Minute, annual herb with a slender tap root and a usually erect stem 0.5–2 cm, unbranched (in Svalbard, often branched in other areas) and with 1–2 pairs of leaves below the inflorescence, mostly on the upper half of the stem. The entire plant is glabrous, red or purplish tinged.

Leaves opposite, fused at base with a 0.5–1 mm sheath surrounding the stem, with an up to 1 mm long and pale brown continuation of the sheath (the ochrea) above the petioles. Leaves 0.5–2 mm, the lower ones with a short petiole 0.2–0.5 mm, blade 1–3 × 0.5–1.5 mm, obovate, oblong or elliptic, obtuse.

Inflorescence a few-flowered, dense cyme surrounded by bracts similar to stem leaves.

Flowers radially symmetric, bisexual, with a perianth of 3 tepals. Tepals 0.6–0.9 × 0.4–0.6 mm, oblong, yellowish green, white or pink. Stamens 3 + 3 staminodia (reduced, non-functional). Gynoecium of 3 fused carpels with one seed.

Fruit a trigonous nut, ca. 1 × 0.5 mm, ovoid in outline, acute, black, emerging well beyond the perianth at the fruit stage.

Sexual reproduction by seed; no vegetative reproduction. Obviously self-pollinated. Fruit set regular. Seeds germinate to ca. 45 % in an experiment (Alsos et al. 2013).

Fruits are spread by surface water and birds.

There is nothing similar in the Svalbard flora.

Wet, open soil on sediment plains, lake, river and brook shores, on open patches in shallow mires and on moist or wet, disturbed ground. The species is an opportunist that easily occupies patches with open, fine-grained substrates where and whenever they appear (e.g., within settlements). It is indifferent as to soil reaction (pH).

In the middle and northern arctic tundra zones and in all sections. Koenigia is frequent on Bjørnøya (Engelskjøn & Schweitzer 1970) and rather frequent in many parts of Spitsbergen, from Sørkapp Land north to the north coast in Haakon VII Land. It is also recorded from Kvalhovden on the east coast of Spitsbergen (Heer Land) and from two localities on Edgeøya.

The global range is arctic–alpine circumpolar, in Europe reaching south to S Scandinavia but in Asia and North America reaching far south, respectively to southernmost Siberia (and perhaps the Himalayas) and to the southern Rocky Mountains.

The vast majority of visitors to Svalbard have probably never noticed this minute plant. However, if you notice some intensely red, minute leaves in a moist or wet moss mat, or on a wet, black surface of hepatics and algal crust, it often is Koenigia islandica. Annuals are rare in the Arctic and disappear when going north. The reason is mainly that the annual lifestyle is very risky in the Arctic. If the season is bad, as it often is, annuals are eradicated because they do not have time and heat enough to reproduce, and the only way of surviving is by a seed bank in the soil. In Svalbard, there are three annuals: Euphrasia wettsteinii and Comastoma tenellum, both very rare and strictly confined to the climatically most favourable sites in the middle arctic tundra zone, and Koenigia, being rather common and reaching well into the northern arctic tundra zone and almost bordering on the polar desert zone. It is thereby the most hardy of annuals in the world. It has therefore been the focus of several studies in plant physiology and population dynamics (e.g., Dahl 1963; Reynolds 1984a, 1984b).

Dahl (1963) showed that a possible reason for the survival of Koenigia in a wide span of temperature regimes was its confinement to wet sites (practically being part of the wet surface), at least during germination and growth until flowering and initial fruit set. The water moderated the effects of the climate in both directions, keeping the substrate warmer during cold spells and cooler during periods with a danger of over-heating. Dahl used Koenigia as a model organism to predict the upper temperature limits of alpine (and arctic) plants, i.e., estimating the risks of lethal over-heating. Koenigia is less susceptible to over-heating after the flowering period; it can then wither away as long as the fruits mature. That is the reason why you often see Koenigia in rather dry moss mats at the end of the season; the plant is functionally dead but its fruits survive.

Quite another matter is what the Svalbard Koenigia really is. Hadač (1942) described it as a new variety, K. islandica var. arctica Hadač, due to its extremely small size. In other arctic regions, Koenigia may become quite large, 5 cm tall or sometimes more, especially in manured sites (seen in 2013 in W Greenland and in earlier years in Siberia, Alaska and Canada, H. Solstad & R. Elven observ.), and often branched from the lower leaf axils. In Svalbard, it is minute and with no branch below the inflorescence, whatever conditions it experiences. It is reported to be a tetraploid (2n = 28) species throughout the world but Löve & Löve (1975) proposed the Svalbard plant as a separate species – Koenigia hadacii (based on the variety of Hadač) – due to their finding a diploid chromosome number (2n = 14) counted on plants from the Isfjorden area in Svalbard ("seeds from Isfjorden"). They later validated this species name by a formal description (Löve & Löve 1976). They reported that there should be a voucher (Löve & Löve G325) for their Svalbard count but this voucher has never been found, and the number has not been confirmed by later counts on plants from the same area (U.-M. Hultgård finding 2n = 28 in plants from the Longyearbyen area, referred by Hultgård 2000). We do not accept K. hadacii. Its only reported morphological difference from K. islandica s. str. is its smaller size. We have compared plants from Svalbard, Scandinavia, and Iceland (type region of the species) and can see no differences whatsoever except for the size, and small-size plants are common also in the majority of other regions, and these correspond closely to the Svalbard plants.

Alsos, I.G., Müller, E. & Eidesen, P.B. 2013. Germinating seeds or bulbils in 87 of 113 tested Arctic species indicate potential for ex situ seed bank storage. – Polar Biology 36: 819–830. Doi 10.1007/s00300-013-1307-7.

Dahl, E. 1963. On the heat exchange of a wet vegetation surface and the ecology of Koenigia islandica. – Oikos 14: 190–211.

Hadač, E. 1942. Notulae ad floram Svalbardiae spectantes. – Studia Botanica Cechica 5(1–2): 1–5.

Hultgård, U.-M. 2000. Koenigia L. – In: Jonsell, B. (ed.) 2000. Flora Nordica, 1, Lycopodiaceae – Polygonaceae: 240–241.

Löve, Á. & Löve, D. 1975. In: IOPB chromosome number reports XLIX. – Taxon 24: 501–516.

Löve, Á. & Löve, D. 1976. Nomenclatural notes on arctic plants. – Botaniska Notiser 128: 497–523.

Reynolds, D.N. 1984a. Alpine annual plants: Phenology, germination, photosynthesis, and growth of three Rocky Mountain species. – Ecology 65: 759–766.

Reynolds, D.N. 1984b. Population dynamics of three annual species of alpine plants in the Rocky Mountains. – Oecologia 62: 250–255.