What Is a Witches' Broom?

Delve into the world of the unusual broom-like deformation and why it is a prized rarity in conifers.

A discussion of the various spellings of witches' broom (witch's broom, witch’es-broom, etc.) is for another article but a learned explanation can be found in the British Conifer Society Journal, Autumn 2015.

A witch’s broom may be a broom used by a witch in folklore (a Besom) but in its horticultural sense it is more familiar as a diseased or mutated mass of dense deformed twigs and foliage forming a birds nest-like structure in a tree or shrub. They are the source of some of our most choice and beautiful dwarf conifers.

Normally in plants, especially evident in trees, the leading shoot will produce an auxin, a plant hormone, which will slow the growth of the secondary and tertiary shoots to prevent them from overgrowing it. Interference in this mechanism by mutations or cytokinins (a phytohormone) induced by fungi, insects, nematodes, phytoplasmas, viruses or other outside agencies can cause plant apices to develop into witches' brooms.

The fungus Taphrina betulina is responsible for witches' brooms on downy and silver birch, and the fir broom rust Melampsorella caryophyllacearum stimulates bud formation to produce large numbers of disfiguring deciduous brooms on Abies concolor and A. lasiocarpa (white and subalpine firs) in the Rockies. A dwarf mistletoe, Arceuthobium douglasii induces massive hanging conglomerations of branches on Douglas firs (Pseudotsuga menziesii) in California and Oregon.

There are many other examples, but we are more concerned in this article with the brooms caused by a genuine genetic mutation in a growing tip, not necessarily the leading shoot. These are likely to be stable and when propagated can make attractive dwarf or colorful new cultivars of horticultural value. Although they can occur in any plant, they are most often associated with conifers.

Witches' Brooms and Relevant Conifer Genera

Witches' brooms in conifers are normally associated with the Pinaceae (Abies, Picea, and Pinus in particular). They undoubtedly occur in other genera, but, maybe not so many, or are overlooked. Soft foliaged conifers like Chamaecyparis and Thuja will rapidly overgrow any mutation and they will be lost if not spotted quickly.

Many of these are color variations such as yellow or variegated white on a green plant; they are normally referred to as sports. Growth tip mutations can be color-changing or distorting, but the classic WB is a slow growing or dwarfing cluster of shoots. These obviously start small, and some stay very small, but in favourable conditions, not being shaded out or blown off the tree, they can reach great ages and size. Typically one will see a ball of irregular foliage a foot or two across. Occasionally, they will reach 4–5 (-6 or more) feet across and may be fifty years old.

Common Conifer Terrain for Witches' Brooms

My favorite was a huge broom over a metre across in a Scots pine by the side of a major ‘A’ road near Bury St Edmunds, Suffolk, England. Obviously, pretty old in 1980, it took a good few years of trying pieces before I managed to get a graft to take. Older brooms are notoriously dry and sometimes difficult to graft successfully. It made a nice little plant, but I am still hoping it will eventually produce little green cones like the ones which studded the parent broom.

Although WB’s are not uncommon in the UK, they are more typically found on conifers in countries with more serious mountainous areas. High altitude seems to trigger more brooms, as you might expect from higher solar radiation causing more mutations. One can drive around in the mountains of Colorado above 8,000 feet and see a broom or two in the roadside conifers every 328 feet. Getting at them is another matter. The best are always out of reach. In the early days, legend has it, that the European collectors would blast the brooms down from high in a tree with a shotgun, a practice continued today in the States.

Witches' Brooms as a Worthwhile Hobby

There have been collectors of WB’s for a long time now, but I suspect the early ones were only on a casual basis because of time and transport limitations in the early 1800’s. More recently, in the last thirty or so years, collecting became more intensive and has started to produce a new generation of really dwarf plants which will eventually be so useful as genuine miniature trees in rock garden work and the smaller gardens of today.

