Introduction

Scotch Broom or Cytisus scoparius was a very familiar sight from my childhood growing up on the Sunshine Coast of British Columbia, Canada. Every May to June the power-lines, gravel pits, logging roads, and other disturbed areas were a ‘sea of yellow’ with their yellow blooms. As a child, I did not realize this was a problem. But I learned when I was still a teenager that this was a highly invasive species that did not belong in my environment. I did not understand why it was left to grow unchecked. Why was it simply allowed to take over and replace the native salmon berries, trailing blackberries, and other delicious native species with significant wildlife values? Why was no one trying to remove the Scotch Broom?

Now I understand that things are more complicated than that. For one thing, invasive species were not really a concern many people had when I was still a child. Now there is so much more awareness. I tried to create awareness in my early 20s when I started my own local “Broom Busters” group. For a couple of years each early spring I would go out with a small handful of volunteers and cut down the broom from roadsides. I also contacted the local parks to report broom infestations and assisted with their removal. In other places I would rip out newly established plants by hand before they had a chance to grow.

Description of Cytisus scoparius

Leaves & Stems

Cytisus scoparius is a semi-deciduous erect or sprawling shrub of the Fabaceae (legume) family. It lives on average 6 – 15 years and sometimes up to 30 years. They are sometimes evergreen but become deciduous when the winters are colder or during summer droughts. It can grow up to 4 m tall and as much as 6 m across. When young its branches are upright but as it matures they often fall prostrate, particularly in areas with snow. Branches are five-angled and green. When young in particular, it photosynthesizes through their stems throughout the growing season. As they mature the branches become more brownish to grayish in color.

Scotch Broom grows from a deeply forked taproot. Like most members of the Fabaceae family, it has nitrogen-fixing bacteria in its root nodules that allow it to grow and thrive in poor soils.

On young growth the leaves are often solitary and sessile (without a leaf stalk). On mature growth the leaves become three-foliate and are either subsessile or have a petiole (leaf stalk). Leaflets are narrowly elliptic to obovate and are 5 – 20 mm long and 1.5 – 8 mm wide. The upper surface has scattered hairs and the lower surface is usually covered with short hairs.

Flowers & Fruits

Flowers are born in the axils of the one-year-old stems. They have pedicels (flower stalks) and are either in pairs or are solitary. The two-lipped calyx is hairless (glabrous) and the upper lip (partially connate sepal) has two teeth while the lower lip has three teeth. The teeth are much shorter than the lip so it makes it difficult to mistake it for a calyx with 5 sepals (lips). The flower itself is golden yellow and 15 – 25 mm long, looking much like a typical legume-type flower.

The fruits are typical legume-type pods except they are quite compressed. They are oblong and green when immature and are 2.5 – 7 cm long and 0.8 – 1.3 cm wide. When they mature they turn black. The pods have brown or white hairs along their margin but otherwise are glabrous. They are explosively dehiscent with a loud snap. In the summer near large patches you can hear the continual snapping as more and more pods split open scattering their seeds.

The seeds are small and black with 3 – 12 seeds per pod and up to 18,000 seeds per plant per year. The seeds can remain viable in the soil seed bank for up to 60 years. They are also resistant to low-temperature fires which may actually help encourage their germination. The seeds are somewhat unique in that have an elaiosome attached to them. An elaiosome is a small appendage that contains an oily substance that makes them attractive to ants. The ants then help to disperse the seeds over short distances.

Toxicity

While Scotch Broom is technically not poisonous it is considered unsafe to consume in any large quantity. This is because of its effect as a cardiac stimulant. It can cause dizziness, heart palpitations, sweating, and dilated pupils. However, no mortalities have been reported.

