The introduction of trees and shrubs of horticultural interest has been an important function of the Arnold Arboretum for many years. Since its founding over seventy five years ago the Arnold Arboretum has been responsible for the introduction of nearly 3000 species and varieties of woody plants new to this country. Many of these have proven to be valuable ornamentals and are now widely distributed— commonly seen around homes, in parks and gardens and featured in other arboreta.

The staff of the Arnold Arboretum has also been actively engaged in the development of new or improved ornamental trees and shrubs through plant breeding. The techniques of the plant breeder: hybridization, back-crossing, selection, used in the past to develop the now numerous varieties of roses, lilacs and rhododendrons, are being applied at the Arnold Arboretum to crab apples, forsythias, magnolias and other ornamental trees and shrubs. The plant explorer will continue to select and introduce plants new to America. The plant breeder will continue to use the best characteristics of each introduction to develop even more desirable ornamentals.

The breeding of new horticultural varieties of trees and shrubs requires much patience for the results may not be evident for many years. The actual transfer of pollen between species or varieties to produce the hybrid seed is only the first of many steps, and detailed records must be kept through all steps. The hybrid seeds are carefully collected and stored. Many must be stratified to insure germination and some require two years for this process. The seeds are planted in the greenhouse in the early spring and the seedlings transferred to the nursery a few months later. After a year or two in the nursery they are planted in rows ten feet apart with the plants 2–5 feet apart in the row. Here they remain until they flower-which may be several years, but often 5–10 years. The more promising types, as measured by leaf and flower characters, are then selected for propagation and again grown in the nursery for several years. They are then planted with sufficient spacing and in several locations, to test for growth habit, size and hardiness. The hybrids which pass this test are then selected and propagated for distribution to the commercial nurserymen, who in turn must propagate them for distribution to the public. Thus it may take 10–20 years or more before the new hybrid becomes available to the horticultural public.

The plant breeding work at the Arboretum has covered a wide range of genera with emphasis in recent years on the production of small or dwarf trees and shrubs to meet modern landscape designs. Since the average home owner—or his wife—is also the gardener, the trees and shrubs which are planted should be hardy and require a minimum amount of pruning, cultivation and spraying.

One of the first hybrids released in recent years by the Arnold Arboretum was an ornamental cherry, named as a tribute to the writer’s wife and given her maiden name “Hally Jolivette.” The French name Jolivette is also appropriate since it means “pretty little one.” This flowering cherry is a hybrid between Prunus subhirtella and a variety of P. apetala. It is a small tree of graceful habit and comes into flower the second or third year from the time of propagation from cuttings or by budding on P. subhirtella rootstocks. The flowers are semidouble, white with a light pink center, and open up in succession so that the tree is in bloom for several weeks. The branches are dark and slender and at dusk the white flowers appear to be floating in air. This variety, like all cherries, likes full sun and well-drained soil. It is now available from a few commercial nurserymen. Mature specimens can be seen on the Bussey grounds and in the cherry collection of the Arboretum just inside the Forest Hills gate.

Neither nature nor the plant breeder has yet produced the ideal variety of ornamental apple. It should be of small or moderate size, have attractive flowers which retain their color and texture until the petals drop, be of graceful growth habit while at the same time requiring little pruning, have foliage that will become colorful in the fall and possess attractive fruit remaining on the tree until late fall.

Most of the apple hybrids made at the Arnold Arboretum have involved parental varieties of hybrid origin such as M. arnoldiana, M. pumila niedzweizkyana, and M. purpurea eleyi. As a result the progeny have been extremely variable providing a great range of genetic recombinations. Many of the best oriental species are apomictic and produce only maternal types of offspring, but M. sargenti proved to be facultatively apomictic and occasional sexual hybrids have been obtained by artificial pollination.

