Sweet-flags
What are they?
For a long time, the sweet-flags were considered to be members of the arum family and they certainly have some similarities in their strange flower spikes. But the flower-spikes of sweet-flags lack the spathe that surrounds the spike of arums and these plants are now put in their own family, the Acoraceae. The taxonomy of sweet-flags is currently marred by much disagreement as to how many species there are and what their native distributions are. Various forms have been introduced throughout much of the world, but Central Asia appears to be the most likely origin of many of the forms.
Where are they found?
Sweet-flags grow on the margins of lakes, rivers and canals, or occasionally in more seasonally wet areas such as ponds or ditches.
Identification
These plants are relatively easy to identify by their combination of iris-like leaves and strange spikes of dense, petalless flowers. A close look at the flower spikes with a hand lens reveals a densely packed mass of flowers, arranged in a spiral. Each flower consists of a central, green stigma, surrounded by six, yellow stamens.
European Sweet-flag Acorus calamus
Introduced from mainland Europe for its herbal qualities. Populations of this plant persist along a few of the regions waterways, especially along the Waveney, Little Ouse and Yare valleys and at scattered lakes and ponds elsewhere. Flowers May to July but often not produced. Leaves 50-125cm long and 7-25mm wide. When in flower, this plant is easy to identify, but flowers can be scarce in the UK and then the plant is easily missed among other, similar-looking plants. A good distinguishing feature is the puckering that appears on one side of the midrib along the leaves. Fruits are not produced by the triploid form of this species that occurs in the UK.
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Slender Sweet-flag Acorus gramineus
Introduced from eastern Asia as an ornamental for garden ponds but surprisingly rarely recorded outside of cultivation. First recorded in East Anglia along the River Yare near Norwich in 2024. Flowers May to July but often not produced. A much smaller plant than European Sweet-flag with leaves 8-50cm long and 2-8mm wide. Garden plants are typically of the cultivar 'Variegatus', with cream-coloured margins to the leaves.
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