Epilobium tetragonum subsp. tournefortii
Tournefort's Willowherb.
First discovered in the UK as recently as 2007 ( Kitchener & Leslie, 2008 Watsonia 27:65-67), at Royston, it has since spread to several sites in Cambridgeshire and Bedford and may yet be found to be more widespread. However, for the non-expert it might be quite easy to miss, as its flowers on average are almost as large as Great Willowherb ( E. hirsutum). This blog attempts to highlight the differences.
Great Willowherb. ( E. hirsutum).
Square-stemmed Willowherb ( E. tetragonum subsp. tetragonum)
Tournefort's Willowherb ( Tetragonum subsp. tournefortii)
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| Great Willowherb. Deep pink flowers and dense leaves. |
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| Tournefort's Willowherb. |
Even at a distance the structure is much more open with less dense leaves than Great Willowherb. The flower colour is less deep a shade of pink without the purple tint and the flowers often are not upright with many out-facing flowers.
A closer inspection shows the stigma is club shaped rather than the cross shaped Great Willowherb.
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| Left, Great Willowherb (E. hirsutum), right Tournefort's Willowherb. Note, the network of veins in Great Willowherb vs. parallel veins in Tournefort's. |
It is regarded as a subspecies of Epilobium tetragonum (Square-stalked Willowherb), so next step is to look at the differences between the more common E. tetragonum subsp. tetragonum and this larger flowered version.
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| E. tetragonum subsp. tournefortii. 14th July 2024. Girton, Cambs. |
The first observation is that the capsule hairs are appressed, held very tight to the capsule and caylx. This is a feature of all tetragonum subspecies and is shared with E. obscurum ( Short-fruited Willowherb),- not covered in today's blog. These appressed hairs are also very dense but this does vary from plant to plant.
Tetragonum subsp. tetragonum can have just a few patent non-glandular hairs sticking out from the lower calyx and capsule occasionally. Also beware fungal growth which can look like glandular hairs, but this is often more dense than the glandular hairs on E. obscurum. Separation of E. obscurum from tetragonum subsp. tetragonum can be difficult.
Second feature is the stigma which is club shaped and also gets slightly broader towards the tip. Flower size is about 30mm across on average, slightly smaller than Great Willowherb. Tetragonum subsp. tetragonum has a stigma which is club shaped but does not get broader towards the tip. Also has flowers that are much smaller with petal length of 5-9mm compared to 10-15mm in tournefortii.
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| E. tetragonum subsp. tournefortii. |
This flower has been cut away to show the stigma and style. It shows the stigma getting broader towards the tip but also the supporting style has tiny hairs that occur up to 75% of the length of the style. This is a feature that does not occur on the more common E. tetragonum subsp. tetragonum. This is an important feature to identify Tournefort's Willowherb.
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| E. tetragonum subsp tournefortii |
The style hairs can been seen looking into the flower using a magnifying lens. Best to pull back the petals to see.
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| E. tetragonum subsp. tournefortii (left), intermediate (centre), subsp. tetragonum (right) |
Above photo shows just how big the tournefortii flower is, compared with the common tetragonum flower on the right. Scale lines are 7mm apart. Although these are sub-species they do hybridise and intermediates can occasional be found, as shown in the centre. Details of this intermediate and other hybrids of tournefortii can be found on the BSBI website, British & Irish Botany 3(4) 469-481, 2021 by Geoffrey Kitchener and Alan Leslie with a few photos from me.
With all rare willowherbs , all details should be checked as other hybrids can occur which also have large flowers.
Photo sheets follow to highlight features.
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Hybrids with large flowers such as between hirsutum x tetragonum subsp tetragonum = Epilobium x brevipilum also need to be considered but these display some form of four-lobed stigma but shape might approach a club shape or be confused.
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| Flower showing four lobed stigma. E. x brevipilum. 10th Aug 24. Rampton, Cambs |
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| Hybrid upper stem has hairs that are not tightly appressed but point up and outwards at up to 45 degrees. They are also dense. |
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| Capsule hairs also stick out at 45 degrees unlike tetragonum which are appressed. |
Hybrids with E. hirsutum as a parent will tend to have flowers slightly smaller than E. hirsutum so would need to be considered before claiming a Tetragonum subsp. tournefortii .
Hopefully the above photos will help raising the profile of this alien from Southern Europe.
Peter Leonard
30th September 2024
Cambridgeshire.
