Mom said it was a bad weed, but I didn't care. It was beautiful.
So I find it understandable that Captain Grant, back in 1850, brought seeds from Hawaii to plant in his garden on Vancouver Island. Only three germinated. And from those three seeds, the "bad weed" jumped his property line, galloped south to Victoria, crossed the Strait to the mainland, and dug itself in, evacuating any of our native flora that stood in its way.
Scotch broom, Cystisus scoparius, is a stiff, dusty-green shrub that grows to about 3 metres tall. It was probably called "broom" because of the long, flexible, but tough new growth; a couple of branches bound together would make a quite acceptable broom.** The leaves are small and inconspicuous; off-season colour is provided by the new green branches. The flowers are a glorious, sunshiny yellow and cheddar cheese orange. In the fall, it produces thousands of seeds in black pea-pods.
Old branches, new branches, dead wood, of Scotch broom. Iona Beach Regional Park. No leaves or flowers yet; it's early spring.
Broom is well equipped for survival in our climate. It grows well on poor or disturbed soil, because it is able to fix nitrogen from the air. Photosynthesis goes on even in the winter in the green branches, giving it a head start on other deciduous plants. It is winter-hardy, tolerating temperatures as low as -25 Celsius. And it produces seed in abundance; up to 3500 seed pods per plant, with about 7 seeds per pod. That comes out to about 20 thousand seeds. Per plant. And it throws them as far as 5 metres from the parent. Then they can wait up to 10 years for conditions to be best for germinating.
Add to this, toxins that protect it from foraging animals, deep roots and a waxy coating that prevents water loss in periods of drought, and a tendency to acidify the soil, which prevents the growth of native meadow plants. No wonder three plants were enough for an invading force!
Mom was right. It is a"bad weed". If it were well-behaved, and stayed in the gardens where we planted it, things would be fine. But rampaging over the fields, it is a destroyer. It crowds out native forage plants, out-competes evergreen seedlings, grows too densely to serve as cover for small animals. It is highly flammable; even a healthy plant always contains dead wood, and the fresh branches are high in natural oils. It is mildly toxic to animals and humans, and unpalatable to foraging animals.
Eradicating a patch, digging it out, leaves a gaping hole where nothing will grow, until the dormant broom seeds take advantage of the empty space. Burning it off stimulates the seeds.
Clearing land, whether for infrastructure, construction projects, or even logging, opens new ground for broom. Once it has a foothold, it prevents the regrowth of forests, even dooming tree-planting operations.
Iona Beach, where I took the photo above, has the worst infestation of Scotch broom in all the Vancouver metropolitan area. In Surrey, we have found it on the cleared strip under the power lines; a great highway for rapid expansion. On the Island, it is moving north rapidly.
Mom's generation had a saying; "Beauty is as beauty does." I guess broom is not so beautiful, after all.
*The hedge on Buena Vista is gone now; I checked. So is the big old farmhouse I lived in, the sloping meadow behind it, the little maple wood we played in, the henhouse and the fox that haunted it. It's all close-packed housing now. We're invasive, too.
**Or, more likely, vice versa. See comment by Christopher Taylor