Cardamine hirsuta L., Hairy Bitter-cress

Account Summary

Growth form and preferred habitats

Characteristically a winter-annual, perennating as a leaf rosette 4-8 cm across and perfectly capable of surviving hard frosts even as very small seedlings (Salisbury 1962, p. 349), C. hirsuta is a very abundant and, indeed, cosmopolitan ruderal weed of disturbed and cultivated ground. The species is particularly loathed by gardeners on account of its short generation time, fecundity and vast seed bank.

Hairy Bitter-cress is very closely associated with damp or dry disturbed ground and with shallow, very dry soils in crevices on walls, paths, car parks, in waste places and hedgerow bottoms. However, it does occur, although less frequently, in more natural settings, eg in the shade of woods and scrub and more often in open well-lit conditions, on cliffs, ledges and screes – particularly in limestone districts. It is also said to frequent sand-dunes (Rich 1991) but, being land-locked, Fermanagh is devoid of these, although there are some local sand quarries where the plant does occur.

Fermanagh occurrence

C. hirsuta has been recorded less than half as frequently in Fermanagh as C. flexuosa (Wavy Bitter-cress) – the latter plant favouring our wetter soils. The two species are closely related (see the C. flexuosa species account above) and they can be difficult to distinguish, but there are records of C. hirsuta from 145 Fermanagh tetrads, 27.5% of those in the VC.

Identification

C. hirsuta and C. flexuosa are closely related (see the C. flexuosa species account above) and they can be difficult to distinguish. Useful field characters that separate them are stamen number (C. hirsuta almost always has four, C. flexuosa almost always has six), the leaf-stalks of the basal rosette are decidedly hairy in C. hirsuta, while C. flexuosa has a hairy, flexuous stem, usually bearing more, and more prominent, stem leaves than C. hirsuta.

Flowering period

C. hirsuta flowers very early in the season − in mild areas of the country from March onwards when temperatures become favourable. In damper soils, however, it often completes two or more generations of its brief life-cycle, flowering from March to May, and then again from fresh crops of plants through from June to October (Salisbury 1964; Grime et al. 1988). Thus, as with other common garden weeds such as Lamium purpureum (Red Dead-nettle) and Euphorbia peplus (Petty Spurge), flowering and fruiting of Hairy Bitter-cress may occur during eight months of the year in many parts of the B & I (Salisbury 1962, p. 350).

Seed production capacity and dispersal

The tiny seeds are released explosively and can travel up to 80 cm in still air whenever the ripe fruit is touched or disturbed even slightly (Salisbury 1964, p. 109). Although normally it is only a small plant, 7-30 cm tall, flower and seed production are high (even on plants dwarfed by regular or occasional disturbance). Typical small sized plants each produce an average of around 600 seeds (Salisbury 1964). Under ± ideal conditions, a single large, branched fruiting individual can shed huge quantities of seed.

As with other members of the genus, the seeds are coated with a layer of mucilage which becomes sticky when wetted, so that secondary dispersal by attachment to animals, including ourselves, and in the garden, by sticking to tools, wheels and so on, helps convey the species over longer distances (Salisbury 1964).

As is always the case, reproductive capacity is extremely variable and dependent upon the environment, especially upon soil conditions. For example, comparison of a sand dune population with an adjacent one growing on clay on a wall top, found the former produced c 98 seeds per plant, while the plants on shallow clay had a mean of 640 seeds (Salisbury 1942). In the same season, a well grown plant in a manured garden soil under little or no competition, produced an estimate of around 52,000 seeds. Under these very favourable conditions this plant showed there were increases in both the number of seeds per pod, as well as in the number of pods per plant (Salisbury 1942, pp. 44-5).

Germination

C. hirsuta seed may or may not undergo a period of after-ripening or natural dormancy. A study by Salisbury (1962, p. 390) suggests there is, if any, a very brief delay − only two to three weeks between seed release and first germination of this species, but according to Grime et al. (1988), after-ripening requires several months, and the main period of Hairy Bitter-cress germination is in the autumn. Roberts & Boddrell (1983) found some seedlings appeared in spring, but most did so in summer. Whatever the case is regarding dormancy, (and it may vary between populations), eventual germination is characteristically intermittent, generally spread over six or more weeks, and the annual's plant growth in spring, or in summer, is rapid, but very dependent on suitable temperature and moisture conditions.