There are three main areas of the world where WB’s are being hunted; The Rocky Mountains and Cascade Ranges in the USA and central Europe. There are some, questionably obsessive, collectors of WB’s who seem to have spent a large part of their lives in the mountains hunting for ever slower growing little plants. Some have found and named or listed over a thousand WB’s and entered conifer folklore: Jerry Morris from Colorado is one, mostly collecting brooms on Abies lasiocarpa, Picea engelmannii, Pinus flexilis, and P. contorta, all at fairly high altitude. In Europe, German, Czech and Polish collectors are combing the Alps and especially the Tatra Mountains for Picea abies and Pinus mugo brooms. This is where the really tiny dwarfs seem to be coming from.

Witches' Broom Nomenclature

Newly discovered brooms in the wild and the subsequent grafts from them are often numbered with a hash number. So, if 9 different brooms have yielded scions in a particular area, they will be numbered #1 to #9. The prime example is the San Seb(SS) series of up to 1,000 numbers given to dwarf plants from different brooms collected by Milan Halada and Jan Beran from trees of Pinus mugo subsp. rotundata in the San Sebastion region of northern Bohemia (Czech Republic).

One suspects that there must inevitably be some duplication with the same mutation occurring more than once. Many will fall by the wayside, but the best will be named and propagated; SS #25 is a choice tight dark green bun now named P. m. subsp. rotundata ‘Beran’. The finder usually coins the name, which accounts for some wonderfully eccentric ones.

Names are often given after the place of origin. One of the most informative and nicest is Pinus flexilis ‘Tioga Pass’ (above Yosemite National Park), a wonderfully evocative place if you have ever been there. The wild brooms themselves are sometimes labelled to ensure they are not collected from on multiple occasions by another collector or two. It is important to take only a small part of the broom and leave some scion wood for another year in case of failure.

Propagation of Witches' Brooms

Brooms can occasionally be propagated by rooting cuttings, Picea abies in particular, but normally they have to be grafted. The normal compatibility rules apply: Picea scions onto Picea abies rootstocks, sometimes P. sitchensis, Abies onto A. alba in the past, but mostly onto A. koreana nowadays. Five needled pines onto P. wallichiana or P. armandii, P. strobus having fallen out of favour due to plants “miffing off” in our mild damp climate. Two needled pines onto P. sylvestris, P. mugo, or P. mugo var. rostrata.

Knowing the origin of the stocks may be important when it comes to siting plants: We received part of a beautiful Scots pine broom from near Madrid, Spain with the comment that it would be a very good plant for a hot dry climate. Maybe it would, but we graft it onto Scots pine rootstocks sourced from a Northern Scottish clone so that its roots will be happy in our long damp UK winters.

Some species are very prolific; others rarely produce WB’s. The European mountain pine, the P. mugo/P.uncinata complex, has thousands of different dwarf “cultivars” derived from wild collected brooms, but there are hardly any named ones of P. pinaster in spite of the millions of trees around the Mediterranean.

The Close Relationship between Witches' Brooms and Dwarf Conifers

One novelty source of new WB’s is to find them on existing dwarf conifers. It’s perhaps not so surprising as the plant must already have had a propensity for mutating. Mature plants grown from WB’s sometimes start to produce cones with viable seed. Even smaller plants have been grown from them.

The Victorian desire for little trees to complement their Lilliputian rock garden landscapes started them looking for new, dwarf, cultivars. Some were selected slow growing seedling mutations, but many were propagations from WB’s. Their penchant for collecting things also fuelled the quest for more variety and ever more dwarf plants, aided by interest from Continental nurserymen, a craze that is continued today.

The best example was seen in the world’s reputed earliest rock garden at Lamport Hall, Northamptonshire, dating from 1820, which, in about 1980, had two specimens of Picea abies ‘Pygmaea’ which had reached over two metres after an estimated 140 years (now, sadly, removed). These were almost certainly derived from a WB, as was the other classic example; the earliest recorded dwarf conifer; Picea abies ‘Clanbrassiliana.'

Seeking Slow-Growers

Planted by Lord Clanbrassil in 1798, the original is still alive in Tollymore Park, Newcastle, County Down, Northern Ireland. The illustration shows one of its earlier plantings growing at Eastnor Castle, Herefordshire. It is now a 17 feet specimen after about 150–200 years. A point that should be noted by all those who ask “How big does it get” when contemplating buying a dwarf conifer.

