THE ORIGIN OF CONIFER CULTIVARS

The Pinetum at Devizes in England once belonged to Humphrey Welch, its creator, who wrote an authoritative book on dwarf conifers. In this picture taken at the arboretum is a most interesting specimen. Just above the golden Lawson's cypress to the left is a Picea abies 'Humilis' 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.

Bob with Humphrey Welch during a visit to England in 1985.

Picea pungens 'St. Mary' is a most attractive, low-mounding form of Colorado spruce that originated as a witches'-broom. Pinus strobus 'Horsford' is a dense bun that was discovered as a seedling growing in Vermont. Pinus strobus 'Sea Urchin' is a dense, bluish bun that came from a witches'-broom seedling. Picea abies 'Little Gem' is a tight, globose form of Norway spruce that originated as a witches'-broom upon Picea abies 'Nidiformis' which itself had originated as a witches'-broom. Picea mariana 'Blue Teardrop' developed as a fast-growing branch on Picea mariana 'Nana'.

Above left shows the original 'St. Mary' broom and above right shows an old specimen of 'St. Mary'.

Obviously all known cultivars had to originate in some manner. The ones just listed are a few examples of the various origins of plants. All of these plants are cultivars. They are selected variants of the normal species that have garden merit and can be propagated asexually to produce duplicates of themselves. Plants that are artificially induced to grow in a desired manner by propagating selected material are not to be included in this class and are considered cultivariants. A good example of a cultivariant is Abies procera 'Glauca Prostrata' which is described as a flat-growing plant, but invariably produces an upright leader and eventually becomes a tree. The grafting of a side branch of Abies will generally produce a cultivariant exhibiting this kind of behavior.

Upper picture is the original broom of Taxodium distichum 'Secrest' and the lower picture is Abies procera 'Glauca Prostrata'.

The mechanisms that produce cultivars are not very well understood, but there are some good observations and interesting 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 mariana 'Blue Teardrop' as originating from a fast-growing branch on Picea mariana 'Nana', itself a slow-growing cultivar. This type of activity is really 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 occurring in growing tissues, they are most susceptible to damage by this radiation. If such damage occurs at the right time and place, a mutation may result. Since a typical plant of Picea mariana 'Nana' has a high number of 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 the shape or growth rate.

Picea mariana 'Nana' is the picture above while Picea mariana 'Blue Teardrop' is pictured to the right.

Pinus strobus 'Horsford' and Pinus strobus 'Sea Urchin' both originated from seed. 'Horsford' was found growing in the wilds of Vermont by William Horsford while '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 as to just when the mutation of each one occurred is not so obvious. 'Horsford' may have resulted from a mutation during the sexual activity that created the seed from which it germinated. However, the mutation may have actually occurred at an earlier time as evidenced by the work of Waxman. For over twenty years Waxman collected seed cones from congested masses of growth called witches'-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.

The picture above left is 'Horsford' and above right is 'Sea Urchin'.

Witches'-broom seedlings are indicative of genetic aberrations in witches'-brooms since such a high percentage of dwarfs is produced. The percentage could easily be much higher except for the fact that almost 100% of witches'-brooms have only female flowers, and the fertilizing pollen must come from male flowers on normal 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 witches'-broom in the region of the seed's origin. If not, then the seed was produced by a genetically damaged sperm, egg cell, or zygote.

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

Picture above is Pinus sylvestris 'Riverside Gem'.

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 dense cushion about three feet across and 18" high when it is twenty years old.

There are several ideas which attempt to explain the origin of a witches'-broom. 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 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 the observation of several brooms within a small area, indicating that the virus spread through the area like a disease.

Picture above left is a broom cut from a Douglas fir and above right shows a broom on an azalea.

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. Cytokinens are found at a higher than normal level in a witches'-broom. Cytokinens are hormones that do not move very freely around the plant. Their presence stimulates cell divisions. The hormone, giberillen, which encourages shoot elongation is present at reduced levels, especially in a virally produced broom.

Picture above shows a broom on Abies concolor that has been difficult to propagate.

