Gardening: Attack of the killer weed

The Kiwi cousin of the Bitter Cress weed is threatening to strangle our gardens. Ursula Buchan offers advice on how to identify and eradicate this deadly plant
I don't want to sound like Corporal Jones in Dad's Army, running round in a tizz shouting "Don't panic", but I think you ought to know that there is an aggressive alien weed now on the loose in British nurseries, which has the capacity to make our gardening lives far more difficult.

If we all take some care to refuse it entry into our gardens, and eradicate it if it does appear, there will be no problem; if not, we are in for a wearing time.

The weed is the New Zealand bitter cress, Cardamine corymbosa, 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, though. It appears to be perennial, with a persistent, flat crown of pale green, round leaves, which are held opposite each other on long stalks. It has an inch-long stem with the usual four-petalled white flowers and one or more taproots. If you try to pull the plant up, the stems usually break off at soil level, leaving the roots behind in the soil, and thus being capable of sprouting once more.

That is the secret of its success, and the reason why it is currently 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 certainly 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 recently at the probable country-wide 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 in 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 one alpine nursery in the Midlands, I have seen how this weed has successfully colonised small pots, topped with grit; it is extremely difficult to clean off without pulling out the grit and disturbing the pot's true occupant. If you touch the seed-heads, they explode, sending seed several feet into 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 now, you should be aware that you may be bringing it home with you.

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 so that you prise up the entire root. Then, just in case there are seeds on the surface that are ready to germinate, remove the top half an inch of compost from the pot, put it into a bag, and then place the bag safely in the dustbin.

Should you then 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 every bit ofthe taproot, Tell all your friends to look out for it, too, and take care when you pass on plants to them. But, whatever you do, don't panic.