A similar, highly recommended, dwarf pine arising from a WB which has been around for many years, and many will be familiar with Pinus sylvestris ‘Beauvronensis.’ A fine example of an old specimen can be seen in the Heather Garden at Saville, Windsor Great Park, England. Although slow, it is now over 17 feet high. There is a tendency for dwarf or slow conifer cultivars derived from WB’s to grow faster over the years as the now missing leading shoot growth hormone inhibition has less effect.

These large ancient specimens have obviously outgrown their miniature tree status even though fresh propagations from them would remain useful slow growing little trees for many years. It illustrates why even slower growers were, and are still, sought after.

Modern Hunters of Witches' Brooms

One of the main objectives of WB collectors today is to find the slowest growing WB to produce the tiniest little plant. The limit seems to have been reached by the discovery of more than one broom consisting of simply a tight cluster of buds with no shoots. You would expect a real carpentry problem grafting a small piece, but to their credit they seem to find ways.

The most enthusiastic collectors are not concerned with aesthetics and will usually graft slightly higher on the stock (6– 9 inches) than looks right. This produces an ugly little lollipop which is easier to keep clean and weed-free, but can take many years to grow into a shapely object of desire for the garden. Having said that, deliberately grafting slightly larger growing, but still dwarf, pines and piceas onto a rootstock at 20–30 inches can produce a really attractive novelty dwarf plant on a stem which can add height and interest to a rock garden or trough, while allowing the under-planting of alpines.

Normally, one would graft as close to ground level as possible, to form a better plant and partly to hide the graft scar. Most WB’s will form a bun with varying degrees of tightness, many very attractive, when propagated. Personally, I prefer a miniature tree, with a visible trunk and some “architectural” qualities along the lines of the original Victorian concept. Either way, the best are ideal for troughs and really miniature gardens or garden railways and can do away with trimming for many years!

A Closer Look at Dwarf Conifers

For the future, there will soon be a new generation of more dwarf pines and spruces than were available in the past. Typical is Pinus mugo ‘Meylan’ a WB found on a plant of the old favorite “dwarf” Pinus mugo ‘Mops’ which nowadays tends to get too large. There are many more of these lovely neat little dwarf buns to come, look out for them in the more specialist nurseries and eventually the garden centers, but don’t expect to find all the names in the literature though.

Not all the plants derived from brooms are just dwarf with little colour variation; there are some good, bright yellow, little pines and dwarf blue spruces or oddly shaped novelties as well. Examples of plants for the future would include Picea pungens ‘Bali’ and ‘Porcupine’, the two little plants illustrated are growing in Jan Beran’s Czech garden from WB’s found in the States.

There are many more of these really dwarf blue spruces slowly becoming available. Pinus contorta ‘Frisian Gold’ found as a WB before 1962 by the Zu Jedelloh nursery is an example of a golden yellow pine. There are quite a few more, mostly P. mugo forms, many much smaller and neater to come in the next few years. A really stunning plant slowly becoming available is Abies koreana ‘Kohouts Icebreaker’. This was found as a WB on a plant of the already popular A. koreana ‘Silberlocke'.

Many of you will be familiar with the bright creamy white of the recently introduced pyramidal Picea glauca ‘Daisy’s White’. For the real enthusiasts there is now a more dwarf little globe of cream, P. glauca ‘Jalako Gold’ which was found as a tiny witches' broom on a plant of ‘Daisy’s White’. It will be scarce for many years.

A Long-Term Passion Project

A note of caution, selecting from all the thousands of witches' brooms can take many years of evaluation and deciding whether a plant is good enough or different enough to be named and propagated. Time moves slowly in this world though and, even after that, it takes many more years to multiply up enough stock and bring something new to the gardening public.

Some, especially the dwarf and choice, will always remain as collectors’ items because there is a limit to the number of scions available every year from a dwarf, slow plant producing only a few tiny branchlets. The slightly larger growers stand a chance of being commercial and setting plants onto the garden centre benches twenty or more years after being found.

Text and photographs by Derek Spicer.

This article was originally published in the Winter 2017 issue of Conifer Quarterly. Join the American Conifer Society to access our extensive library of conifer-related articles and connect to a nationwide group of plant lovers! Become a member for only $40 a year and get discounts with our growing list of participating nurseries in our Nursery Discount Program.