Question: How can I eradicate invasive Scotch broom that’s taking over my property?
Answer: Though often admired for its bright yellow flowers blooming from April to June, Scotch broom (Cytisus scoparius) is actually classified as a noxious weed that costs Oregonians about $40 million each year in lost timber revenue and control efforts.
Scotch broom was introduced as an ornamental in 1850 by Capt. Walter Grant to Vancouver Island, British Columbia. A native of the British Isles and central and southern Europe, Scotch broom was later intentionally planted for erosion control and to stabilize coastal dune areas. Subsequently, it has spread invasively across the whole Pacific Northwest and California.
Scotch broom is extremely hardy, invading open, disturbed sites that have dry, sandy soils, and can live from 15-25 years. It has an aggressive root system with a taproot up to 2 feet in length.
As the shrub grows, its inner stems die back, providing highly flammable fuel for wildfires — just another reason to eradicate this invasive shrub. Scotch broom spreads solely by seeds held in pods which split when dry, popping explosively and ejecting seeds some distance from the plant. Each plant can have 18,000 seeds, which are viable for up to 80 years. Scotch broom also tends to acidify the soil around it, inhibiting native species from establishing.
Even though it can be a pretty daunting task to get rid of this broom, there are things that you can do to bring it under control.
Prevention, of course, is the best method to use where the ground has been disturbed by overgrazing or development. In such cases, you want to quickly re-vegetate the area using fast-growing native trees such as Douglas fir or red alder, shrubs such as currants and snowberry or native grass mixes.
Since you already have Scotch broom on your property, there are mechanical, chemical and Integrated Pest Management methods for eradication.
Mechanical: Pull smaller shrubs (less than one-half inch) by hand or with a weed wrench, making sure to pull up the whole root. Larger shrubs should be cut to the ground before setting seed. Watch out for seedlings each spring and pull them up by the roots while they are small.
Mowing is sometimes done to knock down large patches, but be sure to mow before seed pods form. Keep in mind there is a good chance seeds that are already on the ground could be spread by this method. Mowing will result in dense, multi-stemmed regrowth which is easier to treat chemically.
Chemical: Several broad-spectrum herbicides such as glyphosate (better known as Roundup), Garlon 3A or 4 and Crossbow are all effective, but be sure to read and follow the label.
With any herbicide, be careful to minimize drift and injury to non-targeted plants, applying the herbicide when temperatures are 60-72 degrees on non-windy days that are followed by 12 hours of no rain. Spray is most effective before and after bloom when the Scotch broom is growing vigorously. Treat new seedlings yearly.
IPM: Mow in early spring. Cut shrubs that are 1 inch or larger in diameter in late summer to the ground. These stumps often don’t need herbicide treatment. Treat regrowth in the fall or the following spring with Garlon or Crossbow. The Oregon Department of Agriculture is now experimenting with a biological control — the release of a seed weevil — which is showing some promising results.
Be sure and clean all equipment used at the site. Pulled plants should be burned or tightly bagged prior to disposal at the local landfill.
Scotch broom is very difficult to control because of its longevity, tolerance to drought, lack of natural enemies and the longevity of its seed banks. However, with persistence, vigilance and several years of using the above methods until the seed banks are depleted, it is possible to remove this troublesome shrub.