Similar Species Frequently Confused With

There are other species of broom-type plants that also go by the common name of broom that are non-native but introduced in North America. They are:

• Striated Broom Cytisus striatus – often misidentified as Cytisus scoparius but has a much more restricted range in North America. C. striatus however, is more of a grey-green in color than C. scoparius. C .scoparius also has larger and more abundant flowers. The pods of C. striatus are densely white-hairy compared to only having hairs along the margin in C. scoparius.
• French Broom Genista monspessulana – this superficially looks very similar. However the branches are not ridged, or green which allows for year-round differentiation. It also will not grow as tall, only reaching a maximum height of about 2.5 m. Furthermore, the seed pods are very covered with dense, long white hairs.
• Spanish Broom Spartium junceum – this plant also superficially looks similar and grows to similar heights. However, it is summer deciduous, losing its leaves early on while it photosynthesizes through its stems. The stems are semi-succulent, grey-green, and rush-like in appearance. The pods are similar however to Scotch Broom, so use the branches to differentiate the two species.
• Gorse Ulex spp – multiple species, another woody leguminous shrub with yellow flowers and small leaves. However, these are easily distinguished because they are armed with large thorns while Cytisus scoparius is thornless.

Native Distribution

Cytisus scoparius is native throughout central and western Europe east to Poland and Hungary. It is also native to northern Africa. It is very common in England and Scotland, hence the common name Scotch Broom. In Australia where it is also invasive, it is often referred to as English Broom.

Interestingly, Scotch Broom in its native habitat is capable of reaching densities where it is also treated as a weed. This is likely due to its ability to rapidly invade the disturbed sites that are abundant anywhere humanity is abundant. It has in recent years been listed as an invasive species in many European countries. This is of course controversial, as it should be. We should not consider native species as invasive. Instead, we should address the problems that helped them create monocultures in the first place. If you are interested in hearing more about ‘native invasive species’ check out my article Are Common Cattails an Invasive Native Species or Do Human Perceptions Need to Change?

Habitat Types Where Scotch Broom is Found

In its native environment, Scotch Broom is prolific in open areas, grasslands, heathlands, open forests, and disturbed areas. It is found from low to middle elevations and occasionally subalpine locations but is uncommon at high altitudes. This is likely due to its lack of hardiness to extreme cold. Scotch Broom can however tolerate moderate freezing temperatures. It seems to prefer Mediterranean-style and cool temperate climates.

Cytisus scoparius is usually found in full sun to partial shade. Full shade is not a habitat where Scotch Broom will grow. If it is found in open forest it will die out when the canopy closes as it will not grow in full shade.

Scotch Broom prefers drier sandy soils but can tolerate richer soils as long as they are not too wet. It can also tolerate very acidic soil conditions.

Human Uses of Scotch Broom

Scotch Broom was widely used as an ornamental and often still is. Newer horticultural varieties apparently pose no risk of invasiveness. However, improperly educated people still sell the seeds of the invasive type online. Sometimes inadequate labeling also leads to invasive Cytisus scoparius being sold in garden stores resulting in accidental introduction.

In herbal medicine, Cytisus scoparius is infused or made into a decoction. It is commonly used as a diuretic due to the presence of scoparin and is sometimes used to treat dropsy. Scotch Broom is also cathartic and a cardiac stimulant. It is said to be a better cardiac stimulant than digoxin (from foxglove) because it does not accumulate in the body. The flowers can also be made into a salve to treat gout. It was also sometimes used historically to induce abortions.

Cytisus scoparius is sometimes used as a substitute for hops, coffee, and capers. It was also used in the tanning of leather and as a yellow dye. The branches were used for brooms, for thatch roofs, and the bark was sometimes stripped and made into a rope.

In folklore and myth, the Scotch Broom was said to be a sign of plenty when there were lots of flowers. It was also used in weddings in place of rosemary. Its fragrance was said to tame wild dogs and wild horses. In Italy, it was burned to ward off witches.

