The weed is the New Zealand bitter cress, Cardamine corymbosa, which is very similar in appearance to our native "hairy bitter cress" which explains why it has spread so stealthily. Our British native version is maddening enough, germinating as it does all year round in mild seasons, growing in any soil, however poor, running up to flower and seeding in just a few weeks, and sending out explosions of seeds whenever you brush against it. Fortunately, however, it is easy enough to pull up by the stem, for it is an annual.
The New Zealand version is not so amenable because it appears to be perennial, with a persistent, flat crown of pale green, round leaves, held opposite each other on long stalks and one or more taproots. It has an inch-long stem with the usual four-petalled white flowers. if you try to pull it up, the stems usually break off at soil level, leaving the roots behind in the soil, capable of sprouting once more. That is the secret of its success, and the reason why it is causing nurserymen, from Scotland to the West Country, to tear their hair out.
It colonises plants in pots very quickly, particularly in polythene tunnels, and the fear is that, however careful nurseries are (and it is in their interest to wage a war against it), this weed will inevitably be passed on to customers and end up in private gardens.
Charlotte Evans, of Waterwheel Nursery, near Chepstow, has become so concerned at the probable spread of this alien weed that she has started a public-spirited campaign to alert both nurserymen and the general public to the menace.
She has learnt that the weed was introduced from New Zealand in 1975, to the Royal Botanic Gardens, Edinburgh, and probably also elsewhere in Scotland, and that it is now widespread in Scottish nurseries, from whence it has made its way south, and also across the water to Holland. In onenursery in the Midlands, I have seen how this weed has colonised small pots, topped with grit; it is very difficult to clean off this weed, without pulling out the grit and disturbing the pot's occupant. If you touch the seed- heads, they explode, sending the seed several feet through the air.
This bitter cress sets seed by mid-April in nursery polytunnels, and by mid-May outside, so, if you are buying plants from garden centres or nurseries, you should be aware that you may be bringing it home. If you see a cress-type weed in the pot, which will not pull up easily, remove it with a sharp knife, digging down into the compost to prise up the entire root. Then, just in case there are seeds on the surface remove the top half inch of compost, put it in a bag, and throw it away.
Should you find that the weed does get a hold in the garden, dig up each plant you see with a hand-fork or knife, again being very careful to remove all the taproot. Tell your friends to look out for it, and take care when you pass on plants. But, whatever you do, don't panic.