The Apiaceae, the plant family that gives us carrots, parsnips and parsley, among other staple foods, also contains some horrors. Let me list them: hemlock, hemlock water dropwort, fools' parsley and giant hogweed,and this list is not exhaustive. Heracleum montegazzianum, the giant hogweed, looks pretty, with its foam of white flowers that lure the unsuspecting to draw closer. As the plant looms over you, as it can grow very tall, you might be tempted to pull off a sprig to take home, and then your problems begin. The sap is phytotoxic, and once on your skin it reacts with sunlight to produce a very nasty rash, a painful reddening of the skin that produces an powerful urge to scratch. And you dare not get it in your eyes! The skin needs protection from light for at least several days, if you are lucky, and some think that it might produce lifelong damage.The British and the US authorities are as one on this matter, for they both insist that you are not allowed to plant this weed in your gardens, and in the USA you cannot transport it over state boundaries.Many gardeners, farmers and countryside managers simply destroy the stuff. To be brutally honest, as a grandfather who would like to take his grandchildren walking, I don't want this weed around and am happy that it be eradicated.The plant often grows on riverbanks, so you need to take especial care when walking in such places.
We do not have poison ivy growing in Britain. This plant is in the genus Toxicodendron, whereas our native ivy is in the genus Hedera. We don't normally count ivy as a weed, it is simply part of the landscape, but tree growers might think it a weed sometimes.
There are others that are environmentally destructive, and I am thinking of Japanese knotweed and Himalayan Balsam. I saw a mass of Himalayan balsam while taking a Sunday walk with my wife Maureen. We were strolling down the embankments in the Mersey Valley, but part of our walk took us along the banks of a small stream, which were becoming smothered with the purple flowers of this invasive weed. It is a beauty, never say that weeds are ugly, but it clogs up water courses. Introduced by Victorian plant hunters in the nineteenth century, it has naturalized too well and the blockages that it causes disrupt water flow in places and smother native riverbank flora.
Then there is Japanese knotweed. If you are selling your house, the questionnaire asks whether you have suffered Japanese knotweed. If you have, it is very difficult to sell, as the sale value drops savagely, for it can break through floors and needs specialist treatment. I have seen it kill a small elder tree, as the weed starved the elder of resources when it took over the embankment on which the tree stood, and it can cause riverbanks to collapse. Someone in the nineteenth century introduced it because it looked nice! Thanks! We are having to have a national eradication strategy to deal with it. It is illegal to grow the stuff and you must destroy any plants on site