Distribution of Cytisus scoparius in North America

Scotch Broom was first brought to Virginia for animal fodder in the 1800s. It was introduced to California and British Columbia as an ornamental in the mid-1800s. I was always told that the infestation in coastal BC was the result of 3 seeds planted by a Scottish immigrant on Vancouver Island. While I have no idea if that is true or not, it is a very prolific plant and it certainly could be true that 3 seeds alone caused the invasion. However, I do not blame the immigrant of course. Many invasive species were brought to North America that way. People moving to the new world often wanted to bring something from their homeland. And back then virtually no one understood what an invasive species was anyway so they had no idea of the future impact.

In Canada, Cytisus scoparius has been recorded in British Columbia, Prince Edward Island, and Nova Scotia. I do not know what it looks like in the Atlantic provinces but I have seen firsthand the scope of this plant’s invasiveness on Vancouver Island, the Sunshine Coast, and the Lower Mainland. There are what I call “seas of yellow” in the late spring when the Scotch Broom flowers. It is everywhere. It is also in the Okanagan and the Kootenays just not quite as widespread as it is in the coastal areas.

In the USA, Scotch Broom is found in the western states of Washington, Oregon, California, Idaho, Utah, Montana, as well as Alaska. It is fairly widespread in the eastern states where it is found in Michigan, Ohio, Kentucky, Tennessee, Alabama, Georgia, North Carolina, South Carolina, West Virginia, Virginia, Maryland, Delaware, New York, New Jersey, Washington DC, Pennsylvania, Connecticut, Massachusetts, New Hampshire, and Maine. It is also present in Hawaii. According to iNaturalist it has also been recently discovered and confirmed in Colorado, Kansas, and Texas. It is also suspected of being present in multiple other states so it is definitely still expanding its range.

In Mexico Cytisus scoparius so far has only been reported and confirmed in the Mexico City area. However, it is also present on the northern border on the USA side in San Diego, California so it is likely in northern Baja California, or will be soon. It has also been reported on the USA side of the border in El Paso, Texas. It will likely continue to spread south into northern Mexico as it continues to spread in the southern USA.

Scotch Broom has been introduced on every continent except for Antarctica. It is considered highly invasive in several areas outside of North America.

How Scotch Broom Spreads

It is primarily spread through long-distance by deliberate human introductions in gardens, and accidental introduction through seed transport in clothing, shoes, pets, tools, vehicles, outdoor toys, etc. It can also be easily transported in gravel due to its preference for gravelly habitats.

Ship ballast water is another source of long distance dispersal.

Short-distance dispersal occurs through the ants that are attracted to the elaiosomes (oily appendages) on their seeds. Birds and other wildlife also transport seeds. Short distance dispersal also occurs through humans on clothing and shoes as well as vehicles, pets, etc. It also spreads even more locally due to its explosively dehiscent fruit pods that can send the seeds several meters away from the parent plant.

Habitats at Risk of Invasion in North America

In North America, Scotch Broom seems particularly invasive in western North America. It can sometimes be locally quite common in eastern North America. However, it likely has a harder time competing with the lush and relatively aggressive native and non-native vegetation already present there. Still, it is a common sight along roadsides in particular in eastern North America.

Scotch Broom thrives in disturbed habitats. It grows abundantly on roadsides, clearings, waste areas, power lines, and anywhere the land has been cleared for development or forestry. Roadside patches are problematic because they are an easy vector for accidental long-distance dispersal through their prolific seed production that can get carried on clothes, in shoes, and on pets. Disturbed sites are the most at risk of invasion by Scotch Broom (see the photo above under the power-lines in Sechelt, BC, Canada). This threat could be reduced significantly if replanting programs are immediately implemented after a disturbance.

Cytisus scoparius also invades both disturbed and undisturbed grasslands and replaces native grasses as the dominant vegetation type. It also invades open, mixed, or deciduous forests where there is enough light to gain a foothold.

Dunes are also at risk of invasion because it prefers sandy soils. It was used as a stabilizer for dune slopes and edges in both Canada and the USA before its invasiveness was realized.

