By Amy Grotta, OSU Forestry & Natural Resources Extension – Columbia, Washington & Yamhill Counties

As tree planting season winds down and the weather warms, we are already starting to see buds popping on spring’s earliest bloomers. Soon the spring explosion will be in full force. It won’t be long before the hillsides are brilliant yellow – and not with daffodils.

Colorful and abundant as it is, Scotch broom is one of the more serious forest weeds that we have to contend with. The Oregon Department of Agriculture has estimated the economic impact of Scotch broom on Oregon forestland at $47 million annually – that figure includes lost forest productivity and control costs.

Shade from a closed forest canopy is the ultimate control for Scotch broom, but unless the infestation is minor, using this passive approach is undesirable. Dense Scotch broom outcompetes desired vegetation including tree seedlings, produces seed that persists in the soil for decades, and reduces biodiversity.

Different people have their preferred methods of controlling Scotch broom. Good recommendations can be found in two publications available through the OSU Extension Catalog:

• PNW 103, “Scotch Broom: Biology and Management in the Pacific Northwest”
• EC 1598-E, “Invasive Weeds in Forest Land: Brooms”

Whatever control method you choose (hand pulling, mowing, spraying, etc.), timing is everything. As tempting as it may be to tackle your Scotch broom when it is in full flower and easy to spot, this can be counterproductive in the end if care is not taken. For example:

• Plants that are cut or mowed in spring tend to resprout. If using this control method, it is best to wait until the driest part of the summer, when the plant is stressed and before the seeds have fully matured.

• While Scotch broom is quite susceptible to correctly applied herbicide  during the bloom period, some herbicides can harm conifer seedlings in the springtime, if the buds on the conifers have already begun to swell/break. Consult the Pacific Northwest Weed Management Handbook for specifics.

Scotch broom has a few natural enemies that may aid in the fight. These insect predators, called biological controls, are researched and regulated by the Oregon Department of Agriculture.

One of these, a bud gall mite, just showed up in Oregon within the last decade. Scotch broom plants infested with the bud gall mite would have weirdly deformed buds and little to no flowering, like in the photo. If you suspect that you have seen this, contact your Extension office as there is interest in the spread of this natural enemy.