is not very surprising that such mutations occur quite often in this cultivar. In plants with a more open growth habit (fewer growing tips), such sporting is more uncommon but does occur. Sometimes the sporting affects the color of a plant instead of its shape or growth rate.

Pinus strobus ‘Horsford’ (Horsford eastern white pine) and Pinus strobus ‘Sea Urchin’ (Sea Urchin eastern white pine) both originated from seed. ‘Horsford’ was found growing in the wilds of Vermont by William Horsford. In contrast, ‘Sea Urchin’ was grown in a controlled experiment by Sidney Waxman at the University of Connecticut. Both plants are obviously the products of mutations, but just when the mutation of each occurred is not obvious. ‘Horsford’ may have resulted from a mutation during the sexual activity that created the seed, from which it germinated. However, the change may have occurred at an earlier time, as evidenced by Waxman’s work.

For over twenty years, Waxman collected seed cones from congested masses of growth, called witch’s brooms, and grew seedlings from them. These seedlings had a high percentage of compact and dwarf forms among them. Several exhibited enough merit and individuality to warrant cultivar designation and naming.

Witch’s broom seedlings indicate genetic aberrations since such a high percentage of dwarfs is produced. That percentage could easily be much higher, except that almost 100% of witch’s brooms have only female flowers. The fertilizing pollen must come from male flowers on standard parts of the tree. Other dwarf plants from seed collected in the wild and grown commercially at seedling nurseries and those found in the wild like ‘Horsford’ may often be produced from an unnoticed witch’s broom in the region of the seed’s origin. If not, the seed was created by a genetically damaged sperm, egg cell, or zygote.

Cultivars originating from seed tend to behave stably and are relatively dependable. Those produced from cuttings taken from a witch’s broom are often another story altogether. Take, for example, one plant not yet mentioned, Pinus sylvestris ‘Riverside Gem’ (Riverside Gem Scots pine). This progeny of a witch’s broom develops into a dense, upright plant with a pleasingly conical habit. Interestingly, ‘Riverside Gem’ plants will consistently die after about twenty years, a trait observed in several cultivars propagated from witch’s brooms (with varying life spans). The ‘Riverside Gem’ witch’s broom was shaped like a broad cushion and appeared dense enough for a person to sleep upon. Plants propagated from this broom appear entirely different. They are thick, narrowly conical trees that reach about eight feet and die when they reach twenty years.

The cultivar, Picea pungens ‘St. Mary’, is a much better-behaved plant than ‘Riverside Gem’. It maintains the dense, low habit of its originating broom and is a most desirable plant. It develops into a full cushion about three feet across and 18 inches high, when it is twenty years old.

Several theories attempt to explain the origin of witch’s brooms. Most brooms are thought to be viral in origin. A virus upsets the hormonal balance in an elongating bud, causing it to grow little but produce many lateral branches. Such growth continues until the broom chokes itself or is shaded to death, provided that the hormonal irregularities themselves are not fatal. If this type of broom is propagated, the progeny will fail immediately or within just a few years. One clue that a discovered broom is of this type would be observing several brooms within a small area, indicating that the virus spread through the site like a disease.

Brooms that do propagate successfully are attributed to other causes. These “other causes” have never really been defined. But some interesting facts or clues are known. Cytokinins are found at a higher-than-normal level in witch’s brooms. Cytokinins are hormones that do not move very freely around the plant. Their presence stimulates cell divisions. Another hormone named gibberellin is present at reduced levels. It encourages shoot elongation. This sort of combination would tend to promote the formation of many shoots while keeping them short.

These unknown agents upset the hormonal balances in a bud. How they can persist into the resulting brooms is a question that still needs explanation. Since these agents apparently have a genetic relationship to the broom, the problems are even more complicated than they at first appear. Grafting a small piece of a “non-viral” witch’s broom onto a seedling will generally create a plant with the original broom’s characteristics. The hormonal imbalance apparently remains, even though a new stem and root system with a standard balance have been added. Of course, the broom itself was on a species-normal trunk and root system while attached to the parent tree. Either a causative agent was in the piece of the broom that was grafted, or the genetic structure of its cells was imprinted with a new hormonal code equal to that of the whole broom.

Almost all witch’s brooms that have been observed to flower have been female. Pinus sylvestris ‘Longmore’ (Longmore Scots pine) is a male broom. If the egg cells are fertilized in the strobili of a broom, the resulting seeds produce a high percentage of dwarf plants. Those dwarfs result from the normal sperm cell from the tree's male flowers fertilizing the genetically dwarf eggs (zygotes) of the abnormal witch’s broom. Either the eggs have an altered genetic structure, or the causative agent is somehow encapsulated within the seed. The variation of growth rates exhibited by the seedlings, however, indicates genetic changes. A causative agent would be expected to produce a relatively uniform population of typical species and witch’s broom duplicates, with little or nothing in between.

Some seedlings from witch’s brooms will die at a young age, develop into weak, sickly plants, or consistently exhibit dead areas. Other seedlings from the same source will be standard in all observable ways. Still others will develop into compact or dense plants, and a few will become very dwarf. Such variation within a population is thought to be due to genetic factors.

Many cultivars originate as abnormal seedlings from apparently normal parent plants or as branch mutations on otherwise typical trees. For example, Pinus strobus ‘Fastigiata’ (fastigiate eastern white pine) gets exceptionally large, and the branches widen as it ages. In Vermont, a fastigiate Pinus strobus was found that maintains its spire-like growth habit. Heavy winter and spring snows have had little effect upon its shape. Several similar plants are growing together, but the specimen with the best growth habit was selected and named Pinus strobus ‘Stowe Pillar’ (Stowe Pillar eastern white pine).

Any seedling population will show variations in growth habit, rate of growth, and coloration. This variation is normal but seldom produces anything that varies very much from the species norm.

Color mutations can occur in seedlings or on the branch of an otherwise average tree, such as a yellow branch mutation I found on Picea abies ‘Reflexa’ (reflexed Norway spruce). This branch was the sport that produced a cultivar named Picea abies ‘Gold Drift’ (Gold Drift Norway spruce).

Pinus strobus ‘Hillside Winter Gold’ (Hillside Winter Gold eastern white pine) was discovered growing on a slope next to an interstate highway by Layne Ziegenfuss. There was a large group of yellow trees in the area. They were unnoticeable in the summer because they were green at that time of the year. He selected the one with the best color for propagation. He almost threw the grafts away when they turned green in the propagation house. The trees on the slope were gone the following year. Someone had removed the original grove. Layne searched for the seed source, expecting to find a yellow branch somewhere, but never located a seed source. Many similar variants have been found in other species since Layne’s discovery.

Genetics appear to be a crucial factor affecting the origins of new cultivars. The agents affecting the needed changes in a typical tree's genetics to produce aberrant growth or seed are not entirely understood. However, Nature works to create these mutations, and the process has produced a treasure trove of attractive plants for the modern homeowner.