Rangelands and agricultural lands are also at serious risk of invasion by Scotch Broom, costing ranchers and farmers in labor and money to remove them. They provide little forage value to livestock and they are invasive on farmlands.

Since it is a light-loving species dense closed-canopy forests are not at risk. Wetland habitats are also not apparently at risk due to their preference for drier soils. It is occasionally however found in riparian habitats though does not appear abundant in those areas. It can tolerate some degree of water-logging, as long as it is temporary.

Impacts of Invasion by Cytisus scoparius

Given a chance Cytisus scoparius grows rapidly and prolifically, crowding out other plants to create near monocultures if left unchecked. It destroys habitat and replaces local forage crops used by wild animals, birds, and insects. There are few native species that use Scotch Broom in any capacity.

In grasslands, it threatens our native grasses and significantly reduces species richness. It currently threatens already threatened native butterflies in these habitats. It is very aggressive and tends to outcompete the other native vegetation in these areas as well. To worsen matters, shade-tolerant non-native species such as Kentucky Bluegrass, St. John’s Wort, Wall Bedstraw, and many others will grow under the shade of Scotch Broom. As a result these shade-tolerant non-native species have a much higher proportion of vegetation cover in areas where Scotch Broom has invaded compared to grasslands without it.

In logging patches sometimes the rapid growth of Cytisus scoparius prevents tree seedlings from getting re-established. This costs the logging companies significant losses in terms of invasive species removal and replanting. In Oregon, USA alone it costs an estimated $47 million in lost timber production.

Potential Benefits of Invasion

There are native wildlife species that are known to browse on the leaves and/or seeds including deer, elk, mice, grouse, and quail. However, Scotch Broom dramatically reduces species richness in habitats that it invades, reducing its potential value. Even though native mice have been found to feed on the broom, studies have shown a reduction in deer mice population in habitats where Scotch Broom has invaded.

Methods to Remove Scotch Broom

As always prevention is the preferred method of control. It, like most invasive species, is still widely sold online and in many local garden stores. Do not buy or transport any Scotch Broom. Do not plant it in your yard.

If you see them being sold online or in your local garden stores please inform them of their invasive status and ask them to do their part and cease selling them. Ask them to instead sell more native species as ecologically friendly garden alternatives to invasive species. Cultivars that are supposedly non-invasive have too frequently been found to be mis-labeled or otherwise shown to be invasive when it escapes into the wild.

Physical Control Methods

Once already established, however, physical control is always the most effective means. Physical control is labor-intensive and time-consuming but it usually causes the least amount of environmental damage.

Physical methods to remove Scotch Broom generally involve cutting them down with hand cutters, loppers, hand saws, axes, machetes, or a small power saw, depending on the size of the shrub. Be sure to cut as close to ground level as possible.

Best Time to Remove Scotch Broom

The best time to remove Scotch Broom is anywhere from late fall to early spring before it has a chance to flower again as the seeds mature rapidly and remain viable in the soil bank for up to 60 years. You should never disturb the soil in a Broom patch from late spring to mid-fall. If you disturb the soil it will allow the prolific seeds to germinate and continue to grow the following year.

It is best to pull Scotch Broom after a rain when the ground is wet as the roots will come out of the ground much easier with less disturbance to any surrounding plants that may be present.

In areas of drought cutting in early spring when in flower or when the pods are still very immature can be a very effective method. Cutting followed by drought gives the Scotch Broom much less chance to survive. Simply cut them, dispose of the shrubs and then implement your monitoring program.

Removal Methods to Prevent Resprouting

In wetter areas however they should be removed completely, if at all possible. Cutting alone in these areas results in a higher proportion of plants re-sprouting from the remaining rootstock. Cutting does work better however on larger plants that are woody and no longer green at the base. If you are unable to remove the root them to help ensure against re-sprouting you can either peel the bark on the stump down to below ground level or chop into the stump several times with a hand ax. Both methods help reduce the rate at which they will re-sprout.

