Drosera intermedia Hayne, Oblong-leaved Sundew
Account Summary
Native, occasional. Suboceanic temperate, but also present in N America.
1836; Mackay, J.T.; bogland on Cuilcagh Mountain.
May to October.
Identification and variation
Intermediate in size and leaf shape between D. anglica (Great Sundew) and D. rotundifolia (Round-leaved Sundew), the leaves of D. intermedia are held erect and have a wedge-shaped blade 7-12 mm long and 4-10 mm wide. The leafless flowering stem (ie scape), arises laterally from below the terminal leaf 'rosette'. The scape is decumbent at its base and thus appears to curve up from the side of the plant, whereas in D. anglica the scape originates from the centre of the loose rosette of leaves.
Unfortunately, D. intermedia shows some variation in this often-used identification feature, so plants need to be examined very carefully. Significantly, the scape is usually much shorter in D. intermedia than in D. anglica, often scarcely overtopping the leaves − although this also is not completely reliable, since it can rarely be up to three times as long as the leaves. Having said this, in general the scape is short in comparison with D. anglica, where it is usually two to four times the leaf length (Crowder et al. 1990).
In both D. rotundifolia and D. anglica new leaves grow in the centre of the rosette after flowering, which makes old inflorescences look as if they arise laterally. This can result in possible misidentification if other characters are not carefully checked (A. Culham, in: Rich & Jermy 1998).
On account of the variation involved, RHN and the current author fear that there may be some confusion between D. intermedia and D. anglica. Particularly with respect to leaf shape and size, potential errors also occur between D. intermedia and the hybrid between D. anglica and D. rotundifolia (= D. × obovata), which by its nature is often intermediate between its parent species. This problem is not unique to Fermanagh, however, and we do not believe that the potential for mistake greatly distorts the picture of taxa frequency and occurrence painted here.
Growth and preferred habitats
Oblong-leaved Sundew usually forms rather dense mats of plants up to 100 cm in diameter on bare, wet, acid peat, typically on the bottom of shallow pools on both lowland raised and blanket bogs. Plants in these mats are inevitably entangled, since vegetative propagation by means of axillary buds is undoubtedly involved in their production. Axillary bud formation and colony mats are probably more common in D. intermedia than in the other two sundews. The stem axis and leaf petioles grow strongly and, to a limited extent, this enables the plant to keep up with a rising water level or rapidly growing moss cushion. However, unlike D. rotundifolia, Oblong-leaved Sundew cannot tolerate any degree of shading from shrubs (Crowder et al. 1990).
In terms of habitat preferences, D. intermedia more closely resembles D. rotundifolia than D. anglica, ie in occupying shallow temporary pools in bog hollows and appearing around the edges of deeper, more permanent pools and towards the base of mossy hummocks on valley bogs. Plants appear able to withstand flooding indefinitely and can therefore be considered as aquatics. More rarely D. intermedia appears in wet, flushed seepage areas, or beside paths on blanket bog slopes and on bare peat in disturbed scrapes on wet, peaty heathland. It is more of a lowland species than D. anglica and D. rotundifolia, reaching only 335 m above sea level in Co Donegal (vc unspecified) (Crowder et al. 1990).
D. intermedia appears to combine the ability of D. anglica to tolerate long periods of immersion, with the capacity of D. rotundifolia to survive on substrates that dry out for a time in summer, but D. intermedia does not quite match the competence of either in extreme situations. In the oceanic west of Ireland, however, rain is plentiful and well distributed in time, so that summer droughts are rare and not usually very prolonged.
On most if not all of our lowland bogs, all three Drosera species occur together, often associated with Narthecium ossifragum (Bog Asphodel), Rhynchospora alba (White Beak-sedge), Potamogeton polygonifolius (Bog Pondweed) and either both common Cotton-grasses ((Eriophorum vaginatum (Hare's-tail Cottongrass) and E. angustifolium (Common Cottongrass)), or just the latter on its own.
Insect trapping
D. intermedia is an efficient insect trapper and Robert Northridge and I have seen patches of the plant in Fermanagh covered with the dead bodies of Coenagrion spp., damselflies and, on another occasion, the bodies of Pieris spp., butterflies. Data from three field sites in Britain also showed a correlation between the amount of insects captured and plant growth and reproductive performance. The percentage of plants that had captured prey on their leaves in these studies varied from 55%-96% (Crowder et al. 1990).