Dwarfed plants and seed longevity

The plants have a shallow fibrous rooting system and very often in mid-summer, because of droughting, the plants and their flowers are so tiny that they can readily be overlooked. However, since even dwarfed plants are capable of flowering and the flowers automatically self-pollinate, unless the plants are heavily disturbed, they generally succeed in fruiting and releasing additions to the soil seed bank, where they can persist for five or more years (Roberts & Boddrell 1983; Thompson et al. 1997).

Hybrids

A sterile triploid hybrid between C. hirsuta and C. flexuosa is known to occur. Said to more closely resemble C. flexuosa, it is easily overlooked or very rare (the only BI record is in VC 47 (Montgomeryshire)), and nothing suspiciously like it has been noticed in Fermanagh (Stace 1975; Rich 1991).

British and Irish occurrence

The New Atlas shows C. hirsuta is common and widespread throughout almost the whole of B & I, becoming less frequent in NW Scotland and W Ireland. It is regarded as introduced in some of the most northerly Scottish islands.

Following the advent and increased use of herbicides from the 1960s onwards to control weeds and to maintain parks, gardens and waysides, short-lived ruderals, including C. hirsuta, have tended to increase in these types of artificial habitat, especially if they are sprayed in spring or early summer. The chemical kills off, or severely curtails, the vigour of perennial competitors, thus allowing seedlings of opportunistic, rapidly evolving, annual species like C. hirsuta the chance to fill the liberated ecological space (Warwick 1991). In recent years, Hairy Bitter-cress has become a major weed of horticultural nurseries and garden centres in B & I, since it so rapidly colonises the surface layers of bare soil in plant containers.

In the garden, vigilance and regular (even weekly!) hoeing before the plants fruit is the answer, or mulching with 5 cm of grass cuttings; otherwise it requires a resort to herbicide (Roth 2001).

European and world occurrence

C. hirsuta is an originally Eurasiatic species that has spread widely around world (Hultén & Fries 1986). In Europe, C. hirsuta is very widespread in the W & C, from the Mediterranean basin northwards to around 62°N in Scandinavia and Iceland. However, it rapidly peters out in more easterly areas and is only present beyond longitude 30°E along the northern shore of the Black Sea (Jalas & Suominen 1994, Map 2376). Beyond this, it has spread widely across both hemispheres, so that its distribution has become circumpolar southern temperate and, indeed, it is often now considered a cosmopolitan weed (Rich 1991; Hultén & Fries 1986, Map 935).

Having said this, ten plants collected in S Australia in the middle of the 19th century and previously regarded as native C. hirsuta (Bentham1863), all turned out after modern examination to be an endemic species (Kloot 1983). A large measure of scientific caution is required when assessing the world distribution of any weedy species, since very often closely related yet different taxa occur in similar habitats around the globe. In this respect, we may be talking of anything from varieties to species, or possibly even genera. On the other hand, the pace of modern air transport means that seed can all too readily hitch a ride, so that rapid invasion of new territory is a very real possibility.

Names and uses

The English common 'book-name' is 'Hairy Bitter-cress', but its flavour is not bitter at all. In fact its fresh young leaves are pleasantly tangy and delicious and nutritious in salads and, not surprisingly, reminiscent in flavour of Water-cress (Rorippa nasturtium-aquaticum agg.), but a little sweeter and more fleshy (Mabey 1972). Carl Linnaeus named the related English wildflower Cardamine amara, the Latin specific epithet of which means 'bitter'. No country folk would ever have described these plants as being bitter in flavour. However, despite this the Herbal and Flora writers copied one another and between them managed to transfer this inaccurate name to almost all the species of the genus.

The Latin specific epithet 'hirsuta', of course, means 'hairy', which is rather overstating the case in this instance, although the upper surface of the leaflets is slightly hairy.

Worldwide, C. hirsuta has numerous local English common names such as 'Spit-weed', 'Shot-weed', 'Flick-weed', 'Touch-me-not', 'Popping Cress' and 'Poppits', all of which refer to the very characteristic explosive seed release. Other names listed by Britten & Holland (1886) include 'Lamb's Cress' and 'Land Cress' (as opposed to 'Water-cress') − but the latter name is also sometimes applied to Barbarea vulgaris (Winter-cress). The name 'cress' comes from the Old English 'cærse', 'cerse', or 'cresse', with cognate names in other Germanic languages, and all these words are derived from an Indo-European base meaning 'to nibble' or 'to eat' (Grigson 1974).

Whatever we call it, we can agree that C. hirsuta is generally very abundant and very widespread, easy to weed out in the garden, but virtually impossible to get rid of entirely. Perhaps we should eat more of it in our salads!

Threats

None to it, but the species is spreading with the assistance of the horticultural trade and it is a torture for gardeners.

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