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Plant Answer Line Question

Keywords: Cytisus scoparius, Noxious weeds--Washington, Plant quarantine, Invasive plants

Is it safe to plant Cytisus scoparius 'Moonlight' here on Whidbey Island? I know that Scotch broom is thought to be invasive, but I wonder if maybe this variety is less of a problem.


Some sources (such as the State of Oregon's noxious weed control board) have said that "sterile cultivars" of Cytisus scoparius are exempt from regulations governing noxious weeds. However, the Center for Urban Horticulture's Professor Sarah Reichard, an expert on invasive species, says the following:

"The 'sterile cultivar' issue is huge worldwide. The reality is that sterile cultivars depend on the type of sterility: there are many reasons a plant might be sterile. Only a few of them can be considered to be stable under varying environmental conditions.

Regardless of what is done in Oregon, in Washington it is illegal to sell or grow any cultivars of Cytisus scoparius. Moonlight is less aggressive, but I have definitely seen it seeding out. But it does not matter how aggressive it is: it is still on the quarantine list in this state because that is the way the state law is worded. Island County may not have it on their high profile noxious weed list because it is only a B non-designate there because it is widespread. But our noxious weed (control) and our quarantine lists are two different things in this state and it is quarantined here."

For future reference, here are links to Washington State Plant Quarantine and Noxious Weed lists.

Date 2017-08-15
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Plant Answer Line Question

Keywords: galls, Cytisus scoparius, Rosa nutkana, Insect-plant relationships, Invasive plants--Control

I work on a restoration site and this fall I have been noticing weird fuzzy growths on many of the Nootka roses (Rosa nutkana) growing there. Do you know what is causing this? And is it related to similar strange growths on all of the Scotch broom? In the case of the broom, it actually kills them completely—they turn brown or black, and their roots are pretty much non-existent, which makes them very easy to uproot (which is what we are trying to do). I just don’t want to lose the roses or other desirable plants on the site.


What you are describing on the roses sounds like mossy rose gall (Diplolepis rosae). Washington State University Extension's HortSense page says that these galls which are caused by cynipid wasps will not harm the host plants. You could picky them off the roses, but that seems impractical in a restoration site, and besides they are fairly benign and attractive curiosities. The particular species of cynipid wasps which cause it are unlikely to affect plants which are outside the rose family.

Your other question about dying Cytisus scoparius (Scotch broom) is especially interesting. I think what you are seeing on those browned and blackened plants is also a gall, caused by the Scotch broom gall mite (Aceria genistae). This insect is apparently on the cutting edge of controlling invasive broom. According to this informational page from University of California Statewide Integrated Pest Management Program, the mite was first seen on broom plants in Tacoma, WA and Portland, OR in 2005. It has since spread through the Pacific Northwest. An abstract of an article entitled "The Scotch Broom Gall Mite: Accidental Introduction to Classical Biological Control Agent?" (J. Andreas et al.) appeared in the 2011 XIII International Symposium on Biological Control of Weeds. Studies are underway to see if the mite affects non-target plants such as lupine. For now, you can rejoice in the fact that the mites are curbing the growth and reproduction of the broom, and making your work a little bit easier!

Date 2017-11-03
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August 01 2017 12:36:01