Short, White Brassicas With Long Seed Pods
What are they?
This page covers members of the brassica family that typically grow less than 50cm in height and have white - usually rather small - flowers. They all share the same general appearance of four-petalled, white flowers that are followed by elongated, slender seed capsules. For identification purposes, it is wise to wait until seed capsules have started to develop, as this will narrow your search.
Where are they found?
This is a group of only loosely related plants, so there is much variation in the habitat choices, from urban habitats to roadsides, heaths and saltmarshes. However, the habitat can be a valuable aid to identification for some species - especially the coastal ones, so be sure to check these details in the individual species notes below.
Identification
All these plants have white, four-petalled flowers and elongated seed capsules, so you need to look elsewhere for identification clues. Most important will be details of the leaves - both basal and those on the flowering stems, while tiny differences in the flowers and the capsules can help with closely related species, such as the bitter-cresses. As noted above, the habitat and location is also often useful to note.
Thale Cress Arabidopsis thaliana
A native species which is widespread and common to abundant in a wide range of open habitats, particularly disturbed ground, pavements, walls and other urban habitats. Flowers March to July. A tiny species growing to 30cm, but often much less and often only 2-3cm tall. Basal leaves entire or slightly toothed, grey-green and roughly hairy. Overall very similar to the bitter-cresses, but the whole plant is grey-green and slightly waxy, and the seed capsules are held well away from the main stem (not close to it).
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Hairy Bitter-cress Cardamine hirsuta
Native. Widespread and often abundant as a weed of any type of open ground, especially in gardens, nurseries and similar places. Flowers mostly March to July but a few plants can be found in flower more or less throughout the year. Flowers rather small and often not opening fully; only four stamens, with the two shorter ones being absent. Leaves compound, the basal ones usually with a few bristly hairs on the petiole (hand lens may be needed!). Stems relatively straight (not clearly zigzagging), hairless. Seed capsules long and numerous and carried erect and quite close to the main stem. Compare very carefully with Wavy Bitter-cress (some individuals may be impossible to correctly identify between these two species).
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Wavy Bitter-cress Cardamine flexuosa
Native. Typically a plant of wet flushes and muddy streamsides, especially in shady places, but increasingly becoming a plant of gardens and urban areas. Flowers mostly May to September but occasionally other times. Flowers rather small and often not opening fully; six stamens (two short, four long). Leaves compound, the basal ones usually with hairless petioles (hand lens may be needed!). Stems usually clearly zigzagging, hairy. Seed capsules long and numerous and carried erect and quite close to the main stem. Compare very carefully with Hairy Bitter-cress (some individuals may be impossible to correctly identify between these two species).
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Cryptic Bitter-cress Cardamine occulta
Introduced from eastern Asia, probably indirectly from various countries, via the nursery trade. Flowers probably sporadically throughout the year, but most often in spring and summer. Plants are only sparsely hairy and usually have few basal leaves at flowering time and don't show a clear, basal rosette. Flowers have six anthers and the leaves typically have rather angular, three-pointed, leaflets, most importantly the terminal one.
Only first officially recorded in Europe in 1993 and in the UK as recently as 2014. However it has been noted elsewhere as possibly present in Europe (and perhaps the UK) since at least the late 1960s and it should be noted that the first UK record came from nursery stock in Coventry that had been purchased from Norfolk! The uncertainty surrounding this plant due to its similarity to other bitter-cresses is the reason for the rather unusual English (and scientific) name.
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Large Bitter-cress Cardamine amara
A native of damp soils, usually avoiding chalky areas. Widespread, though local, along streams and ditches and especially around damp places in woodland and scrub. Flowers April to June. A showy species and much larger than other bitter-cresses; often grows in quite large colonies along muddy waterside edges. Leaves are compound, the basal leaves with rounded lobes and those of the stems more angular and narrower. Seed capsules long, linear and upright. When in full flower, the violet (not yellow) anthers are very distinctive.
