Wednesday, June 17, 2020

Those tricky willowherbs (Epilobium species)

Epilobium montanum. Swindon, 17 June 2020.

1. OK, let's start in my garden, and probably in yours too, if you live in the UK or Ireland. This is Broad-leaved Willowherb (Epilobium montanum, Sw: Bergdunört) growing out of a flower-pot. It's a well-chosen name. The shape of the broad leaves, with their broad bases appearing heart-shaped (though they aren't), is unlike any of the others. The leaves on this specimen have gone a bit bronzy. 4-part stigma, capsules about 7cm.

[Very common throughout the British Isles. Also throughout Sweden but only common in the southern half. Incidentally the Swedish name for willowherb, dunört, means "down-wort" -- as in feather-down, referring to the feathery seeds. It's a mystery to me why Linnaeus named this species montanum (which berg translates).]

Epilobium montanum. Swindon, 17 June 2020.

Broad-leaved Willowherb. This specimen has more typical coloration: green leaves and red stems. The stems are round and often finely hairy, like this one, but the hairs are small and don't stick out much. You can see one of the leaf-stalks (petioles): on this species it's pretty short, but it's there.

The lower leaves are always opposite each other, but further up the stem they become alternate.

Epilobium montanum. Swindon, 17 June 2020.

Broad-leaved Willowherb. Don't expect to see the leaf stalks from above!

Epilobium montanum. Swindon, 30 May 2020.

Broad-leaved Willowherb. Here are the flowers. The petals are pure pink (not purplish) and, at 8-10mm, a little larger than any of the other medium-size willowherbs. 4-lobed stigmas.

Epilobium montanum. Swindon, 20 June 2020.

Broad-leaved Willowherb. I guess you don't really need this photo, but I think it's interesting. It shows a bizarre group of Broad-leaved Willowherbs near to where I live; their leaves are all in threes rather than twos. 

Epilobium montanum. Frome, 30 July 2023.

Warning! The smallish leaves on late growth can look very untypical. They may have narrowed bases and they may lack the normal toothed edges. The photo above, I promise you, is Broad-leaved Willowherb! Keep your eye on the other features (e.g. four-lobed stigmas, relatively large flowers) and you won't go far wrong.

Epilobium obscurum. Swindon, 17 June 2020.

[I'm leaving this section as is, for now, but alas I think this is a misidentification. As time went by the fruits exceeded 7cm which never happens with obscurum. So I don't know. These willowherbs really are tricky!]

2. This is the other species currently in my garden: Short-fruited Willowherb (Epilobium obscurum, Sw: Mörk dunört). The petals are pinkish-purple and only 4-7mm. The stigmas are clavate (club-shaped), not divided into 4 lobes. But the key feature is the capsule length, only 4-6 cm. Another well-chosen name!  

(Yes, if you want to distinguish Epilobium species there's no getting around it, you do need to carry a measuring tape. Sometimes you'll be measuring fruits and sometimes the open flowers.)

[Common throughout British Isles. Uncommon in the southern half of Sweden.]

Epilobium obscurum. Swindon, 17 June 2020.

Short-fruited Willowherb. On my specimens the surfaces of the upper part of the plant are minutely hairy, giving the effect of a whitish bloom. The hairs don't stick out. (Apparently they can do on the hypanthium -- a very small area just below the flower --, but my plants don't have that feature.)

Epilobium obscurum. Swindon, 17 June 2020.
Short-fruited Willowherb. The lower part of the plant is more or less hairless, except on the slightly raised ridges coming down from the leaf-bases. The leaves are quite narrow and more or less sessile (stalkless), but not so narrow as the next species).

Epilobium tetragonum. Swindon, 17 June 2020.

3. Moving 100 meters up the road, this one is Square-stalked Willowherb (Epilobium tetragonum, Sw: Kantdunört). The flowers and stigmas are similar to Epilobium obscurum, and so is the minute not-sticking-out hairiness. Fortunately there's more to go on.

[Common in the southern British Isles, mostly England and Wales. Rare in the south of Sweden.]

Epilobium tetragonum. Swindon, 17 June 2020.

Square-stalked Willowherb. What immediately distinguishes it are the longer capsules, typically 7-8cm but often reaching 10cm.

The technical term for the hairs is "appressed". You may shrewdly suspect that they are not actually stuck to the stem surface, and you're right, as you'll see if you look through a x20 lens. But they do stay remarkably close to it. Get familiar with this lens view, to avoid confusion with late-season American Willowherb, whose reduced hairiness may look similar to the naked eye, but where the lens reveals patent (sticking-out) hairs.

Epilobium tetragonum. Swindon, 17 June 2020.

