Look out of any New York window, and you’re likely to see one, but you’d be hard-pressed to identify it. The reason: ailanthus altissima, or the “tree of heaven,” made famous by Betty Smith’s 1943 book, A Tree Grows in Brooklyn, hasn’t technically been planted by anyone. And because its placement was unintentional, it isn’t counted in street-tree inventories. Still, it grows, and at a staggering rate of five feet per year and up to 49 feet tall. It’s these sorts of plants, and their smaller relatives, that we refer to as “weeds.”
I have never much noticed this species of tree, but will be looking out for it from now on. But I had already noticed its just as invasive "cousin". During a visit to London in 2010 I was amazed by the presence of the buddleja, the "butterfly bush". This "urban weed" grew literally all over Hackney.
The tree growing out of these walls must have been several years old, but no one had noticed them in time and no one had bothered to remove them. The erosion of these old (Victorian?) structures was a sad sight. But the vitality of the plant was admirable.
The damaging properties of this plant are mentioned on several botanical sites. If London is ever hit with a Ballardian catastrophe we can predict what plant will erase all traces of human civilization.
The worst invasive characteristics occur in disturbed sites, especially if the disturbance is continued or repeated. Typical areas invaded are quarries, urban wastelands, railways, gravel workings, and building sites. In the UK it has spread by wind borne seeds, following the low pressure drag created by trains, throughout the rail network past and present. Here the loose surfacings of stone and soil embankments form a happy substitute for native slopes and screes, and the thickets formed can encroach on safety zones and hamper access for maintenance etc.
Much damage is also caused to built structures in the railway environment, where any minute crack or softening of mortar, which can admit a seed, is as suitable for germination and growth as any fissure in a rock face. Deeply penetrating and thickening roots and woody stems soon force masonry apart to costly effect. It is, of course, a widely established plant also on waste ground and in many other disturbed habitats.
Strangely enough - here in the Netherlands the plant is also common, but it does not act so destructively. At least - I have not noticed it here yet.
bobulate.com - the-taxonomy-of-the-invisible
www.flickr.com - spike55151
en.wikipedia.org - buddleja
www.weed-killer.net - buddleia
Decay of a city