Wavy St. John's-wort. Hypericum undulatum
Wavy St. John's-wort was a species I had never seen. It has a distribution in Cornwall and Devon with some in Pembrokeshire and a very few near the coast in Mid-Wales. A trip to Ireland via Fishguard provided a chance see it, although mid-September was getting rather late in the season, some records seemed to suggest this species could still flower through September.
We failed to find any at the first site near Newgale, where we were staying. It was late by the time we arrived and started raining, so it was not a intensive search. The next day we headed off to a site near Carnhedryn; a large area of rough wet pasture with just a few cattle.
This provided a suitable habitat and previous records suggested it might be a good place. We only found three plants despite an intensive search but that might have been due to it being late in the season. Later that day a search on the coastal path near Tresinwen also failed to locate any plants. The site looked rather overgrown.
H. undulatum shares some features with H. tetrapterum ( Square-stalked SJW) both having a square stem with wings and these two species can have intermediate forms in the western Mediterranean. This is according to the best article on St. John's-worts, British and Irish St. John's-worts by Dr N. K. B. Robson, published by the Wild Flower Society in 'A Guide to some difficult plants'.
In the UK hybrids have been found although they are very rare.
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| Wavy SJW hiding in long grass. 13Sept21 |
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| Wavy SJW- top view of flower showing the usual yellow and black anthers. |
Note the sepal like bracts (with a thick main vein) just below the flower where the second stalk breaks off the to the left. This is also present on the bud on the left and made photographing the sepals difficult.
Although several species of St. John's-worts can have red on the underside of the petals, which give the unopened buds a red look, none have the extent of red which Wavy SJW exhibits, as shown below. This and the square stem are good features to identify this species. The petals are showing none or maybe just one black dot near the margin suggesting that black dots are few.
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| Wavy SJW 13th Sept 2021 |
Sepals are both pointed with a distinct tip and oval in shape, with the widest point in the middle. They are also quite wide. Compare with Square-stalked SJW below.
Square-stalked SJW sepals. Narrower and very pointed with a more tapering shape. Same pale perforation lines and veins just visible and just a few black glands. A maximum of three glands is suggested in the literature for Square-stalked SJW , whereas Wavy SJW has three plus.
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| Wavy SJW different plant showing wide sepals. |
A different example shows sepals with more black glands on the surface. Note the oval shape, where the widest point is about half way on the sepal on the middle bud. Some tips have been lost on the sepals on the open flower.
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| Wavy SJW showing sepals and bracts. 13 Sept 21 |
Here again there is a sepal like bract getting in the way and hiding one of the sepals on the open flower.
The shape of these sepals is still wide and oval and they do not just taper down from a wide base as in Square-stalked SJW.
Stem.
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| Wavy SJW square stem. |
Stem shape at mid level shows a square stem with wings. Sometimes these wings have black edges.
This is similar to Square-stalked SJW which can have slightly larger wings apparently.
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| Wavy SJW Leaf. |
Leaf showing wavy edge, a feature that was not as easy to see on these late season plants, partly due to the lack of lower leaves at this late date. The leaf is sessile but not clasping. It has many quite prominent perforations and some net veins can be seen between the major veins. It also has black dots mainly along the edge.
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| Wavy SJW leaf showing net veins. |
Some comparison shots of Square-stalked SJW.
Flowers
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| Square-stalked SJW flowers. |
Flowers are crowded at the top of plant. Petals are pale yellow with no or very limited red.
Sepals are tapering and pointed and can have a few black glands, if any.
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| Square-stalked SJW. |
Stem is square and winged. The leaf is sessile but not clasping. It has many quite prominent perforations and some net veins can be seen between the major veins. It also has black dots mainly along the edge. All similar to Wavy SJW except the perforations are smaller and denser but this difference is tiny and may be too subject to variation.
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| Detail of leaf of Square-stalked SJW showing density of perforations. |
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| Detail of leaf section of Wavy SJW showing lower density and slightly larger size perforations . |
The combination of lots of red on the petals, less crowded flowers and a wavy edge to some of the leaves, especially lower down, separate Wavy St. John's-Wort from Square-stalked SJW. The size of the leaf perforations and density are also a feature, with Square-stalked having smaller and denser perforations but as the photos show above, this is a subtle difference and may not be reliable.
The literature suggests that the winged square stems have more prominent wings in Square-stalked but this is hard to judge and there may be some overlap in this feature. On the sample of only three plants, it is hard to judge the sepal shape but it does appear that the sepals are more oval in shape than most Square-stalked SJW .
Separation of these two closely related species is therefore fairly straightforward however, finding a hybrid between them would be quite difficult and would require very close inspection. We are fortunate that flowers in Britain have the prominent red flushing because plants in for example Portugal, have just yellow petals.
The literature suggests that the flower diameter is 12-20mm for Wavy SJW and 9-13mm for Square-stalked SJW. A featureI did not check on my visit.
Peter Leonard
Rampton , Cambridgeshire.
15 March 2022
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| Square-stalked SJW leaf showing small and dense perforations. |
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| Wavy SJW leaf showing slightly less dense perforations. |
Note the wavy edge to these leaves especially near the base of the leaf. Not easy to see.
