Grooming Brooming

Witch's Broom on Norway Spruce
Picture courtesy of Garden Professor Bert Cregg

A commenter over at the Garden Professor’s Facebook page linked to this article about Witch’s Broom, and the intrepid conifer enthusiasts who climb trees, risking life and limb to harvest them, with the hope of finding a source for the next great dwarf cultivar, so beloved of Railway, and Fairy Garden enthusiasts, and even home landscapers wanting a slower growing, denser leaved, and smaller specimen for their landscapes. From the article:

Hermsen and Larson are part of a tiny group of plant fanatics — they estimate they number about 200 nationwide — who traipse through forests collecting witch's brooms. The term, which dates to the Middle Ages, refers to tree mutations made up of dense masses of shoots growing from a single point. They look like balls of twigs woven together and can grow to several feet across. Each is the only one of its kind in the world. They are genetically unique.

They were dubbed "witch's brooms" in Medieval Europe because it was believed witches placed them high in trees, and even rested in them.

Intrigued, I started googling around, and found some interesting stuff about Witch’s Brooms.

Here’s Garden Professor Bert Cregg:

Witch’s brooms are growth anomalies that occur on various trees, most commonly conifers, Brooms can be caused by a variety of factors including diseases, aphids, environmental stress and random mutations. In some cases the growth defect is only present when the causal agent, say, a pathogen is present. In others, however, the growth mutation can be propagated by grafting scion wood from the witch’s broom onto a regular rootstock. In fact, this is the origin of many forms of dwarf and unusual ornamental conifers. Because of this, brooms are often a prized commodity and ‘Broom hunting’ is an active past-time for conifer enthusiasts such as members of the American Conifer Society.

Here’s his Fact Sheet on Spruces, and some of the cultivars originally gleaned from a naturally occurring Witch’s Broom.

‘Nidiformis’, the Nest Spruce, which itself was a witches’ broom that had been found on a Norway Spruce. Photo Credit Bill Barge

Whenever someone walks through a collection of dwarf, contorted, drooping, variegated, or otherwise unusual conifers such as the Harper Collection at Hidden Lake Gardens, one of the first questions is “Where did these trees come from?” In a few cases the trees represent relatively rare species that we don’t see often in this part of the world. However, in most cases these unusual trees represent genetic mutants of otherwise common conifers such as Pinus strobus, Picea glauca, Picea abies, Tsuga canadensis or other run-of-the-mill species. The trees we see in collections that are prized by conifer connoisseurs are the result of genetic mutations that result in unusual color or variegation, extremely slow growth rate, or loss of apical dominance. The mutations usually happen one of two ways: seedling mutants or witch’s brooms. Seedling growers that grow tens of thousands of seedlings from a given species will occasionally find individuals with unusual growth characteristics such as variegated or weeping form. Scions from these trees can be grafted onto other seedlings to propagate trees with the unusual trait. Likewise, mutations can arise within an individual tree. These are called witch’s brooms or “sports”. The unusual trees are propagated by grafting scions from the witch’s broom onto seedlings. If the mutation results in a weeping form, different forms of the tree can be produced depending upon where the scion is grafted. Grafting high on the normal tree (or standard) results in a weeping form, whereas grafting at ground level results in a prostrate or low growing form.

It’s not just conifers, either. The spotlight specimen in the Gass Garden, is a Vine Maple, Acer circinatum, first described during the Lewis and Clarke Expedition.

Witches Broom of Vine Maple - Photo Credit Harold Greer for Oregon State University

There’s a dwarf cultivar of A. circinatum named ‘Little Gem’ that was originally discovered as a witch’s broom.

A. circinatum 'Little Gem' - Picture courtesy of Oregon State University

Serendipitous web surfing can be very enlightening.