Source: Knapp, Alan. “Notes on the identification of some difficult species.” Sussex Botanical Recording Society Newsletter, no. 63 (January 2007). http://sussexflora.org.uk/wp-content/uploads/2016/03/Newsletter_Jan_2007.pdf.
This is the first of a series of notes which will be appearing in the Newsletter aimed at helping with the identification of species which cause problems, especially where there are pairs of closely related species which can easily be confused. These notes should be used in conjunction with other information, especially the Floras by Stace and the Plant Crib. NB that parts of the Plant Crib are now available on the BSBI website: www.bsbi.org.uk/identification.html, and the appropriate sections are referred to below.
Epilobium obscurum / Epilobium tetragonum
These are both quite common but can be hard to distinguish. The key difference between them is the presence of glandular hairs just around the base of the calyx and sometimes on the fruit in E. obscurum. They are, however, easy to miss, so look very carefully with at least a x10 lens. If you can’t find any hairs and think you have E. tetragonum check more than one flower. If you find glandular hairs then it is clearly E. obscurum. (assuming you have already eliminated all other Epilobium species!)
Epilobium palustre / other Epilobium species.
Epilobium palustre is extremely scarce in Sussex and some (or possibly many) past records are errors (probably for E. obscurum) but it may be overlooked, so please examine Epilobium plants with pale flowers found in very wet areas carefully. A useful initial indication is that the heads of E. palustre are very droopy and the flowers are a very pale colour. To be certain, in addition to other features mentioned in most keys, you must examine the seeds. The seeds of E. palustre have a small appendage between the top of the seed and the hairs. Stace shows a picture of the seeds of E. palustre in which this appendage can be clearly seen.
Juncus acutiflorus / Juncus articulatus
These species cannot be reliably distinguished by vegetative characters – you must have flowers and/or fruits. The shape of the tepals (see Stace key) is the best character but can be quite difficult. The shape of the fruiting capsule in good condition can also be used. The tepals in the species are in two sets of three (inner and outer). All of the tepals of J. acutiflorus are acuminate and have more or less recurved apical points (sometimes the recurving is pretty obscure). Also the fruiting capsule is acuminate. In J. articulatus the tepals do not have recurved points, the outer tepals are acute but the inner tepals are often less acute. The fruiting capsule is mucronate, i.e. the top is rounded to acute with a distinct short point (see drawings in Plant Crib 1998). These distinctions need careful observation but do seem to work well once you become familiar with the species. Finally it should be remembered that these two hybridise and the resulting hybrid, Juncus x surrejanus, is quite probably common although we have very few records for it. See notes on p.330 of Plant Crib 1998 or section 5 of: www.bsbi.org.uk/Juncus_Crib.pdf for more information
Lemna minor / Lemna minuta
There are problems here, and populations of smaller plants found in autumn or winter cannot be reliably identified. Plants of L. minor with larger fronds can be easily differentiated from L. minuta: if the fronds are >3.5 mm long then it is L. minor. However, differentiating young L. minor from L. minuta is difficult. The fact that L. minor fronds have three veins but L. minuta fronds have only one would be a good character if you could see the veins. It is very difficult to see the three veins in healthy plants of L. minor. However the single vein of L. minuta is a little easier to make out. Look carefully from various angles in good light and you may see a single line or low ridge down the middle of the upper surface of the frond.
Typha angustifolia / Typha x glauca (T.latifolia x T.angustifolia)
Although locally common in a few places, Typha angustifolia is rather scarce in most of Sussex while the hybrid is commoner than you may think. The presence of a gap between the male and female parts of the inflorescence only means that the plant is not Typha latifolia. To be sure that it is T. angustifolia you must first measure the leaf width. If it is 6mm or less (which is very narrow) then you have T. angustifolia but if it’s a bit wider it could be either. Towards the end of the summer you can check if the heads are fertile as T. x glauca.is sterile. Collect a head or two, make a clean cut across the diameter and look at the cut face for the presence of seeds near the centre (you need to look very carefully with a lens as they are small and can be hard to see). If they are present the plant is definitely T. angustifolia. If absent, try cutting in other places and look again to confirm sterilty.