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Field Pennycress

Thlaspi arvense

  • Name also: Field Pennygrass, Field Penny-cress
  • Family: Mustard Family – Brassicaceae (Cruciferae)
  • Growing form: Annual herb.
  • Height: 20–40 cm (8–16 in.). Stem unbranched–sparsely branched, bristly, glabrous. Mustard-like fragrance.
  • Flower: Corolla regular (actinomorphic), white, approx. 0.5 cm (0.2 in.) wide; petals four, 3–4 mm (0.12–0.16 in.) in.) long. Sepals 4. Stamens 6, of which 4 long and 2 short, anthers yellow. Gynoecium fused, a single carpel. Inflorescence a dense raceme, extending in fruiting stage.
  • Leaves: Lacking basal rosette, leaves alternate, basal leaves stalked, soon withering, stem leaves stalkless, amplexicaul. Blade long–narrowly triangular, with sagittate base, margins curved–sparsely large-toothed, glabrous, lime green.
  • Fruit: Many-seeded, oval, flat, widely winged, 10–15 mm (0.4–0.6 in.) long silicula, notched at apex. Bristle tip shorter than wing’s breadth. Stalk approx. 15 mm (0.6 in.).
  • Habitat: Gardens, yards, flower beds, arable land, fallow land, paths, waste ground, heaps of earth, rubbish tips, seaweed piles.
  • Flowering time: June–September.

Field pennycress is probably native to central or western Asia, but it travelled to Finland with people in ancient times. The plant is mentioned in the law of 1734: “Let he who throws wild oats, field pennycress or any other weed into a neighbour’s field lose his honour and be fined… and pay compensation for damage”. Nowadays it grows commonly as a weed in inhabited areas in virtually all of Finland, especially in rich, loamy ground that has been worked. It cannot thrive among areas of dense vegetation, so is not found growing wild.

Field pennycress’s oval, flat and widely-winged silicula makes it easy to recognize. When it is flowering the lowest leaves on the stem begin to go yellow, and in dry places they can ripen their seeds even if they are almost leafless. The siliculae stay green for a long time and its long, narrow wing increases its surface area, so the responsibility for assimilation is partly transferred to the fruit towards the end of the plant’s life-cycle. The siliculae develop abundantly and each of them contains dozens of seeds. The dried, rigid stem holds the withered siliculae and they spread over the snow and ice in the winter.

A whole bunch of names for field pennycress have been inspired by its appearance when it is fruiting: the form of the silicula has been likened to e.g. a coin, a purse and a pocket. Other names also refer to its onion-like (or mustard-like) fragrance, which if eaten by cows can flavour the milk or butter. On the other hand, like many other Mustard family plants, young field pennycress shoots can be added to salads, and the seeds can be used in the same way as mustard seeds.

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