Square-stalked Willowherb. The other easy-to-spot feature is the the very narrow strap-shaped leaves, more or less stalkless. Also, the four ridges on the stem are more pronounced, so you can see why it's called "square-stalked", though that's really a bit of an exaggeration. The stalk is still basically round, but with four pronounced ridges.

Epilobium parviflorum. Swindon, 17 June 2020.

4. Right next to our previous plant, this very different one: Hoary Willowherb (Epilobium parviflorum, Sw: Luddunört). Small pinkish-purple flowers like Epilobium obscurum and Epilobium tetragonum, but this time the stigmas are four-lobed. And of course the whole plant is densely hairy.

[Common in most of the British Isles, except the Scottish uplands. Common in some parts of Sweden: e.g. the far south, the Baltic islands and around Stockholm.]

Epilobium parviflorum. Swindon, 17 June 2020.
Hoary Willowherb. Emerging raceme, everything looking felty.

Epilobium parviflorum. Swindon, 17 June 2020.
Hoary Willowherb. Stems and leaves: all surfaces with sticking-out hairs. Stems round and unridged. Leaves becoming slightly narrower towards the base (unlike Epilobium montanum), and sessile (stalkless) but, unlike the next species, not clasping.

Epilobium parviflorum. Swindon, 20 June 2020.

Hoary Willowherb. An open flower, showing the four-lobed stigma.

Epilobium parviflorum. Swindon, 20 June 2020.

Hoary Willowherb. A typical well-grown plant: erect and quite tall, up to a meter.

Epilobium hirsutum. Swindon, 15 June 2020.

5. Another couple of hundred meters away, and here's the first flower of the year on this specimen of Great Willowherb (Epilobium hirsutum, Sw: Rosendunört), much the most noticeable of the group and the only one you might describe as "well-loved". A usually tall plant (to 1.8m), though you often see runty specimens growing out of kerb-sides. All surfaces noticeably hairy, though not quite as "felty" as the previous species. Clasping leaves. Big showy flowers (petals 10-16mm) Stigma four-lobed.

[Very common throughout the British Isles, except in the Scottish highlands. Only common in the far south of Sweden.]

Epilobium hirsutum. Swindon, 20 June 2020.

Great Willowherb. Five days later, and a lot more flowers.

Epilobium hirsutum. Swindon, 20 June 2020.

Great Willowherb. Close-up of stem and leaves, the leaf-bases clasping the stem. If you zoom in on the edge of the stem you can clearly see the two kinds of hairs; one kind is "patent" (sticks out sideways), the other kind stays close to the stem surface.

Epilobium tetragonum (left) and Epilobium ciliatum (right). Swindon, 23 June 2020.

6. Just round the corner. The plant on the right is American Willowherb (Epilobium ciliatum. Sw: Amerikansk Dunört or Vit Dunört). The stigmas are clavate like Short-fruited Willowherb (2) and Square-stalked Willowherb (3), but unlike them the stem, fruits and sepals are covered with small patent hairs, many with minute shining beads of liquid on the tips ("glandular"). In late season the liquid beads may have disappeared and the sticking-out hairs may be much less apparent to the naked eye, but you'll see them through a x20 lens.

[An introduced species but now common throughout the British Isles. Fairly common in southern and central Sweden.]

(The plant on the left is Square-stalked Willowherb!)

Epilobium ciliatum. Swindon, 23 June 2020.

American Willowherb. Close-up showing the stems covered with patent hairs, many of them glandular. The other feature that differentiates it from Short-fruited Willowherb (2) and Square-stalked Willowherb (3) is the short but definite leaf-stalk (petiole), 1.5-4mm.

Epilobium ciliatum. Swindon, 26 June 2020.

American Willowherb. A flower, showing the clavate (club-shaped) stigma. The flowers are only 3-6mm, the smallest among the medium-sized willowherbs. They're often pinkish-purple but these ones opened very pale, virtually white (only turning pink as they faded).

That is pretty normal (hence the alternative Swedish name Vit Dunört, White Willowherb) but not so normal as on another species, Pale Willowherb (not shown here yet). 

Anyway, below is an example of a pink-flowered American Willowherb (note clumsy use of the hand to stop the phone camera from bleaching out the colour!).

Epilobium ciliatum. Berkley Marsh (Somerset), 29 July 2023.


This post (which I'll continue to add to) is intended to be a user-friendly and non-technical guide to these common but confusing plants, focussing on the features that I've tended to find useful. If you want to see me struggling to get to grips with the more technical aspects, see this post from a year ago:

Here's a really helpful BSBI PDF by Bob Leaney in Norfolk, about common problems with identifying these willowherbs. He points out, for instance, that the glandular hairs on willowherbs don't actually have a glandular cell on the tip. The point is that once the beads of liquid are washed off or rubbed off they do not reappear. Hence it's best to check on freshly emerged growth.



At 2:04 pm, Anonymous Anonymous said...

Thank you


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