Brassicaceae - UK wildflowers - Five-minute families

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Learn how to identify common plant families in (more or less) five minutes.

In this series, I aim to give you a quick introduction to some of the common plant families found in the UK. Learning to identify plant families is a great help in identifying individual plant species you come across – if you know the family, then you know where to start looking for the species in a guide, and identifying the family is an achievement in itself. Plants are classified into families based on characteristics that they share; this is then narrowed down further into genera and then into species. So, the way to identify a plant’s family is to know these characteristic features.

Today we're going to look at the Brassicaceae or cabbage family. This family is still sometimes referred to by its old name of Cruciferae, so called after the pattern of the plants’ flower petals. The 4000 or so species of mostly annual and perennial herbs (plus a few shrubs) are found all across the world, though predominantly in the Northern Hemisphere.

Many of our vegetables and greens come from the Brassicaceae, such as cabbage, kale, Brussels sprouts and cauliflower - all varieties of (Brassica oleracea) - radish (Raphanus sativus), turnip (Brassica rapa) and watercress (Rorippa nasturtium-aquatica), as well as ornamentals like stock (Matthiola) and wallflower (Erysimum cheiri). Brassicaceous wildflowers include honesty (Lunaria annua), garlic mustard (Alliaria petiolata) and cuckooflower (Cardamine pratensis), as well as some usually considered weedy - for example, hairy bittercress (Cardamine hirsuta) and shepherd’s purse (Capsella bursa-pastoris).

The UK species are annual or perennial herbs.

The flowers of the Brassicaceae are borne in clusters and are usually white- or yellow-petalled, sometimes pinkish or purplish. The flowers have four petals arranged in a distinct pattern like a cross, with four sepals that tend to alternate with the petals. The petals are usually equal in size. There are normally six stamens. The flowers are insect pollinated.

The ovary is attached above the rest of the floral parts, so is a superior ovary. This ripens to a fruit that resembles a capsule with two sections, one on each side of a membrane. These fruits split along two valves, away from the membrane, releasing the seeds. The fruits of some species have what's called a beak on the end. The shape and size of the fruits and whether or not they have a beak are important id features for individual species.

Brassicaceae leaves may be entire, lobed or almost divided and are arranged alternately up the stems, although some species have a basal rosette of leaves. Compounds in their tissues give many Brassicaceae a cabbagey or mustard smell when crushed. This is also what gives them their recognisable flavour.

So, a plant with flowers that have four equal petals in the shape of a cross or square, with four sepals, and which has alternate leaves and fruits with valves is most likely to be in the Brassicaceae.

Look out for these three species in your local area:
• Hairy bittercress (Cardamine hirsuta)
• Honesty (Lunaria annua)
• Shepherd's purse (Capsella bursa-pastoris)