Hand-operated weed pullers work very well to remove the entire plant though they are labor-intensive. You can use loppers to cut off some of the lower branches first before using the weed puller. The largest ones may require a shovel to dig them out completely. Alternatively you can chop the stumps and leave them in the ground and let the yearly monitoring program deal with them.

Low-temperature fires are not an effective means of control because this actually encourages the germination of the seeds in the soil seed bank. Furthermore, sprouts will re-grow from burned stumps left in the ground, particularly in wetter climates. However, prescribed burning can be beneficial after physical removal to stimulate the seed germination followed by physical removal of the seedlings. This is a great way to help reduce the seed bank present in the soil.

Mulching in winter or early spring may help prevent seedling germination from areas where the Scotch Broom removal involves uprooting and disturbs the soil.

Mechanical Methods

Tractor-mounted mowers and other mechanical means are not recommended for large infestations as this typically spreads the seeds around and will destroy any native vegetation that may have still been growing there. Physical removal by manual means is labor-intensive, but remains the most viable method.

Disposal of the Shrubs Once Removed

Once the Scotch Broom has been cut down they should be burned on the spot if possible. Never just dump the shrubs with yard waste, even if they were not in seed when cut down due to the tenacity of the seeds. Yard waste dumping piles are often one of the worst sources of invasive species in our environment.

If you are unable to burn them in your area due to fire bans you can solarize them. To solarize you can put the shrubs under a thick black tarp or into black garbage bags. Then leave them in full sun for 10-12 weeks to ensure they are no longer viable. Some sources recommend shorter times for solarization. However, Scotch Broom seeds are notoriously difficult to destroy so it is important they are solarized for as long as possible. Then they can be disposed of. When bringing them to a local garbage dump make sure you tell them they are invasive species so that they will bury them right away.

Chemical Control Methods

Chemical applications are almost never an ideal method of control for any invasive species. That is because chemical alteration of the environment often makes the environment more suitable for invasive species than native species. Furthermore, it is often difficult to keep the chemical control method contained so that it does not directly affect any native species that are there during the application process itself. As a result, plots where chemical control is used usually show a decrease in species richness. On the other hand, in plots where only physical control is used species riches significantly increases.

Furthermore, there are no chemical control methods that effectively target only Scotch Broom. And due to the nature of Scotch Broom, multiple applications are always needed.

Chemical control is not recommended.

Biological Control Methods

Biological control involves the use of a predator, herbivore, disease, or some other biological agent to control an invasive species once it is established in the environment. The problem with biological control is that the agent used must be entirely specific to only the target organism. This is often difficult to determine since the agent of control is also not native to the environment and could behave differently when released there. Take the old example of the mongoose and the rat. The mongoose was released in Hawaii in the late 1800s to help control the rat. To this day there are still rats in Hawaii but the mongoose has helped to decimate many native bird populations.

Biological control methods are extremely risky and should only be carried out by professionals after years of rigorous study. The use of biological control methods can never be used alone. They must be part of an integrated pest management approach. This is because the control agent would need to effectively destroy over 99% of seeds to actually control the Scotch Broom on their own. Results this high are seldom achieved in the field. However, using biological control as part of an integrated management approach can be very effective. Following is a list of biological control methods that have been used in North America to reduce the population densities of Cytisus scoparius.

Insects Used for Biological Control

The broom twig mining moth Leucoptera spartifoliella was first released in California in 1960 where it reportedly did significant damage, though the plant is still highly invasive there today. It was later discovered to be already present in Washington state through accidental introduction back in 1941 and the plant is still highly invasive in that region as well. There have been better results with Leucoptera spartifoliella reported in Australia and New Zealand.

The broom seed beetle Bruchidius villosus has been used with some success in New Zealand and Australia where up to 60% of seeds were seen to be eaten. It was accidentally introduced in eastern North America and then later moved to western North America where it has reportedly caused significant damage to Scotch Broom populations. In North Carolina, USA, where it was introduced some years ago it has been found to reduce seed production by 80%. The broom seed beetle is useful in that it feeds on the seeds of both Scotch Broom and Striated Broom so it would be useful in areas such as California where both are present in abundance.

