Sphagnum palustre is one of only six species in our islands that are within the sub-genus (or Section) Sphagnum. They are all large plants with obviously concave and hooded branch leaves. But Sphagnum species are so inherently variable that other species (in different Sections) can sometimes be confused with it – think straight-leaved forms of S. auriculatum as well as S. compactum and S. squarrosum.
If you can place a Sphagnum in its Section it means you can take short-cuts with the keys by ruling out most other species. There is an easy way of telling in the field if you have picked up S. palustre or any of the other Section Sphagnum species. Break a shoot in two to expose a stub of the stem (you may need to remove a few branches to see it). Then squeeze the exposed bit of stem firmly between thumb and forefinger to get rid of excess water and examine it end-on with a x 20 hand-lens. The stem will have a dark core surrounded by a very wide white or pale outer cortex. This combination is only seen in Section Sphagnum. S. auriculatum, for example, typically has a narrow dark outer ring with a wide, paler inner cylinder. S. palustre has a very wide pale cortex because its stems are sheathed in 3-4 layers of large, transparent hyaline cells strengthened by spiral fibrils.
S. palustre isn’t a hummock-forming species, unlike S. papillosum, with which it can be confused in the field sometimes (especially when plants are orange-brown). You can sometimes resolve intermediate-looking plants (if all else fails) by counting the number of branches in the fascicles (check a few to be sure). S. palustre typically has 4-6 branches whereas S. papillosum won’t have more than 4.
Two varieties are currently accepted in Britain and Ireland, although unless you are in the habit of sectioning branch leaves you are unlikely to identify either.
- Sphagnum palustre var. palustre – the nominate variety; most plants are this. In section, its chlorocysts are triangular, with straight sides.
- Sphagnum palustre var. centrale – its distribution is poorly known. Chlorocysts are barrel or lens-shaped, with bulging sides.