About
Tower mustard is classified as vulnerable in Great Britain. An 'opportunistic species', tower mustard is able to grow in areas which are extensively grazed, occasionally managed as arable land, or in conifer plantations being clear-felled. However, its usual habitat is dry sandy hedgebanks, grasslands and dry woods, thriving in full sun light and on free-draining, sandy soils. The plant also produces abundant seeds which sometimes last for many years, meaning it can often miraculously reappear on old sites after long periods of absence.
Pressures
Since the 1930s, tower mustard has been declining in number across Britain. Only 30 populations now remain in Britain, with London holding one of largest, most important populations on the concrete banks of the Stain Hill Reservoirs (near Kempton, in the London borough of Richmond upon Thames). This decline is mainly due to the destruction of the plant’s natural habitat through agricultural intensification and development. Moreover, existing habitats are often neglected, resulting in the growth of coarse vegetation that out-competes tower mustard, a plant requiring open spaces to reproduce.
How to spot
A slim, grey green plant, tower mustard’s small pale yellow flowers sit at the top of its tall stem. It is a very distinct plant which flowers from May to June. It has an unbranched stem, sometimes tinged with violet, and is slightly hairy towards the base. The base rosette of leaves resemble a dandelion and the flower petals are pale yellow.
Appearance and confusing species
Tower mustard is notoriously difficult to distinguish from the rest of its widespread family of rockcresses.
Where and when to see them
One of the last remaining large populations of tower mustard in the UK can be found at Stain Hill Reservoir, near Kempton. With careful monitoring it may be possible to re-introduce this species to areas across the UK where it was found historically.
Where
Tower mustard prefers free-draining, sandy soils in grassy and waste places over chalk or limestone. Habitats crucial to its survival include lowland calcareous grassland, lowland heathland and lowland dry acid grassland. It seems able to colonise areas which are extensively grazed and occasionally managed as arable land, such as were traditionally found in the Breckland region of East Anglia. It is also found in conifer plantations which are being clear-felled.
When
This biennial plant flowers May to July. Germination generally takes place in the spring and for the first year plants exist as basal rosettes with flower stems formed the following spring. Seedpods ripen by July and then can remain in place producing seed for quite some time (up to February has been recorded).