How these unknown agents upset the hormonal balances in a bud and how they are able to persist into the resulting brooms are questions that still need explanation. Since these agents apparently have a genetic influence as well, the questions are even more complex than they first appear. Grafting a small piece of a 'nonviral' witches'-broom onto a seedling will generally create a plant with the characteristics of the original broom. The hormonal imbalance apparently remains, even though a new stem and root system 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 broom that was grafted or the genetic structure of the cells was imprinted with a new hormonal code equal to that of the whole broom.

Picture above left is the broom on Pinus mugo 'Mops' that I offer for sale and to the right is a broom on Picea abies in Germany at the Horstmann Nursery.

Almost all witches'-brooms that have been observed to flower have been female. (Pinus sylvestris 'Longmore' is a male broom). If the egg cells are fertilized, the resulting seeds produce a high percentage of dwarf plants. Those dwarfs result from the normal sperm cell from the male flowers of the tree fertilizing the eggs of the abnormal witches'-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 product a relatively uniform population of species normal and witches'-broom duplicates, with little or nothing in between.

Some seedlings from witches'-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 normal in all observable ways while others will develop into compact or dense plants and a few will become quite dwarf. Such variation within a population is thought to be due to genetic factors.

A cultivar produced from a witches'-broom is not free from the possibility that it may one day produce a witches'-broom itself. There are several very good examples of such an occurrence. Picea abies 'Little Gem' was propagated from a broom on Picea abies 'Nidiformis' which was itself propagated from a broom on Picea abies.

Pictured above is a group of Picea abies 'Little Gem' and below is a picture of a plant that is provisionally called Pinus strobus 'Kate Brook'.

Many cultivars originate as abnormal seedlings from apparently normal parent plants or as branch mutations on otherwise normal trees. For example, Pinus strobus 'Fastigiata' gets very large and the branches widen as it ages. In Vermont a fastigiate Pinus strobus was found that maintains its spirelike growth habit in spite of heavy winter and spring snows. There are several similar plants growing near the specimen pictured to the left, but this particular specimen has the best growth habit.

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. These Tsuga mertensiana in the picture to the right were photographed at the Rhododendron Species Foundation, Federal Way, Washington and are all natural seedling originations.

Color mutations can occur in seedlings or on the branch of an otherwise normal tree as shown here to the right with this branch mutation on Picea abies 'Reflexa'. In the bottom center of this photograph is a gold branchlet. This branchlet is producing golden plants of this cultivar. Sometimes the mutation results in a variegated plant like the Pinus peuce 'Variegated' pictured on the left.

Genetics appears to be a crucial factor affecting the origins of new cultivars although the agents affecting the needed changes in the genetics of a normal tree to product aberrant growth or seed are not completely understood. However nature works to produce these mutations, the process has produced a treasure trove of attractive plants for the modern homeowner. A few additional cultivars that originated from witches'-brooms are pictured below.

ABIES PROCERA ‘BLAUE HEXE’
Broad and globose; short, dense branches; short, wide needles. (6)

ABIES VEITCHII ‘HEDERGOTT’
A dwarf form that develops into an irregular mound. (3)

Abies lasiocarpa 'Lopalpun' is an exceptionally miniature selection with foliage that is a pleasant shade of blue. 

Microbiota decussata 'Jacobsen' is a rarity. It was found as a broom on Microbiota. Its foliage is ascending and slightly twisted, not at all spreading like that of the typical decussata. 

ABIES CONCOLOR ‘MORA’
A dwarf blue mound with exceptionally dense branching; grows about 3/4” per year. (4)

ABIES KOREANA ‘PINOCCHIO’
Another of the small, bun-shaped forms of Korean Fir. Its foliage is green, but not as green as ‘Cis’, which it does resemble. (5)

PICEA OMORIKA ‘PIMOKO’
Dwarf, dense, broadly globose; twigs with numerous brown buds and typical silver and blue foliage. (4)

PINUS MUGO ‘'Little Delight'
It is ultradense and grows about 3/4” per year. Each shoot has up to ten tiny buds near its tip with an occasional 2” shoot having over 20 buds. The original broom on 'Mops' is shown in the article on this page. (2)

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