We normally associate the words ‘alien invaders’ with the world of science fiction … of Dan Dare (for older readers) of Star Trek and of ET among myriad others. Yet the plant world is full of alien invaders, as I often point out; and here is yet another, although I doubt whether many residents of central Scotland would think of broom as an alien invader.
Broom? - that wonderful shrub which is so common in parts of Scotland, where it covering whole tracts of Spring and Summer countryside with masses of magnificent golden-yellow flowers. Broom? - that fascinating plant which produces these intriguing seed-pods, like blackened, shrivelled green pea pods, which burst open on hot late summer days with audible pops to spread their seed. Broom - an alien invader? But what about this description of the ecological impacts of broom? “This aggressive, rapid spreading plant can grow 3 feet in the first year. It can form dense impenetrable stands that degrade rangeland, prevent forest regeneration, and create fire hazards. It invades rapidly following logging, land clearing, and burning. It can form pure dense stands for miles along highway and country roads to crowd out native species and destroy wildlife habitat.”
There are a couple of clues about the geographical origin of these sentences … ‘3 feet’ and ‘highway’ … for ‘3 feet’ means, nowadays, that we are not in Europe; and ‘highway’ points in the direction of another continent, in this case North America. This description of the ecological impacts of broom is taken from the ‘Weed of the Week’ feature in the website of the United States Department of Agriculture (USDA) Forest Service, Forest Health Staff, Newtown Square, PA. ‘Weed of the Week’ - broom? Surely this is a mistake!
No, there’s no mistake. The USDA National Agricultural Library offers as good a description of invasive plants as I have come across. “Invasive plants are introduced species that can thrive in areas beyond their natural range of dispersal. These plants are characteristically adaptable, aggressive, and have a high reproductive capacity. Their vigor combined with a lack of natural enemies often leads to outbreak populations.” And broom … or Scotch Broom (Cytisus scoparius) as USDA pefers to call it … finds itself in the company of well-known villains such as Giant Hogweed (Heracleum mantegazzianum), Japanese Knotweed (Fallopia japonica), Leafy Spurge (Euphorbia esula) and the graphically-named Mile-A-Minute Weed (Persicaria perfoliata) in a list of invasive species.
Broom is, in US terms, a plant introduced to their country by well-meaning horticulturists, a plant that has found the environment very much to its liking and which, lacking natural enemies there, has become invasive - an alien invader. And the United States Department of Agriculture offers advice on trying to control this invader. “Hand pull, hand hoe or dig out small plants; brush cutters, power saws, axes, machetes, loppers, clippers, and mowers can be used to cut shrubs; where appropriate, burning is effective to deplete the seedbank but must be repeated in two to four years.” If the American farmer or land manager doesn’t fancy such hard work, “It can be effectively controlled using any of several readily available general use herbicides such as glyphosate, 2,4-Dor triclophyr when used just after plants have flowered.” He might try biological competition by “sowing native plant species that have the potential to out-compete this shrub.” Or, like the use, in this country, of the Azolla vine weevil that I wrote about two weeks ago, he might try the Scotch broom seed weevil, the twig miner or the Gorse or broom tip moth. Broom! Broom!
Sandy Simpson, Polmont Horticultural Society