Cytisus scoparius -- Nevada

Cytisus scoparius is native to Europe, from Ireland to west-central Ukraine and from southern Spain to southern Sweden (GBIF). Cytisus scoparius was introduced to several areas as an ornamental. It was introduced as an ornamental to CA in the 1850s in the Sierra Nevada foothills (Bossard et al 2000). Cytisus scoparius was used to prevent erosion and stabilize dunes (Bossard et al 2000). It has naturalized in many areas in the US including California, Oregon, Washington, and the East Coast (Calflora; iNaturalist; Zouhar 2005). Cytisus scoparius is also well documented as naturalized in Japan, South Africa, southwestern Australia, southwestern Canada, and Chile (GBIF; Zouhar 2005).

Cytisus scoparius has naturalized in areas in the US with a similar climate including eastern Washington, eastern Oregon, the Tahoe border of California, southern Utah, and western Texas. (Calflora; iNaturalist; Zouhar 2005). Outside of the US, there is also an occurrence in a climate matching area in Mexico (GBIF).

Cytisus scoparius is invasive is many places. Cytisus scoparius is classified as a noxious weed in Hawaii, California, Idaho, Oregon, and Washington (Zouhar 2005; WA State Noxious Weed Control Board). It is also classified as a "Category 3," widespread nonnative species by the USDA, Forest Service, Eastern Region (Zouhar 2005). Cytisus scoparius is a major forestry problem in the US, Canada, Chile, and New Zealand (Zouhar 2005; Balocchi and Sanfuentes 2024). Cytisus scoparius is also invasive in Australia, Argentina, Canada, India, Iran, Japan and South Africa (CABI).

Cytisus scoparius is classified as a noxious weed in California, Oregon and Washington; all of which have occurrences overlapping with the climate matching region (Zouhar 2005; WA State Noxious Weed Control Board). Cytisus scoparius is a major forestry problem in the US (Zouhar 2005). In Washington it displaces native and beneficial plants, causing loss of grassland and open forest (Washington State Noxious Weed Control Board). Nevada is on average much higher elevation than the area the occurrences in California, Oregon, and Washington have been found. Nevada averages over 1,000 meters in elevation. Cytisus scoparius produced less than five seed pods in sites over 1,000 meters elevation in New Zealand (Williams 1981). As a result, Cytisus scoparius may not compete as well in Nevada.

There are several invasive brooms, but I could not find many examples with occurrences within the similar climate. Cytisus multiflorus and Cytisus striatus, of the same genus, are not invasive with occurrences in a similar climate (GBIF). Spartium junceum is another broom that is invasive in California and there are occurrences within the climate matching region of California (Cal-IPC; GBIF).

Most Cytisus scoparius occurrences are outside the climate matching region. Cytisus scoparius only has a few occurrences in the climate matching areas in the US in Utah, New Mexico, Oregon and Washington. Most occurrences in the US alone are outside the climate matching areas in areas with more precipitation such as the US Pacific Northwest coastline and the Eastern US coastline. There is one occurrence outside the US that is within a climate matching area and this occurrence is in Mexico. The most dense occurrences of Cytisus scoparius are across western Europe in its native range, which is outside the climate matching region. There are also several occurrences outside the climate matching region in New Zealand, southwestern Australia, Chile, and South Africa.

Cytisus scoparius can dominate a plant community, forming a dense monoculture, replacing native vegetation (Zouhar 2005; WA State Noxious Weed Control Board). Cytisus scoparius in Washington displaces native and beneficial plants, causing loss of grassland and open forest (WA State Noxious Weed Control Board). Cytisus scoparius is a major forestry problem in the US, particularly in reforestation after logging, as it can quickly overtop commercial crop trees (Zouhar 2005). Cytisus scoparius also acidifies the soil, inhibiting establishment of other species (Bossard et al 2000).

Cytisus scoparius is a shrub, 2 to 2.5 meters tall (Jepson eFlora). Cytisus scoparius forms dense monoculture, replacing native vegetation and alterring the fuel structure (Zouhar 2005; WA State Noxious Weed Control Board). Several reviews suggest that Cytisus scoparius creates a fire hazard in Australia and California forests (Zouhar 2005). Cytisus scoparius invasions may also increase fire intensity and frequency in invaded Oregon white oak communities by increases fire hazard by creating extensive areas with large amounts of dead wood (Zouhar 2005). Cytisus scoparius is reported to increase the frequency and intensity of fires (Parsons 1992 as cited in Bossard et al 2000).