One of the first apple hybrids released by the Arboretum is a hybrid between M. arnoldiana and M. pumila niedzweizkyana, distributed under the number 19039. It has been named “Henrietta Crosby” in honor of Mrs. V. R. Crosby,* who has long been a member of the Arnold Arboretum Visiting Committee. This variety is of moderate size, of graceful growth habit, with large single pink (China Rose 025/1) flowers which hold their color well. The fruits are about an inch in diameter, and bright red in color. Mature trees of this variety are on the Bussey grounds adjacent to the Arboretum and at the Case Estate in Weston. It has been distributed to commercial nurserymen, but has not yet been placed on the market.

Another attractive ornamental apple is an open pollinated offspring of M. spectabilis riversi distributed under number 6639. This variety has been named “Blanche Ames” in honor of Mrs. Oakes Ames, a distinguished botanical artist and wife of the late Oakes Ames, former administrator of the Arnold Arboretum. This variety is a rather small tree which has a most attractive growth habit. The flower buds are pink and open up as semidouble white blossoms with a tinge of pink on the outer petals. The flowers are of delicate, but durable, texture and are most attractive. The fruits are small, yellow and drop rather early, but the graceful growth habit and delicate flowers make this variety unique. It is not yet available from commercial sources. Good specimens of “Blanche Ames” are growing on the Bussey grounds, and in the Arnold Arboretum on the bank just inside the Forest Hills gate.

A more recent hybrid apple appears so promising that it has been named before it has been thoroughly tested. It is a seedling from “Henrietta Crosby” and bears the Arnold Arboretum number 22957. The flowers are large— up to 1. ~5 inches in diameter—and are a brilliant red (Spinel red-0023/I). The fruits, which are about half an inch in diameter are bright red and hold their color and texture well into the winter. Although these trees are still young the spreading growth habit is already evident and the mature trees should possess a desirable form. This hybrid has been named “Henry F. duPont” in honor of a discriminating horticulturist who has long been a faithful member of the Arnold Arboretum’s Visiting Committee. A small specimen of “Henry F. duPont” is on the Bussey bank inside the Forest Hills gate. It has been propagated for distribution, but will not be available to the public for at least several years.

Some of the most exciting apple hybrids have been obtained by using Malus sargenti rosea as the seed parent. The hybrids resemble M. sargenti in growth habit and in leaf shape, but some have much larger and more colorful flowers and larger fruits. One of the first of these hybrids, made by George Skirm in 1938, resembles M. sargenti in many respects but is more vigorous. The pink buds open into white blossoms in great abundance. The small fruits are a bright red, but do not last long since they are eaten by the birds and tend to drop early in the fall. This hybrid is a cross between M. sargenti rosea and M. astrosanguinea and since it is a daughter of M. sargenti it has been named “Mary Potter” for one of Professor Sargent’s daughters. It has been distributed to the commercial growers and should soon be available from at least one nurseryman. Excellent specimens are on the Bussey grounds and on the bank inside the Forest Hills gate.

Two other promising new crab apple varieties have been introduced to the trade by the Arnold Arboretum. The first is “Katherine,” originated as a seedling at Rochester Park by B. H. Slavin about 25 years ago. It has been named, at Mr. Slavin’s request, in honor of his daughter-in-law Katherine Clark Slavin. Malus spectabilis is probably one of the parents. This variety has large flowers which are double, with about 20 petals. The white and pink blossoms are most attractive and the growth habit of the tree is quite good. There is some tendency for it to flower only in alternate years and the small yellow fruits are not outstanding. We have progeny of “Katherine” with attractive red fruits which are now being tested. Small specimens of “Katherine” may be found in the Arboretum on Peters Hill, and it is available from several nurserymen.

A spectacular variety of ornamental apple was found as a chance seedling by Dr. Donald Wyman on the Arboretum grounds. The flowers are white, deeply tinged with pink, with about 16 petals and are about 5 centimeters in diameter. The orange yellow fruits are about 1.5 centimeters in diameter and are most colorful in the fall. It flowers when only two or three years old and is an annual bearer. It was named in honor of Dr. Wyman’s older daughter Dorothea. Small specimens of “Dorothea” may be seen on the Bussey grounds and on Peters Hill in the Arboretum. It is available from commercial sources.

