JARS v48n2 - Shortcuts to Short Azaleas

Shortcuts to Short Azaleas
L. Clarence Towe
Walhalla, South Carolina

It is probable that most rhododendron and azalea fans would like to find, breed, hybridize or otherwise develop a truly unique plant. While some are fortunate enough to do so, there is an opportunity for the home propagator to select a new plant which has ornamental and perhaps commercial value without years of evaluation.
The source of this opportunity is that of the witches'-broom that occurs on evergreen azaleas. To say there are already enough azaleas is probably an understatement, but superior dwarf and compact selections are increasingly popular for rock gardens, bonsai and patio plantings.
Since a broom frequently carries many traits of the plant to which it is attached, an azalea propagated from a broom, except for size, will be similar to the parent plant and should perform in a similar manner. Therefore, brooms found on better azaleas should have the most potential. A commercially available azalea should have been evaluated for certain attributes prior to its selection and release and is therefore a good choice.
Why brooms occur is not entirely understood, but they are probably caused by a variety of factors. It has been demonstrated that leafhoppers infected with mycoplasma (virus-like organisms) can cause bud mutations in some pine species and are also suspected to be a cause of brooms in azaleas. While brooms on most broad leaf plants revert to their normal growth habit when rooted or grafted, azalea brooms are usually stable.
An azalea broom typically occurs as a tight, congested ball or mound of growth (Fig. 1). Its source of origin was a mutated vegetative bud from which all subsequent growth was abnormally small. It may be healthy or may have diseased areas due to the crowded foliage. While attached to the parent plant, the broom may not flower or may flower sparingly. The flowers will usually be disproportionately large to the tiny foliage, but should be smaller than those of the parent plant.

Witches'-broom on 'Pink Cascade'
Witches'-broom on 'Pink Cascade'*

It is not unusual to find more than one broom on an azalea and not unusual for multiple brooms to vary slightly in compactness. After being rooted, this difference may disappear; thus the size difference could be due to the degree of restriction of nutrients at the point of attachment of the broom to the parent plant.
Sports, sometimes referred to as bud or branch sports, also occur on azaleas. A sport is similar in leaf or flower size to the parent plant but with a different flower color, leaf color or leaf shape. When propagated, a sport will develop into a plant slightly smaller than or equal to the parent plant. There are, however, mutations intermediate between brooms and sports.

Witches'-broom on 'Silver Sword'
Figure 2. Witches'-broom on 'Silver Sword'

As an example, a large shipment of azalea 'Silver Sword' to a garden center was found to have one plant with a single broom (Fig. 2) and one plant with two brooms, each with tiny leaves about one-fourth the length of those of the parent plants. A "broom" on a third plant had leaves and flowers about two-thirds the size of the parent plant but with heavy wood and an open growth habit. Each of the four had the same leaf pattern and flower color of the parent plants. When told that the growths were caused by disease, the owner was most happy to have all four plants carted away, at no charge, to keep the outbreak at bay. Cuttings taken from a broom are usually short and have to be stuck carefully to insure contact with the rooting medium. After rooting, a cutting will increase in leaf size and internode length and will develop into a slightly larger, more open plant than the broom from which it originated.
While dwarf and compact azaleas can be developed through selective breeding and lengthy evaluations, brooms are frequently ignored. Two popular azaleas that originated as brooms are 'Rukizon'* and 'Chinzan'. An azalea propagated from a broom must be given a new name, but registration and subsequent descriptions should reference the parent plant and its origin as a broom.

Clarence Towe, a member of the Azalea Chapter, is a frequent contributor to the Journal.

Editor's Notes: * Name is unregistered but not in conflict with a registered name.