Getting a handle on broom (part II)

John LeBlanc

Scotch broom (Cytisus scoparius), French broom (Genista monspessulana), Spanish broom (Spartium junceum), and Portuguese or striated broom (Cytisus striatus) were introduced to California in the mid 1800s for landscape planting, mine tailings stabilization, and roadside erosion control.

Toxicity
Brooms contain alkaloids and hydroxytyramine and should not be ingested in any form. While various parts of the plant are said to be cathartic, diuretic, and emetic, large doses can cause vomiting, purging, and low blood pressure. Advanced stages of toxicity can cause complete respiratory collapse and speed up the heartbeat. Large doses have been reported to cause fatal poisoning.

Ecology
Scotch broom is native to the British Isles and central and southern Europe. Today it is estimated to infest more than 600,000 acres in California.

French and Spanish broom are native to the Mediterranean region and Canary Islands. French broom is the most widespread of the three species, reported in at least 23 counties in California. Spanish broom is less widely distributed, but appears to be expanding its range.

Distribution of all species of broom in the south and central valley is limited by dry conditions. At higher elevations, all are limited by cold winter temperatures.

Soils and Root System
Brooms can successfully invade pastures, fields, and grasslands. All grow best in dry sandy soils in full sunlight; they also grow well in a variety of soil textures and a wide pH range.

A common feature of the brooms is an aggressive root system characterized by a taproot that can exceed 2 feet in length and an extensive, many branched, shallow lateral root system. As with most legume species, brooms can fix nitrogen from the atmosphere which allows them to grow on otherwise poor quality soils.

Growth
Stem growth can be rapid (3–4 feet) in the first year. Initial rapid growth during the first 4 or 5 years is followed by 6 to 8 years of relatively slow growth followed by a period of senescence where there is more dead woody than green tissue. As plants grow, the inner stems die back providing a highly flammable fuel.

Green broom stems are photosynthetic. On mature Scotch, Portuguese, and Spanish broom, the stems are responsible for almost all of the plant¹s photosynthetic capacity. French broom also has photosynthetic stems, but the persistent leaves also contribute significant photosynthetic ability.

Reproduction and Sprouting
Brooms are often found along roadsides in nearly pure stands where soils are frequently disturbed. Equipment and contaminated road surfacing materials can spread seeds. While brooms do not establish well in highly shaded areas, they initially invade open sites such as logging roads, landings, skid trails, and harvest areas.

A mature Scotch broom plant can produce about 15,000 seeds per year. On average less than 5% of the seeds are lost to insect predation, and 98% of surviving seeds are viable.

Scotch broom, like other pioneer species, uses a variety of tactics to spread. About 40% of the seeds in any year will germinate immediately after dispersal. Another 25% germinate the second year. The majority of the remaining seed have an impervious seed coat that requires scarification to germinate. Movement along waterways, transportation in gravel and road surface materials, or exposure to low intensity grass or brush fires can stimulate germination of these seeds.

Seeds will germinate for many years even after the parent plants are removed. Seeds stored in glass jars at Kew Gardens in England remained viable after 81 years. In the field, seeds were found to germinate at least 4 years after removal.

In the Sierra Nevada, few animals eat Scotch broom seeds. In coastal conditions, quail and grouse can be relatively effective seed predators, digesting enough of the seed to effectively kill them.

Scotch broom also sprouts readily. Plants cut during the growing season can grow back to their original size in 1 to 2 years.

Control Strategies
Integrated pest management (IPM) is an ecosystem-based strategy that focuses on long-term prevention of pests through a combination of techniques. Pesticides are used only after monitoring indicates they are needed according to established guidelines, and treatments are made with the goal of removing only the target organism. Pest control chemicals are selected and applied in a manner that minimizes risks to human health, beneficial and non-target organisms, and the environment.

Through IPM practices, we can develop a strategy for controlling brooms.

Heavy shade is an effective long-term treatment. Broom is moderately shade intolerant, needing nearly full sunlight to thrive. It will grow in partial shade, but the plants grow slower. French broom appears to be more tolerant of shade than Scotch or Spanish broom. Competition from grasses and other vegetation may control broom seedlings so removal of broom should be followed by propagation of preferred plants.

Though about a dozen potential insect biological control agents have been identified and are under investigation, no releases have yet been approved.

Mechanical Control
Hand pulling of young plants is an effective strategy when the infestation is relatively small. Hand pulling is easiest when the soil is moist and is most effective before the plants produce seed, in the first two years following germination. Large plants can be mechanically removed using specialized tools such as a Weed Wrench™. When attached to the base of the plant the handle acts as a lever to remove the plant and its root system. Hoeing very young plants can also be effective. Both of these techniques cause some soil disturbance that can lead to reinvasion.

Soil disturbance should be avoided as much as possible. Seeds will only geminate in the top 2.4 inches of the soil but disturbance can bring these long-lived seeds close enough to the surface to germinate.

Only about 10% of Scotch broom stems will resprout when lopped near the base during the driest period, which generally extends from the end of July until the first rains in October. Lopping at other times can lead to vigorous sprouting. For most effective results, lop within 3 inches of the soil surface.

Mowing is also more effective when Scotch broom is under drought stress. Since drought stress and high fire danger occur together, care should be taken to avoid causing sparks with the equipment. Mowing close to the ground results in the least amount of sprouting.

Deer, rabbits, and other herbivores do not readily graze brooms, possibly due to the bitter taste of the stems and the availability of more palatable forage. However, in other parts of the world goats control Scotch broom through grazing.

Fire for Control
Using fire for Scotch and French broom control has had varied results. Some researchers suggest frequent prescribed fires to encourage regeneration and to deplete the seed bank over time. Cooler fires can encourage seed germination, followed by prescribed fires that kill the young seedlings before they generate seeds. Soil temperatures between 130°F–300°F in moist conditions stimulated seed germination. Hot fires that generated soil temperatures over 300°F killed Scotch broom seed. Getting soil temperatures this high and deep enough to effectively deplete the seed bank is difficult to achieve safely.

Chemical Control
Many herbicides are effective on broom. The concentration, timing, and method of application determine which herbicide and method of application is most appropriate. Since herbicide formulations and recommendations are subject to change, check with your Agricultural Commissioner or Pest Control Advisor for current recommendations.

Of the common herbicides available to landowners, glyphosate at a solution of 1.5–2% of the concentrated product is mixed in water. Spray the plant until it is thoroughly wet. Apply this mixture just as the flowers are blooming for most effective control.

Painting the cambium region of cut stumps with glyphosate or triclopyr at 50% of concentrated material can also be effective. Stump treatments are most effective when applied within a few minutes of cutting.

A new publication on brooms, written by John LeBlanc, Getting a Handle on Broom (ANR Publication 8049), is now available for download.