Medium-size Willowherbs (Epilobium)

This post is about identifying medium-size UK Epilobium (willowherb) species, and is basically for my own use in sorting out my thoughts, but if anyone else finds it helpful, you're welcome!

By medium-size, I mean those willowherbs, with relatively small flowers, that grow to about 60-70cm, rarely more. These include all the willowherbs that spring up as weeds in virtually every British garden. But for many years I have resolutely turned away from them, because I never feel confident that I know what I'm looking at. Finally I've decided I want to get to grips with this confusingly similar-looking group of species.

My definition excludes the two beautiful, common and highly recognizable species Rosebay Willowherb (Chamerion angustifolium -- this related genus has alternate leaves, not opposite leaves) and Great Willowherb (Epilobium hirsutum) -- both normally tall plants with big showy flowers. The latter plant does, however, hybridize with the medium-size willowherb species (as they do with each other), but I'm nowhere near being able to cope with hybrids! [Since its flowers, usually 10-16mm across, can occasionally be  less than 10mm, and E. montanum flowers (the largest of the medium-size willowherbs) are 8-10mm across, it's worth adding that E. hirsutum is densely hairy and has sessile clasping leaves.]

My definition also excludes the five small willowherbs, all only about 20-25cm tall, and not very likely to be found wild in your garden. They are: Alpine Willowherb (Epilobium anagallidifolium, native - mountain flushes); Chickweed Willowherb (Epilobium alsinifolium, native - mountain flushes); New Zealand Willowherb (Epilobium brunnescens - New Zealand alien, damp barish ground); Rockery Willowherb (Epilobium pedunculare - New Zealand alien, rare garden escape); and Bronzy Willowherb (Epilobium komarovianum - New Zealand alien, rare garden escape).

That leaves the following eight species to contend with... but for most of us,  it's really just the first five, and those are the ones I'm going to be concentrating on.

Broad-leaved Willowherb (Epilobium montanum) Common throughout UK. [BSBI account including map.]
American Willowherb (Epilobium ciliatum) Not recognized until the 1930s (alien from N. America), but common almost throughout UK.  [BSBI account including map.]
Square-stalked Willowherb (Epilobium tetragonum) Common throughout lowland England and S. Wales.  [BSBI account including map.]
Short-fruited Willowherb (Epilobium obscurum) - Common almost throughout UK.  [BSBI account including map.]
Hoary Willowherb (Epilobium parviflorum) Common throughout UK, except Scotland.  [BSBI account including map.]

Marsh Willowherb (Epilobium palustre)  Common, but only in very wet acidic habitats, and consequently absent from much of SE UK. Perennates by aquatic buds ("turions") on thread-like rhizomes.  [BSBI account including map.]
Pale Willowherb (Epilobium roseum) A rather local species of damp disturbed places, with strongholds in NW England, W. Midlands, S. Wales, Cornwall, New Forest, London. Long petioles (4-15mm)! Distinctive once seen, says BSBI.  [BSBI account including map.]
Spear-leaved Willowherb (Epilobium lanceolatum) Dry habitats; mainly restricted to SW England (Cornwall, Devon, W. Somerset, Bristol area). Long petioles (3-10mm).  [BSBI account including map.]

ID features. (concentrating on the five likely species)

Flower-colour isn't a very reliable feature, but it's worth noting that E. montanum is simply "pink", the other four being variously described as "pinkish-purple" or "purplish-pink".

The least disputable ID feature, so long as your plant has at least one open flower, is the shape of the creamy stigma in the centre. This can be either four-lobed, or club-shaped (clavate).

E. montanum and E. parviflorum are four-lobed. E. ciliatum, E. tetragonum and E. obscurum are clavate.

The first pair can be distinguished from each other reasonably easily, in theory. E. montanum is nearly hairless ("rather sparsely pubescent", Stace says) while E. parviflorum is just the opposite: densely hairy, rather matted.

In addition, E. montanum has petioles, though rather small ones (2-6mm), the leaf-base being abruptly delimited from the petioles; while E. parviflorum leaves are sessile (though not clasping as in E. hirsutum).