The Magnolias are among our finest ornamental trees, particularly M. stellata, the Star Magnolia. Unfortunately it blooms so early in the spring that the flowers are often injured by cold weather. We have not yet been able to combine the small attractive growth habit and attractive flowers with a later blooming variety, but one of the M. stellata hybrids has proved to be outstanding for gardens where a relatively large specimen can be used. This hybrid, a cross between M. stellata and M. kobus, has been named the “Merrill” Magnolia, in honor of Dr. E. D. Merrill, former director of the Arnold Arboretum. It has flowers much like those of M. stellata and blooms early in the spring, but it is a symmetrical, vigorous tree and at the age of 15 years is larger than the 60-year-old M. stellata specimens near by. An excellent specimen is growing near the Administration building. It has been propagated and was offered for distribution by a large nursery last year.

The Forsythias herald spring in New England. They are tough, hardy, easy to grow and are spectacular in early spring, but most of the varieties have little to offer the rest of the year. In cold open winters the flower buds are often killed. We have tried to produce new compact hardy types using Forsythia ovata, one of the hardiest species, as one of the parents, but without much success to date.

In 1939, George Skirm produced a tetraploid seedling of Forsythia intermedia spectabilis which was named “Arnold Giant.” The growth habit is rather stiff. The large deep yellow flowers are attractive, but are rather shy and do not turn their faces to the sun. The late Lord Aberconway grew some plants of “Arnold Giant” in England where it was given an award of merit by the Royal Horticultural Society, but in this country most people prefer the lighter colored flowers. “Arnold Giant” is, however, an excellent variety for forcing as a cut flower, and as a parent in the production of triploid varieties.

“Arnold Dwarf” forsythia is an odd segregate from a cross between F. intermedia and F. japonica saxatilis made in 1942. It grows only a few feet tall and the spreading branches root readily in contact with the ground. Its compact spreading growth habit makes it a good ground cover. It has been slow in coming into flower and as yet the flowers have not been borne abundantly. Perhaps propagation from flowering plants will overcome the delay and paucity of flowering. Several collections of “Arnold Dwarf” are growing on the Bussey grounds and several specimens have been planted at the lower end of the forsythia collection in the Arboretum. It is available from some nurseries.

Considerable work has been done with the lilacs, to get a low-spreading type of bush with flowers like those of the Vulgaris lilacs, but with no success. The small-spreading species such as Syringa microphylla can not be crossed with the Vulgaris lilacs, and crosses between Vulgaris varieties and S. laciniata or S. pinnatifolia produce only sterile hybrids. The vulgaris laciniata hybrids produced in Europe have given us the “Chinese” lilacs which have the grace of the Chinese species, S. laciniata, in growth habit and inflorescence, and the flowers have been increased in size by the Balkan parent S. vulgaris. A number of new hybrids between these parental species have been grown, but none has yet been better than the older varieties of S. chinensis.

There are probably thousands of varieties of rhododendron, yet few of them are entirely happy in New England with the cold winters and hot summers. We have grown a number of hybrid segregates from crosses between the hardier Catawbiense varieties and the more tender but more attractive Fortunei hybrids. We have selected several dozen from among more than 300 segregates and they are being tested for hardiness and insect resistance on the Bussey grounds.

The artificial doubling of chromosomes has been a promising technique in producing new varieties of forsythia and other genera. In 1939, seedlings of F. intermedia spectabilis were treated with colchicine and produced a tetraploid plant. This was pollinated with pollen from nearby species, including F. ovata, and gave rise to several dozen triploids. Several of these had exceptionally large flowers—up to 2 ½ in diameter—and flowered freely. One was selected for propagation and has been named in honor of Beatrix Farrand, one of the leading landscape gardeners of this country and former student of Professor Sargent. The “Farrand” Forsythia has been distributed to a number of commercial growers.