The Scotch Broom seed weevil Exapion fuscirostre is specific to Scotch Broom. It was released in California in 1964 where it was seen destroying 60-90% of seeds produced on site. In Oregon, it was found to attack 40-60% of pods and of those pods, it destroyed an average of 85% of the seeds contained within.

The broom gall mite Aceria genistae was approved for release in New Zealand in 2001 but its maximum dispersal rate has been shown too slow to be of likely benefit to land uses threatened by Scotch Broom. It was recently found in King County, Washington but was not approved for release there so studies are still ongoing.

The sap-sucking broom psyllid Arytainilla spartiophila was released in New Zealand and Australia in the 1990s with some success. In North America, however, it was accidentally introduced into California and showed little success, likely due to the high mortality observed in the psyllid.

Mycoherbicides Used for Biological Control

Mycoherbicides, the use of fungi to harm or destroy the invasive plants are currently being investigated. Chondrostereum purpureum has been field-tested in Canada. Fusarium tumidum is being developed as a mycoherbicide in New Zealand.

Animals Used for Biological Control

Domestic goats and sheep will eat Scotch Broom. Grazing animals are only a viable option when they are to remain in place for many years. Domestic goats in particular will eat just about anything, but my readers told me that Sheep will eat this too. Grazers are especially useful in young patches rather than large, mature, well-established stands of Scotch Broom. The animals would need to be contained in a fence or on a lead that keeps them only in the Scotch Broom patch so that they cannot eat the native species that might be present there. However, as the stumps will continue to regrow for many years if the animals are removed the patch will very quickly be reestablished. This is why the grazers would either need to remain for many years or be used as part of an Integrated Management approach.

Integrated Pest Management & Ongoing Monitoring

Integrated management is usually the best approach. In its simplest and least impactful form this involves physical removal methods, possibly biological control methods, replanting, and ongoing monitoring.

Replanting With Native Species is Crucial

In all cases of Scotch Broom removal, the site should be replanted immediately. This is because the bare soil resulting from the removal of Scotch Broom will allow the seed bank in the soil to rapidly germinate and re-invade the patch. A replanting program should already be planned and ready to implement immediately upon removal of the Scotch Broom. Replanting should be timed appropriately for the region in terms of establishment success (usually early or mid spring). Furthermore, the species planted should be local ecotypes of native species already present in the area if at all possible. As the native species grow and fill in the area it provides less available habitat for Scotch Broom. Replanting must be followed by ongoing monitoring.

Ongoing Monitoring is Essential for Success

In all cases of invasive Scotch Broom removal, ongoing monitoring is absolutely essential. This is regardless of the type of control method used and whether or not replanting was also done. Yearly monitoring programs should be put in place to ensure that any surviving individuals are removed so that the population is not able to recover. It is also necessary to remove any seedlings that sprout in the newly disturbed soil. Scotch Broom is aggressive and prolific and will quickly outcompete planted or natural vegetation if yearly monitoring is not put in place.

References and Resources

CABI on Cytisus scoparius https://www.cabi.org/isc/datasheet/17610

Canadensys Plant Search https://data.canadensys.net/vascan/search

Dictionary of Botanical Terms – Lyrae’s Nature Blog Dictionary of Botanical Terms

Fire Effects Information System on Scotch Broom https://www.fs.fed.us/database/feis/plants/shrub/cytspp/all.html

iNaturalist Plant Search https://www.inaturalist.org/home

USDA Plants Database https://plants.sc.egov.usda.gov/home

Wikipedia French Broom https://en.wikipedia.org/wiki/Genista_monspessulana

Wikipedia Spanish Broom https://en.wikipedia.org/wiki/Spartium

Willis, Lyrae (2021).  Plant Families of North America. Not yet published.

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