Reproduction
As in D. rotundifolia, flowers are bisexual and are usually self-fertilised. Some flowers do not open (ie they are cleistogamous). However, the flowers are not as tightly closed as is the case in D. rotundifolia and all of them appear capable of opening when exposed to bright sunlight. Normally, however, cleistogamous flowers fail to open and they self-fertilise while in bud.
Seeds of D. intermedia are more ovoid than the other two British species and they are dark in colour and warted. The rough, warted nature of the seed coat makes them stick together, reducing their ability to disperse by wind. In water the seeds can float for several months and probably local water movement represents the best chance there is for dispersal.
The proportion of plants flowering, capsule numbers and seed production per capsule are all highly variable from year to year and place to place making general statements pointless. The proportion of seeds showing viable germination in experimental studies is often very low − in several studies it being around a mere 2%. When seeds are given a prolonged period of cold stratification of up to eight weeks, followed by a diurnal temperature regime and 14 hour daylength, however, germination rates were improved up to 78-90% (Crowder et al. 1990).
Fermanagh occurrence

D. intermedia is more local and appears scarcer in Fermanagh than the other two native sundew species. Its frequency is merely occasional and it is mainly found on the Pettigo Plateau bogland and around the Five Points area of lowland bogs W of Enniskillen. D. intermedia has been recorded in a total of 31 tetrads (5.9% of the total in the VC), but has not been seen in six of these during the post-1975 period.
British and Irish occurrence and decline
John Harron in his 1986 Flora of Lough Neagh noted a similar decline of D. intermedia populations around that large Ulster lake, where he described it as, "very local and becoming rare". This decline in frequency is reflected on the larger scale when one examines the B & I distribution of D. intermedia displayed in the New Atlas hectad map, or indeed when the world distribution of the three species native to these isles is considered (Crowder et al. 1990).
D. intermedia is local and scattered throughout B & I and displays a strongly marked western tendency of occurrence, more obvious than that shown by D. anglica and D. rotundifolia. It has suffered a long period of decline in both islands, dating from at least the end of the 19th century onwards. This is particularly obvious in lowland C & SE England and in C Ireland, where it has become locally extinct in many previously occupied sites, but the phenomenon is much more widespread than this. Losses appear to follow drainage, peat cutting and afforestation operations, affecting in particular the destruction of lowland heaths and bogs. The result of this has been to make the plant even more definitely western in occurrence in both B & I than previously was the case (F.J. Rumsey, in: Preston et al. 2002).
Sinker et al. (1985) suggested that D. intermedia is the Sundew in these isles least tolerant of competition, but it is uncertain on what scientific basis this verdict is given. It may simply be a subjective comment based on field observation, such as I have made above. On the other hand, an experimental culture study made in a bog lake in Ontario by Wilson & Keddy (1986) examined the competitive ability of D. intermedia when it was paired in turn with six non-insectivorous species in containers (no species names given when reported in Crowder et al. 1990); it was found to be a poor competitor, unable to tolerate much natural stress or disturbance in the particular conditions involved.
European and world occurrence
Beyond B & I, D. intermedia has a much more restricted occurrence than either D. rotundifolia or D. anglica. It is present in NW and C Europe, but is absent from N Scandinavia and from most of the Mediterranean area, except a small area in Tuscan Italy (Jalas et al. 1999, Map 2964). It is also rare in E Europe, although it has been recorded from the eastern end of the Black Sea (Hultén & Fries 1986; Crowder et al. 1990). Other sources also mention Turkey (Sell & Murrell 2018), although there is not unanimity about the more eastern occurrences in the published maps and other literature. The overall phytogeographic distribution is described as belonging to the Suboceanic Temperate element (Preston & Hill 1997).
D. intermedia is also amphi-Atlantic, being widespread in eastern N America. However, there are only a few isolated records from the N and NW of that continental landmass, although to the south it is recorded down into Florida and on the island of Cuba (Hultén & Fries 1986, Map 1001).
Threats
As with all Sundew species, drainage, cutting of peat bogs and afforestation are the major problems restricting available habitats.