by Andrea Williams
I’ll have to work on the song lyrics—
Oh, it grows like a weed
With its pods full of seeds
That’s a broom plant!
—but “broom” is a non-specific term for any number of leguminous (in the pea family) shrubs with a very upright and twiggy habit. Supposedly they look like upside-down brooms, and an uprooted shrub could be used as a sweeping implement. Up here on the Mt. Tamalpais Watershed, of course, we’re more interested in sweeping broom off our lands.
We have three non-native brooms on the watershed that infest about five percent of our lands, but the three species are actually quite different in some ways. Our major weedy broom is French broom (Genista monspessulana), which keeps its three-parted leaves year-round. It does much better in the shade and has a harder time establishing in grasslands than the other two, probably for that very reason: The leaves can “catch” more sunlight but they will also lose water (and be in danger of dehydrating) from that larger surface area. French broom is closely related to sweet broom (Genista canariensis), which is still occasionally sold in nurseries even though it has been shown to hybridize with French broom—its genes were found in broom on Mt. Tam! Studies are still ongoing to see if this contributes to “hybrid vigor” and makes our broom more invasive than most.
Our second species is Scotch broom (or, as one of my college professors used to correct, Scot’s broom), Cytisus scoparius. Scotch broom begins the year with leaflets as well, but soon loses them. Its deep green, ridged stems do the photosynthesizing for the plant. The flowers of Scotch broom are larger and deeper yellow than French, and ornamental varieties (prohibited but still seen in the wild as “sports”) can be white, pale yellow, or pink and red. To me, it’s the broomiest of brooms, and I can imagine using it to sweep a dusty stoop on a late afternoon—maybe after a day of pulling it from the grasslands it tends to invade here.
Our third species is Spanish broom (Spartium junceum), which for a very brief period has tiny leaves, but soon loses them. Like Scotch broom, its stems do the photosynthesizing for the plant. The fat, blue-green stems of Spanish broom are tipped by large, deep yellow flowers in June and July which make it easy to spot in the dense patches it tends to form. It grows well in even the harshest spots and is harder to pull than the others because the stems tend to break off, leaving the root in the ground to sprout anew. Spanish broom has a rounded shape that reminds me of green coral for some reason.
But we have native “broom” plants too. Coyote brush (Baccharis pilularis) is sometimes called chaparral broom, although I think that name is better applied to what is commonly called chaparral pea, Pickeringia montana.
Our broomiest native, at least as far as looks, is deerweed or California broom (Lotus scoparius). Since it has deep green stems, tiny sparse leaflets and yellow pea flowers, it can be hard to tell apart from non-native brooms; but it’s smaller (one to two feet tall), and the fruits curl into pointy-toed elfin shoes, much different from the flat pods of non-native brooms.
We know non-native brooms have a lot of detrimental effects on our lands, from changing the soil chemistry and habitat structure to blocking animal (including human) passage and increasing maintenance needs to keep roads, trails and fuelbreaks open. We also know the seeds in the soil may outlive us, and broom removal is a long slog. The best thing to do is keep it out of areas it’s not in yet, and pull it quickly before it reproduces. That’s where you can help: Many of our regular habitat restoration events involve pulling broom, and brooms are three of our target plants for our hike-and-map squad of weed watchers.
Weed Watchers is a program to look for the worst plants in the best places. We know where the big patches of broom are already, but finding new sites while they are still small saves time, money, and prevents the damage these plants can do; and knowing where weeds aren’t is important in setting priorities. By signing up to be a weed watcher, you get training on the plants we’re most concerned about and how to map them (on paper maps or with a GPS). Then, you go out and hike! You can go with another weed watcher, by yourself, or with staff on a monthly guided saunter. You should have some knowledge of plants, be willing to collect data, and deal with the paradox that a successful day means not finding anything! Weed watching is seasonal, and trainings are happening on the 15th and 26th of this month. Click here for more on the program.