Separating the other three species (E. ciliatum, E.tetragonum, E. obscurum) is a bit more tricky.

E. obscurum is easy to pick out if you can see the perennation: elongated leafy stolons (the others are sessile or subsessile leaf rosettes).

Otherwise you have to look at more disputable matters of stem, petiole, leaf-shape, capsule-length:

E. ciliatum has short (1.5-4mm) petioles, the others are more or less sessile.

E. ciliatum has at least some and often many patent hairs (=at right-angles to stem), E. tetragonum has all hairs appressed, E. obscurum has patent hairs only on the hypanthium (=the short region between the long ovary and the sepals) and sometimes a few on the capsule.

E. ciliatum has four raised ridges on the stem, E. tetragonum four conspicuous raised ridges (hence "Square-stalked"), E. obscurum stems are basically round but with raised lines coming down from each leaf.

E. ciliatum leaf-shape is oblong-lanceoloate with a rounded to subcordate base; E. tetragonum leaf-shape is narrowly oblong or oblong-lanceolate ("strap-shaped"); E. obscurum is narrowly elliptic-ovate to lanceolate ( but not oblong). [NB "Oblong"  in  this context means that the middle section of the leaf is parallel sided and the leaf is about three times as long as wide. "Lanceolate" means that the middle section of the leaf is parallel sided and the leaf is about six times as long as wide. These terms like elliptic, ovate, obovate, linear etc describe the overall dimensions without reference to the shape of the apex or leaf-base, or whether the margins are sinuous, toothed, etc ]

E. ciliatum capsule length to 10cm (?), E. tetragonum 6.5-8cm, E. obscurum 4-6cm (hence "Short-fruited").

Simples! Too late to identify the plants below, seen on a walk this morning. Time to go back out there and see if any of this works...

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Follow-up post, with some photos of the species I've learnt to give names to:

https://michaelpeverett.blogspot.com/2020/06/those-tricky-willowherbs-epilobium.html

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Two brief asides of a more personal nature:

1. Before I began to study them, I considered these medium-size willowherbs as rather dingy and boring plants. Now that I can't stop looking at them, my eyes are constantly picking out beauty in the form of patterns, shapes and colours, and the subtle but eloquent variations between individuals. I suppose this aesthetic revolution occurs whenever we begin to look regularly at something new. I remember saying the same thing about grasses, many years back, but maybe it bears repeating.

2. Medium-size willowherbs are so utterly commonplace that it's a surprise to find them absent from anywhere. But such is the case. Go for a walk in the countryside, or in a grassy park, and you likely won't spot any at all. In town there are many, but they are rather apt to be in highly public places such as on the edges of walkways and car-parks, where I feel rather self-conscious about inspecting an insignificant plant on hands and knees. It's a great temptation to pick some plants and take them home for inspection, but this is something I'm becoming less and less willing to do. I no longer feel as persuaded as I once did by rationalizations such as "they're all very common" and "no-one will miss them". I have no issue with weeding or clearing, but I do have an issue with killing things merely to enjoy studying them. Perhaps that seems senseless, but so it is. I've only picked two plants so far, and both of them I replanted in a pot in my garden, where they're getting on very well! (One of the best places I've found to study willowherbs in seclusion is in woodland: many of them like shady places.)

Links:

Useful cribsheet: How to spot hybrids, distinguishing obscurum from tetragonum, distinguishing upland willowherbs.

https://bsbi.org/wp-content/uploads/dlm_uploads/Epilobium_Crib_3.pdf

Some more useful ID tips and good photos. Covers parviflorum, ciliatum, montanum, obscurum, brunnescens. 

https://www.vc77botany.org/epilobium-species-willowherbs

On tetragonum ssp lamyi. Also notes that roseum and lanceolatum flowers are always white in bud and early flower. However others are often white, e.g. ciliatum.

https://bsbi.org/wp-content/uploads/dlm_uploads/Epilobium_Recorders_Conf_2015.pdf

Patent hairs!