QBARS - v24n1 Breeding Azaleodendrons

Breeding Azaleodendrons
Alexander C. Martin, Los Gatos, Calif.

My efforts to produce azaleodendrons began in 1955. They continued on a fairly extensive scale during the next decade, the crosses in some years totaling 100 or more. By 1965, having tried hundreds of combinations and found a few that succeeded to some extent, my operations became less diffuse and concentrated instead on lines that seemed to hold promise. To date, attempted crosses approximate 1,000. They consist of two main types: those involving evergreen azaleas, and those with deciduous azaleas aimed at creating evergreen yellow flowered azaleodendrons.

During the first few years of azaleodendron experiments, pollen was transferred both ways, to and from azaleas and rhododendrons. But after 177 crosses involving transfer of rhododendron pollen onto azaleas had proved largely negative, hybridizing thereafter was confined entirely to the more productive route: transferring azalea pollen onto rhododendrons.

Crosses with Evergreen Azaleas

This project began in 1955 and terminated in 1962, after about 670 crossing attempts had been made. Numerous azaleodendrons resulted but most of them died or were discarded because of insufficient vigor. Many were feeble dwarfs. However, a few healthy seedlings of fair to excellent quality have been produced.

The best rhododendron parentage for this purpose seems to be 'Margaret Dunn', 'Vulcan', 'John Waterer', and 'Vulcan's Flame'. And the most effective pollen sources were R. mucronatum , and hybrids such as 'Gunrei'.

Seedlings from two of the crosses were vigorous but lacked sufficient foliage. One of these, 'John Waterer' x 'Lilacina', 1956, was a strong grower with fairly attractive pink flowers, but because it became leggy with relatively few leaves, it was discarded after a few years. The other, 'Vulcan x 'Gunrei', 1961, is moderately vigorous and regularly produces an abundance of attractive bright red (Munsell hue: strong purplish red 10 RP 5/12 and 4/12) blossoms of delicate texture, but its small, hard leaves are rather scant. It is still retained.

A third, and by far the best plant, tentatively called 'Abundant Life', is not only thrifty but also has fair foliage and habit, and its large (3-1/2 to 4-inch) flowers are very attractive. However, it may be questioned as an azaleodendron.

It came from a cross of 'Margaret Dunn' pollinated by an unnamed Belgian-Kurume-mucronatum semi-double, rose-red azalea and all of the approximately fifty seedlings resembled rhododendrons in their thick leaves and habit, rather than azaleodendrons. Furthermore, most of them have borne flowers which, in color and form, are suggestive of their rhododendron ancestry. So, except for 'Abundant Life' and a couple of other plants, I have assumed that they were apomicts.

Without understanding how it could be possible for most of the seedlings to be apomicts, with one or more of the lot azaleodendrons, several characteristics of 'Abundant Life' do seem to indicate it is an azaleodendron instead of simply a rhododendron. These are: (1) its rosy-pink color (Munsell hue: deep purplish pink 7.5 RP 6/12) which resembles color of the azalea from which pollen was obtained, and also is different not only from all the other seedlings but also from the rhododendron parentage; (2) the flower form is azalea-like in its wide-open, somewhat recurved, deeply cleft corol­la; (3) as in the semi-double pollen source azalea, small extra petals have occurred occasionally within the corolla; (4) in contrast with the other seedlings, this one seems susceptible to azalea flower spot; (5) in contrast with the other seedlings, the corolla of this one is glossy on the exterior near the base; (6) unlike other seedlings of the lot, this one blooms sporadically in the fall. It sets seed readily with pollen of Exbury azaleas and I have "selfed" it extensively.

In addition, the lot of 'Margaret Dunn' derivatives includes three compact plants with small oval leaves, about 1½ inches long. Two of these have not flowered yet though they are nine years old. The third flowered sparsely last fall and again this spring, with wide-open (almost flat) pale pink flowers, each with seven petals and usually with a split between two of them. Might these also be azaleodendrons?

Crosses with Yellow-flowered Azaleas

According to my experience, de­ciduous azaleas cross much more readily with rhododendrons than do ever­green azaleas. Nevertheless, most of my seedlings from such crosses have been defective in one respect or another, if not in several, and the chances of producing satisfactory azaleodendrons of this type seem small. All of which underscores the significance of 'Broughtoni Aureum' as a fine production of about 140 years ago.

The main problem with most of my seedlings was lack of sufficient vigor, as usually denoted by their light green color. Plants of this sort were commonly subject to severe attacks of mildew on their leaves. Furthermore, seedlings from some crosses, such as with 'CIS', proved deciduous and un­satisfactory for my objective. Also, there is a tendency in the leaves of many azaleodendrons derived from deciduous azaleas to curl downward or inward, or to twist, a peculiarity seemingly associated with the presence of cork-like material locally on the underside of the midrib.

During the past few years, azalea pollen used has been almost entirely from strains of Exbury and Mollis plants though earlier trials included Ghents, R. molle , and Knaphills. Rhododendrons tested include a great many varieties and species, with general preference for R. dichroanthum derivatives and others having light colored, creamy, or yellowish flowers.

The only yellow-flowered azaleodendron that has proved satisfactory thus far is a 1963 cross of 'Little Pudding' x 'Exbury' - a cross that generally yields only about one dark green seedling in several thousand, the remainder being either albinistic or pale green. Tentatively, the plant is called 'In Tune' since it does seem in tune despite its parentage. The seedling is now about two feet high, has lanceolate-oblong soft leaves of good color, and in the first year of flowering, 1968, the blossoms measured 3¼ inches across. The petals vary from 5 to 7, with the flowers in trusses ranging from 10 to 12. Their color is orange yellow (Munsell hue: moderate orange yellow 10YR 8/10 and vivid yellow 2.5Y 8/12) with the margins orange (strong reddish orange 7.511 6/12 and strong yellowish pink 7.511 7/9). The flowers are quite fragrant.

Additional thrifty seedlings from several crosses are growing and with­in the next few years should flower to prove or disprove their merit. A partial listing of these follows:

'Little Pudding' x Exbury, 1966; one especially fine-appearing seedling;
R. carolinianum x Mollis (the hybrid state of seedlings verified by their modified scales)
2 plants; one has light yellow flower;
1969; Crossing tried on R. carolinianum this spring with Exbury and Mollis pollen
and additional trials are planned with "yellow tinge" and "peach" strains of R. carolinianum .
'Goldfort' x Exbury, 1967, 1968: 8 plants;
'King Of Shrubs' x Exbury, 1968: 7 plants;
Undet. rhodo. (Fabia type) x Ex­bury, 1968: 3 plants.