The triploid forsythias are relatively sterile, but an occasional fertile seed is formed. These seeds often have an unbalanced chromosome number and produce aneuploid seedlings which are occasionally so unlike the mother plant that they hardly look like forsythias. Many of these aneuploid seedlings are dwarf types and should begin to flower this spring.

The progeny of triploids have also proved to be unique in Philadelphus. Several of the most attractive varieties of mock oranges are triploids which originated in Europe from segregates of species hybrids. The variety “Bicolore” in the Arboretum produces some viable seed and we have grown more than a hundred progeny. They are extremely variable and may provide some unique segregates of horticultural interest.

Triploid magnolias are also being grown. An artificial tetraploid was produced in 1939, but was of no value because the petals were too thick to open properly. It was back-crossed to normal diploids to produce triploids. We have several dozen plants which are still in the nursery. They will be moved to the testing plots at the Case Estates in Weston in another year or two.

Other Methods Of Producing New Ornamental Plants

It has long been known that the mutation rate in plants and animals could be greatly increased by exposure to X-rays or other ionizing radiation. Horticulturists in Sweden and in Canada have been able to increase the frequency of bud sports in apples by irradiating scions used in grafting. We have done some work along this line with a portable radiation source provided by the Brookhaven National Laboratories. We have also cooperated with this Laboratory by providing ornamental trees and shrubs which have been planted in the “gamma field” at Upton, Long Island. Various intensities of chronic irradiation are provided by planting the trees and shrubs at varying distances from the radioactive cobalt source in the center of the field. As these ornamentals reach the flowering stage they will be examined for desirable mutations or “bud sports.”

Ionizing radiation from X-rays, radium or radioactive cobalt has also been used to stimulate plant growth, but with very few exceptions the experiments conducted in Europe and the United States have not shown any favorable results. We have, however, been able to induce earlier flowering in gladiolus by irradiating the corms with 3000 roentgens of X-rays. This experiment was conducted at the Bussey Institution and will be repeated this summer.

The need for smaller trees for the modern home and the home orchard has stimulated work on dwarfing techniques so that standard varieties of ornamental and fruit trees can be grown as dwarfs. For hundreds of years the European horticulturists have used special rootstocks to dwarf apples and pears. We have used Prunus tomentosa as a dwarfing stock for peaches and plums, and these dwarfed trees should soon be available from commercial sources. Some of the dwarf fruit trees are nice ornamentals and could serve a dual purpose.

Work is in progress to find suitable dwarfing stocks for standard ornamental trees. Malus sargenti and other species of apples are being tested as dwarfing stocks for ornamental crabs. Preliminary tests indicate that the Silver Maple when used as a rootstock dwarfs the Red Maple and the new “Crimson King” maple.

Citation: Sax, K. 1955. Plant breeding at the Arnold Arboretum. Arnoldia, 15(20): 5–12.

New techniques for dwarfing trees and inducing earlier flowering have been developed at the Arnold Arboretum and the Bussey Institution during recent years. The inversion of a ring of bark on the trunk of a young tree has the same effect as girdling, but without the danger of killing or infecting the tree. The flow of elaborated sap down the bark is checked by inverting the bark because the phloem cells are “polarized”-they permit flow in only one direction. This operation is performed in June and preferably on trees several years old, although we have inverted the bark of an ornamental apple with a trunk diameter of about 5 inches. Perhaps one could let a tree reach the desired size and check further growth by inverting a ring of bark. These experiments are still in the preliminary stages, but if anyone is interested he can see this and the other experimental work on the Bussey Institution grounds adjacent to the Arnold Arboretum, where a demonstration dwarf orchard has been planted adjacent to the Bussey greenhouses.

The cultivars being named for the first time in this paper are Malus× “Henrietta Crosby,” Malus×”Blanche Ames,” Malus× “Henry F. du Pont,” and Malus× “Mary Potter.”