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New Zealand Bitter-cress Cardamine corymbosa
Introduced from New Zealand. First recorded in the British Isles in 1985 and slowly spreading as a horticultural contaminant to paths and flowerbeds in urban areas. Still only a handful of records for our region but likely to increase, although its tiny size makes it easily overlooked. Flowers March to June and perhaps at other times. A very tiny (no more than 10cm and often much less) and rather peculiar plant that may grow as simple stems with no leaves and a single, sometimes petalless, flower. Leaves, when present, are compound but only with three to seven leaflets. When present, mature flowers are larger than those of other, small bitter-cress species, with pure white, fully-opening petals, borne singly.
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Sand Rock-cress Arabidopsis arenosa
Introduced from mainland Europe and once recorded in Suffolk in 1959. Flowers March to June. Very similar to Thale Cress but petals more than 5mm long, white or pale pink. Basal leaves have roughly toothed or lobed margins.
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Hairy Rock-cress Arabis hirsuta
Native in areas of chalky grassland. In our region, largely confined to Breckland, where it can be locally common, with a scattering of smaller populations on chalky grassland elsewhere. Flowers April to September. A small plant with basal leaves very similar to those of Thale Cress, but the stem leaves are also hairy. The long seed capsules are held erect, like those of bitter-cresses, but are flattened in one plane.
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Caucasian Arabis Arabis caucasica
Introduced from eastern Europe and popular as a trailing plant on rockeries and garden walls. Occasionally found on walls or banks away from gardens, perhaps where garden waste has been tipped. Flowers March to May. Very like a white-flowered Aubretia and often grown in gardens with that species. Differs from Aubretia in its elongated seed capsules and its larger leaves.
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Common Water-cress Nasturtium officinale
Widespread throughout our region and often common in streams, ditches and along smaller rivers. A native species, but also much cultivated. Flowers May to October. Forms low mats of full, leafy vegetation in usually flowing water, with clustered heads of white flowers. Water-cresses as a group are easily recognised but the two species are very difficult to tell apart and often indistinguishable. To be sure of the species, select mature seed capsules and check if the seeds inside are in a single or a double row. In this species, the seeds are in a double row.
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Narrow-fruited Water-cress Nasturtium microphyllum
A native species, but the distribution is hard to assess due to confusion with the far more plentiful Common Water-cress. Flowers May to October. Forms low mats of leafy vegetation in usually flowing water, with clustered heads of white flowers. Water-cresses as a group are easily recognised but the two species are very difficult to tell apart and often indistinguishable. To be sure of the species, select mature seed capsules and check if the seeds inside are in a single or a double row.
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Wild Radish Raphanus raphanistrum subsp. raphanistrum
Probably an ancient introduction from Neolithic times and now a widespread and common plant throughout most of our region on a wide range of disturbed ground, field margins, waste places, tips and similar locations. Flowers May to September. Once known, this is an easy plant to recognise, with its coarse, bristly leaves and stems and its seed capsules that have faint ribbing between the developing seeds. Plants are often relatively short, but usually have spreading branches, making them more noticeable and obvious than the species covered under the small, white brassicas. Flowers usually have dark veins and populations usually have a mix of both white and pale yellow flowered plants among them. The petals have long bases to them, forming a distinct cross shape to the flower.
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White Wall-rocket Diplotaxis erucoides
Introduced from southern Europe. Very rare, with the Norfolk, Suffolk and Cambridgeshire floras all recording a single record each. There is evidence tat it may currently be increasing, however and a small population currently persists on the North Norfolk coast. Flowers May to September. The white flowers sometimes have purplish veins like those of Wild Radish, but note the distinctive, blunt-ended seed capsules.
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Tower Cress Pseudoturritis turrita
Introduced as a garden plant from mainland Europe. Suffolk has an old record from 1910 and there is an established population growing on old walls in Cambridge. Flowers May to July. Flowers are pale, yelowish-cream to white and followed by remarkably long seed capsules which grow to 8-12cm in length and all hang to one side like a horse mane.
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