Cytisus scoparius contains toxic quinolizidine alkaloids (Burrows and Tyrl 2013 as cited in WA State Noxious Weed Control 2014). Cytisus scoparius is listed as an unsafe herb and can cause nausea and vomiting in humans. Its seeds are also toxic to ungulates, and mature plants are unpalatable and can cause digestive problems and neurologic dysfunction in horses (Burrows and Tyrl 2013 as cited in WA State Noxious Weed Control 2014). Although there have been reports of livestock loss due to ingestion of toxic quinolizidine alkaloids produced by Cytisus scoparius, it is uncommon for livestock to consume enough due to its bitter taste which can make it unpalatable to animals (Zouhar 2006; WA State Noxious Weed Control 2014). I found no evidence on whether the plant was a health risk to fish. Some animals, including deer, rabbits, and mice, have been reported to browse on Cytisus scoparius (Zouhar 2006).

Cytisus scoparius is a shrub, 2 to 2.5 meters tall (Jepson eFlora). Cytisus scoparius can dominate a plant community, forming dense monospecific stands (Downey and Smith 2000; WA State Noxious Weed Control Board 2014; Zouhar 2005).

Although Cytisus scoparius can sprout from the root crown after cutting, freezing, or fire, there is no evidence this species reproduces and spreads vegetatively from its original location (Zouhar 2005).

Although Cytisus scoparius can sprout from the root crown after cutting, freezing, or fire, there is no evidence this species spreads from detached fragments (Zouhar 2005). This species is only known to reproduce from seed.

Cytisus scoparius spreads by seed (WA State Noxious Weed Control Board 2014; Downey 2000). Studies carried out on Scotch broom seeds from a foothill site on the El Dorado National Forest in California showed that fresh seed was 98% viable (Zouhar 2005). There was a study in New Zealand that showed Cytisus scoparius produced few seeds at higher elevation, especially at sites over 1,000 meters in elevation (Williams 1981). Nevada averages over 1,000 meters in elevation. As a result, Cytisus scoparius may not produce many viable seeds in Nevada.

There are many reports of Cytisus scoparius producing well over 1,000 seeds (Zouhar 2005; Whatcom County Noxious Weed Control Board). Shrubs produce an average of 9,650 seeds per year (Bossard and Rejmánek 1994 as cited in WA State Noxious Weed Control Board 2014). Studies carried out on Scotch broom seeds from a foothill site on the El Dorado National Forest in California showed that fresh seed was 98% viable (Zouhar 2006). Seed production is lowered by drought conditions (Bossard and Rejmánek 1993 in Peterson and Prasard 1998 as cited in WA State Noxious Weed Control Board 2014). There was a study in New Zealand that showed Cytisus scoparius produced less seeds at higher elevation, especially at sites over 1,000 meters in elevation (Williams 1981). Nevada averages over 1,000 meters in elevation. As a result, Cytisus scoparius may not produce as many seeds in Nevada.

Cytisus scoparius have hard seed coats that delay germination and enable seeds to survive in the soil seed bank for at least 5 years and possibly as long as 30 years (Zouhar 2005). Scarification is required before germination can occur (Zouhar 2005). Mechanical, heat, and chemical scarification may induce germination (Zouhar 2005).

Cytisus scoparius becomes reproductive on reaching an age of 2 to 3 years and a height of about 2 to 3 feet (Zouhar 2005).

Cytisus scoparius flowers for over three months in California (Jepson eFlora; UCANR 2018). On some sites a second, less prolific flowering occurs in summer (Zouhar 2005).

Most Cytisus scoparius seed falls within 1 meter of parent plants and rarely beyond 10 feet (WA State Noxious Weed Control Board 2014; Zouhar 2005). Ants may take Cytisus scoparius seeds back to their nests, but this is unlikely to account for long distances (Zouhar 2005).

Most Cytisus scoparius seed falls within 1 meter of parent plants, and rarely beyond 10 feet (WA State Noxious Weed Control Board 2014; Zouhar 2005). Although Cytisus scoparius has a hard seed coat that can survive long distance transport in rivers and streams, it is unlikely to land in water given the short dispersal distance and Cytisus scoparius is not a riparian plant (Williams 1981; Zouhar 2005). Cytisus scoparius seed has been recovered from stream sediments up to 50 meters downstream from Cytisus scoparius shrubs, but this may be uncommon and falls short of the 100 meters to qualify for long distance (Zouhar 2005).

Cytisus scoparius is commonly used as an ornamental and its seeds can remain viable in the soil for over ten years, making it likely to come into regular contact with humans (WA State Noxious Weed Control Board; Zouhar 2005). Cytisus scoparius has a hard seed coat that allows the seed to survive for years including through rough transport (WA State Noxious Weed Control Board). Cytisus scoparius seeds have been found in samples taken from recreation vehicles (inside and out) and shoes (Zouhar 2005). Construction crews can disperse broom seeds long distances by transporting contaminated soil or gravel (Zouhar 2005).