Scotch broom (Cytisus scoparius)—a pest for all seasons
This information was compiled in April 2011 by GaLTT member Rufus Churcher. He describes Scotch broom and its habitat, explains why GaLTT is advocating its island-wide eradication, and suggests the best methods for its removal.
What is Scotch broom and how did it get here?
Scotch broom is a plant of European origin, probably from Scotland. Linneaus (Carl von Linné) described it botanically as Cytisus scoparius—a member of a very successful temperate genus. The name ‘broom’ is said to derive from the tight bundle of twigs that makes a besom or whisk broom, but this origin is unsure.
Scotch broom is said to have been introduced to Vancouver Island from Hawaii in the 1850s by Capt. Walter Calhoun Grant who planted it on his farm near Sooke. It is also said to make good winter-feed for horses but I have never known it fed to horses either in Scotland, England or Canada. The use of its light foliage as fodder would be surprising as all parts are heavily alkaloidal and poisonous to many vertebrates, including humans, especially small children.
Habitats and local range
This invasive plant is common on highway shoulders, roadsides, abandoned logging trails, forest clearings and waste and open areas on Vancouver Island and smaller Gulf Islands, north to Campbell River, and it is still extending its range further north. It will seed in under second growth natural forest margins and fringes of canopy shadows of larger trees. Its seeds get included in the mud in the treads of tires, especially of heavy and off-road vehicles, and so is distributed throughout our highway network. Birds, when they consume the seeds, seldom can crack them all, and so carry fertile seeds far and wide. This accounts for the occurrences of ‘spot’ growths of broom in isolated roadsides, and in cleared forest areas such as the Weldwood Lands and 707-Acre Park.
The threat to the natural ecosystems of Gabriola
and Vancouver Island
People often admire broom’s glorious spring show of yellow flowers, comparing it to those of daffodils and others, but many do not realize that, as a plant, it is too successful and able to out-compete the indigenous plants in reproducing, in taking over space, chemically inhibiting neighbouring vegetation, discouraging browsing, and holding viable seeds till the opportune moment to germinate arrives. It can actually replace the natural forest. The only plant that I have seen out-compete broom is another European invading immigrant, the European or Himalayan blackberry when a wall of blackberry vines 3-3.5 m high killed the broom by depriving it of light.
Habitat, growth, and spread of Scotch broom
Broom prefers acid soil and good drainage, but is not particular about soil fertility. It withstands drought. Broom is a bush, with vigorous growth, strong branches and leaders adorned with thin, near parallel, lateral twigs. It matures into a strong shrub with main stems up to 7.5 cm in diameter. The twigs are green, hairless, and grooved. It will regrow from cut stumps if green bark is present and the cut is clean. Old plants become leggy at 1.8 metres and higher. In winter it is generally leafless but with green stems. Wild Scotch broom is usually over-mature after 10 years and plants or branches begin to die and stands to thin. New generations of seedlings eventually replace mature stands.
Leaves: New spring leaves are small, pointed ovals, less than 2.5 cm long, and in summer they trifoliate with three leaflets at end of short petiole/rachis or single pointed leafs on stem. The leaves are often shed in full summer (July-August).
Flowers: The flowers are pea-like, in bracts, usually yellow in wild Scotch broom in Britain and North America, appearing in May-June. But white (C. leucanthus or albus) and purple flowered (C. purpureus) varieties exist, and a Canadian mutant or sport has paired liver red inner petals hooding the stamen and anthers and yellow wing petals.
Garden cultivars have white or cream flowers and more compact growth habits. Such a form is used in highway plantings (along the main road into Victoria) and is feral in our 707-Acre Park. It appears to be not as vigorous as the natural species. Hybrids have also been observed. Other species and varieties exist of this highly successful genus, including the trailing white flowered C. kewensis.
Fruits: The small pea-like pods are about 4 cm long and 0.5 cm broad. They are uninflated, with seeds showing as bumps along the spine. Their colour is leaf-green when developing and fully developed, turning black and hard as they dry, between July and September. The seeds are small (around 3 mm diameter) and very hard. They are high in alkyloids and not attractive to birds, especially North American birds, and are toxic to many mammals, including humans.
