Dr. Bob Gilbert – Columnist
Have you ever looked up into a tree canopy and spotted what
appears to be squirrel’s nest, but on closer inspection you find it alive with atypical needles or leaves and abnormal growth patterns? These masses are called witches’ brooms. There are various causative agents. The most commonly seen brooms are on hackberry (Celtis). In fact, multi-brooms on a deciduous tree is a good clue it may be a hackberry. A mite and a mildew fungus cause these brooms (1). Other brooms are caused by different sources. Cherry and blueberry brooms are caused by a fungus and pine brooms by a rust, while viruses on peaches and black locust can create abnormal growth (2). The name evolved in ancient times when brooms were often found in old trees in very old cemeteries. It was believed they occurred where a witch had rested during her nightly travels (3). Now that witches are almost extinct it is understood that stresses from both environmental factors and parasites can induce these curious growths. Also, a broom that originates from one central bud on a pine can result from a genetic change (3).
Another feature about brooms is that pieces can be rooted or grafted and rarely revert back to normal growth patterns. Most stay small, becoming dwarf or miniature plants. They can produce seeds that may have further variations. When you find miniature plants, especially conifers, in a nursery almost always they were started from a broom. In fact, a whole new industry has evolved of miniature, semi-dwarf or dwarf conifers that appeal to people with rock gardens and bonsai collections.
How do you collect a broom, as they are often high up in an old tree? The tree could be climbed and the broom cut down or a crane would enable the same collecting technique. But most often a shotgun is used. A shower of broken pieces rains to the ground. The entire broom mass would not survive left whole as there are no roots. So, roots have to be created. Trees like pines do not produce roots easily. So, the small portions (scions) are grated on rootstocks of the same type. Most commonly pine brooms are grafted on Western White Pine rootstock because it is very vigorous in our climate. In fact, there are seedlings grown by specialized nurseries for rootstock grafting.
Deciduous trees and shrubs can be rooted in a greenhouse. Just about any evergreen or deciduous plant can produce a broom. Just recently I found several in a deciduous tree whose identity is not clear, likely a hackberry.
When a broom has been rooted and grown it can produce seeds. Most of the resultant seedlings maintain the parental growth patterns. Does this mean that the seeds have undergone a genetic change or has the causative agent attached itself to the seed? In fact, has the broom growth undergone genetic changes as well? Perhaps a reader can unravel this mystery?
Another source of miniature plants is seeds. Growers will set out thousands of seeds to germinate. A very small percentage of the seedlings will be different. I once visited a nursery in Tennessee that was interested in developing mildew resistant dogwoods. Thousands of native dogwood seeds were collected, germinated and planted in long double rows. When the seedlings grew big enough the owner would drive down row after row looking for mildew resistant plants or variations. He would tag the trees that looked promising and move them to a nursery bed for further evaluation. The remainders were plowed under making room to repeat the process. The Cherokee series of dogwoods came from Shadow Nursery by this method.
Years ago, a broom was noticed high up in a white pine, Pinus strobus, at Biltmore Estate. With a shotgun, portions were collected. These were grafted onto the roots of a Mexican White Pine, Pinus strobiformis. This Witches’ Broom has been named Pinus strobes “Biltmore Blue.” Flo Chafin, owner of Specialty Ornamentals in Watkinsville, Ga., has recently donated this rare grafted selection to Smith Gilbert Gardens in Kennesaw, Ga.
This article first appeared in Chinquapin, the newsletter of The Southern Appalachian Botanical Society, winter 2012, Volume 20 (4).
Dr. Bob Gilbert, cofounder of Smith Gilbert Gardens in Kennesaw, Ga.
Karen Lawrence is a professional horticulture and wild life photographer from Franklin.
1. Purdue University Witches’ Boom, Cherry
Plant and Pest Digital and Digitally Assisted
Diagnosis, West Lafayette, Indiana. www.ppdl.
2. Henderson State University, Witches Broom,
3. Alfred J. Fordham, 1967. Dwarf Conifers from
Witches’-Brooms, Arnoldia 27 (3, 4).