Plants of Iceland: Equisetum sylvaticum, Wood Horsetail

Equisetum sylvaticum; The Wood Horsetail is very rare on Iceland. It is known only from a few (remote) localities. However, it can easily be identified: it is the only horsetail that has the primary side branches split into several secondary side branches. All other horsetails either have no side branches from the main stem or only unbranched primary side branches. The Icelandic name is Skógelfting.

Some horsetails can easily be identified while others resemble each other very much. Here are features to look for when trying to identify the species (note there are many more identifying features discussed on the specific species-pages - below are quick identification markers).

The horsetail has side branches, these side branches themselves are split in sub-branches. This feature is found only in the Wood Horsetail.
The horsetail has thick (up to 5mm) unbranched stems (except occasionally some basal side branches). The stems have a central cavity of about 2/3 of the stem. Foremost the stems are very rough. The largest of all Icelandic horsetails: Rough Horsetail.
The horsetail is unbranched (except for occasional basal side branches) with a central city that is never more than 1/3 the stem. It has a very characteristic sharp-tipped sporangium. This is the Variegated Horsetail.
The horsetail has a stem with very large central cavity, 4/5th of the width of the stem. As a result the stem is easily crunched when pressed by fingers holding the stem. It grows in very wet conditions. Normally the stems are unbranched, however note that at times they can be heavily branched! This is the Water Horsetail.

The species listed above are identified by a single characteristic. The three, all of them common, species below are more difficult to distinguish. A technical feature often used is the relative length of the first segment of the side branch to the length of the segment of the central stem that tops the branching node. Do not confuse true segments with sheaths when the segments are small.

The Field Horsetail: in early spring (Iceland: April early May) unbranched non-green (pale) stems appear with sporangia at the top. A very characteristic feature for this species. However as these fertile stems wither later on in spring the green shoots appear, never bearing sporangia, always with side branches. These side-branches grow upwards. The first segment of the side branches are longer than the adjacent main stem segment.
The Shady Horsetail: first segment of the side branches are shorter than the adjacent main stem segment. The side branches grow horizontally first, later bending downwards. In spring the young shoots don't have side branches yet but the sporangia are ripe. At this point the this species can differentiated from the Marsh Horsetail by the teeth of the sheaths: the Shady Horsetail has 10-16 teeth (as many as side branches on each whorl). The Marsh Horsetail has less: 6 to 8 at each branching whorl. The sporangium withers quickly during summer.
The Marsh Horsetail: like the Shady Horsetail it emerges unbranched. However, they tend to stay unbranched, or only with short side-branches, for a longer time. The sporangia don't wither as fast and are formed newly throughout the summer. Like the Shady Horsetail it has shorter first segments of side branches corresponding to the main stem segment above the branching node, a differentiating trait to the Field Horsetail. However, like the Field Horsetail - and unlike the Shady Horsetail - the side branches - once fully developed - also grow upwards.