Seek out a garden anomaly, the witches’ broom

We often see witches’ brooms in winter when looking into the tree canopy of an otherwise barren landscape. There is an abnormal cluster of growth, with shortened stems and needles, maybe in the shape of a ball.

It is a mutation that occurs along the branch of a tree when the growth hormones become overactive. Typically, branches grow from the tip outward. A hormone, called auxin, prevents other buds along the branch from growing at the same time. In a witches’ broom, all of the buds grow at once, producing a tightly congested collection of short, thick stems and foliage (only in summer) or needles. So the two primary features are increased growth and dwarfism.

The genetic change or mutation is caused by stress on the plant. This stress could be an insect such as aphids or mites, or it could be a disease such as a virus or a bacteria. We know for certain that the dwarfism, or smallness, is related to genetic factors. When cuttings from the witches’ broom are grafted onto normal rootstock of the same plant, they do not grow out of the condition! And will continue to produce dwarf plants in future generations at a rate of 50 percent.

Plant collectors, or in this case, “broom hunters” or “broomers,” have developed a significant group of dwarf plants derived from witches’ brooms, primarily from pines and spruces. Picea glauca “Montgomery” (Montgomery Dwarf Blue Spruce) and Pinus taeda “JC Raulston” are two examples. The cultivar names are often taken from the property owners where the tree with the witches’ broom was found. Such hunting is not without some risk. The brooms are frequently located 50 or 60 feet from the ground. The hunters often climb the tree and cut the broom out, or they might use a shotgun to blast it apart from the branch.

The entire broom can be grafted onto a rootstock and maintained as a dwarf plant or it can be grafted onto a 2- to 3-foot trunk and kept in a pompom form. Dwarf conifers have a very slow growth rate and tend to be expensive plants at your garden center. While the round headed shape such as the pompom is most common, others may be prostrate spreaders, upright verticals, or weeping forms.

Collectors take cuttings from the broom and share those with their colleagues.

At Hidden Lake Gardens, a botanical park maintained by Michigan State University Extension, each cutting is grafted onto a seedling tree of the same genera. So a pine broom cutting goes on a pine seedling tree or a spruce broom cutting goes on a spruce seedling tree. When the cutting begins to exhibit new growth, they know the graft has taken and the rest of that original seedling tree is cut away. What is left is a witches’ broom that grows independently and from which additional cuttings can be taken. Hidden Lake has a world-renowned collection of over 600 rare and dwarf conifers, and its staff actively searches out witches’ broom for propagation.

Naturalists and plantsmen have noticed brooms for centuries. Historically, they were thought to be lairs for witches and hobgoblins. But in 1891, Pinus sylvestris “Beauvronensis” was grown from a witches’ broom of Scotch pine in Beauvronne, France. Earlier, Picea abies “Maxwelii” came from a Norway spruce in 1874.

Dwarf conifers make excellent plants for suburban gardens. They will not outgrow their allotted space, as so many of the other conifers do. Most require about 6 hours of direct sunlight and well-drained soil of average fertility.

All of them will need to be watered regularly in their first year until they are established. Check with plant fact sheets to see about drought tolerance.

Ann Auerbach is a Rutgers Master Gardener in Camden County. Send your lawn and garden questions to njgarden@camden county.com, and include ‘Courier-Post’ in the subject line if you’d like to be considered for write-up in the column. A Rutgers Master Gardener will respond to all questions received. Visit our offices at the Camden County Environmental Center, 1301 Park Blvd., Cherry Hill, 08002, 856-216-7130. Master Gardeners are there from 9 a.m. to noon, Monday through Friday.