Regeneration: When a broom bush is damaged or cut, woody regeneration can occur. New plants will not come up from seed when in the shade of mature broom plants as broom’s fallen litter inhibits the germination of its seeds. However the seeds are often flung beyond the parent plant, up to 3 m away. Seeds will germinate under other types of cover, especially broad leaved litter, e.g., big-leaf maple. When mature broom cover is removed, new plants will appear only after the old litter has rotted (2-4 years).
Germination can be encouraged by breaking the surface to help break the seeds’ dormancy, so strong raking of a cleared stand encourages new germination. Seeds are long-lived and instances of Scotch broom germinating in places where it formerly had been abundant some 10 or more years previous are noted, with instances of 20, 30 and even 60 years reported. So, once broom is established it is almost impossible to eradicate it entirely.
Eradication of Scotch broom
Removal is best done by hand or with hand tools; heavy mechanical treatments seldom remove all the plants or regenerating shoots, and never remove all dormant seeds.
First remove mature plants
The first step is to cut the large above-ground stems and pull/extract the smaller plants with their roots, before the pods have matured. If a stump is impossible to pull and the cut surface is in green wood, cut the wood away into the root so that no green-barked wood remains.
Next it is advisable to pile the cut tops on a patch of ground from which all broom has been cleared. Let the tops dry thoroughly over the summer (you will hear mature seedpods ‘popping’ open and spitting the seeds up to three metres away); burn when dry. This will also bake the soil below the fire and kill any seeds that are in it.
Do not, in the aim of tidiness, collect the tops and truck them to another burn site as there is a high probability that there are some branches that still carry dry pods with fertile seeds and these will drop along the removal route and the burn site become an inoculation centre for a new broom population.
Do not, out of economy, use the cut branches for light fencing or pea sticks, etc., as they may carry live seeds in dry pods. And, if the weather and soil are suitable, the smaller pieces may act as cuttings and take root!
Continue to remove new plants
As seedlings will continue to appear on an old broom site for many years, and as new seedlings can bear fertile fruit in their first year, it is necessary to remove all broom plants every year, usually with visual sweeps more than once a year. Germination may take place in the fall (a benefit of the second ‘fall’ spring that the Gulf Islands enjoy) and as broom may grow slowly throughout our winter, whenever temperatures are above freezing and if there is no lying snow, new seedlings may bear a few flowers and fruit early in the season. They may easily escape notice and the eradication will then be unsuccessful!
Note that:
Broom seeds from a single year don’t germinate in a single spring. Farmers and gardeners want as many seeds as possible to germinate in one season to give as uniform and dense a crop as possible. Wild plants want their seeds to germinate over a few years so that a failing spring does not mean total death of one seed crop. The oldest seeds known to have germinated are a water-lily (about 2000 years, Egypt) and a date palm (more than 2500 years, Palestine).
Broom rootlets bear clusters of small spherical nodules with brown or dark centres. These are nitrogen-fixing nodules like those found on the roots of clover. (Both broom and clover are legumes.) Atmospheric nitrogen is fixed by the bacterium Azobacter as ammonia and thence into a nitrate fertilizer for the broom plant. Unfortunately little of this nutrition reaches neighboring plants—this is part of broom’s chemical warfare to outcompete other plants.
The broom genus Cytisus and its near relatives are indigenous to the Old World in the Mediterranean and Atlantic coasts of Europe and North Africa from Morocco to Scotland. Gabriola, from broom’s viewpoint is like its home in Ireland, Scotland or Spain. Thus it thrives.
Broom is susceptible to broadleaf plant poisons but the cuticle is thick and waxy and most of the chemical runs off. This treatment is inefficient as it kills all the neighboring plants, which, unless they happen to be broom, constitute collateral damage.
Selected References:
Reader’s Digest ‘Illustrated Guide to Gardening in Canada, 1979.
Originally published by The Reader’s Digest Association Limited, London, 1975.
‘Trees, Shrubs & Flowers to Know in British Columbia and Washington’, by C.P. Lyons and B. Merrilees. Lone Pine Publishing: Vancouver, 1995.
‘[A Field Guide to] Trees and Shrubs’ by G.A. Petrides. Peterson Field Guides. Houghton Mifflin Co.: Boston, 1986.