Field penny-cress

Field penny-cress (Thlaspi arvense) is found on wasteland throughout Great Britain, although it is a non-native species. It is possible that the plant came into the country accidentally alongside a consignment of crop seeds.

It is an unwelcome weed when it invades farmland, especially as grazing animals refuse to feed on it due to its unpleasant smell when crushed or bitten.

Field penny-cress has upright leafy stems, sometimes with branches, that grow to a maximum height of 24 inches (60 centimetres) when flowers and then fruits are produced. The leaves are narrow, toothed and stalkless, grasping the stem at their base.

The small white flowers appear from May to July. They grow on stalks and in clusters. The fruit are what give the plant its name, because they are almost circular and flat, with broad wings, albeit with a notch at the top. The size of the fruits, at up to three-quarters of an inch across, is not far short of that of a modern British penny, but it is very much smaller than the pre-decimalisation pennies that were around when the plant would have acquired its name. Curiously enough, the plant has a similar name derivation in German, being the “feld pfennigkraut” – maybe the old pfennig was closer in size than the old penny!