Black mustard new to the North-west Highlands

November 27th 2015

Black mustard new to the North-west Highlands

Back in July, I noticed an unfamiliar large-leaved plant in one of the flower beds in my garden at Solus, Torbreck (NC085242). It rapidly developed into a very large member of the cress family (Brassicaceae), standing over 2m high until blown over. The widely branched inflorescence bore large numbers of yellow flowers which smelled strongly of honey and were very attractive to insects such as hoverflies.

I collected a sample on 19th August to show to Ian, but he was unable to identify it in the absence of mature fruits, which are diagnostic in this family. However, fruits had appeared by mid-November and he was then able to name it as black mustard Brassica nigra. The important characters are a combination of yellow flowers and short-beaked seed pods closely appressed to the stem. This is a crop plant elsewhere in the world, but is now rarely grown for seed in the British Isles. It is regarded as a native species, occurring on sea cliffs, riverbanks, rough ground and waste places. A current distribution map shows it to be widespread in England and around the coast of Wales. It is much rarer in Scotland, where it has been recorded from the Borders and Central Belt, with five older records from the south side of the Moray Firth (together with one on Fair Isle and one in Shetland). It has not previously been recorded from anywhere in the North-west Highlands.

I have no idea how the plant arrived in my garden; I did not plant it, and so far as I know it is not a constituent of either ‘wild bird seed’ or ‘wild flower mixes’. Ian is going to record it, with voucher specimens, as an addition to the native flora of Assynt and West Sutherland.

Gwen Richards
24.11.2015

Whole black mustard plant. Photo G. Richards
Whole black mustard plant. Photo G. Richards
Hoverflies on black mustard. Photo G. Richards
Hoverflies on black mustard. Photo G. Richards
Black mustard seed pods. Photo G. Richards
Black mustard seed pods. Photo G. Richards

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