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Myriophyllum verticillatum L., Whorled Water-milfoil

Account Summary

Native, occasional. Circumpolar temperate.

1939; Praeger, R.Ll.; lakelet west of Lough Galliagh.

May to September.

Growth form and preferred habitats

Whorled Water-milfoil is a submerged aquatic perennial with long, flaccid leaves that are markedly longer than the stem internodes that bear them. Mid-stem leaves are 15-45 mm long, 1.4-4(-5.5) times as long as the adjacent internodes and are borne in whorls of four or five. M. verticillatum grows best in still to slow, sheltered waters, over fine inorganic or peaty soils at shallow depths down to 100 cm (Haslam et al. 1975; C.D. Preston, in: Rich & Jermy 1998). It rapidly becomes scarce in deeper waters down to about 3 m, depending upon levels of clarity and light penetration. M. verticillatum can tolerate some degree of water turbidity in shallow situations, but it grows in a much more restricted pH range than M. spicatum (Spiked Water-milfoil), being more or less confined to moderately acid to near-neutral, moderately productive, meso-eutrophic conditions (Preston & Croft 1997). When water levels drop during drier periods in summer, provided this is a gradual process, Whorled Water-milfoil very readily develops an emergent, dwarf, terrestrial growth form that gradually becomes stranded on the exposed mud, and it persists in this form until re-submerged (Aiken et al. 1979).

Fossil evidence and status

In common with the other two Water-milfoil species in Fermanagh (M. spicatum and M. alterniflorum (Alternate Water-milfoil)), fossil evidence suggests that M. verticillatum is a very ancient native species, probably having survived throughout the last full-glacial period in situ in B & I (Godwin 1975).

Vegetative reproduction

As is general in all aquatic species, reproduction of M. verticillatum is heavily weighted towards asexual means, involving in this case the detachment of specialised autumn-produced, robust, club-shaped, dark green, leafy vegetative buds (turions), borne either in leaf axils or terminal on stems, plus stem fragmentation. Detached turions demonstrate dormancy which can be broken by a cold treatment (4°C) (Weber & Noodén 1976). Both of these vegetative methods enable potential rapid increase and an easy means of dispersal in already colonised water systems.

In Ireland, boating and fishing on the more northerly Royal Canal can be affected by dense stands of this aquatic and weed control involves mechanical removal in the autumn, after the overwintering turion buds have been formed on the lower parts of the stems (Caffrey & Monaghan 1995).

Flowering reproduction

Flowering and seed production certainly occurs, but as with other aquatics, seedlings are not readily observed. M. verticillatum, like the other two native members of the genus already mentioned, has a terminal inflorescence, 7-25 cm long, of small, whorled, unisexual and bisexual flowers held above the water surface (ie emergent). The unisexual flowers have reduced petals and sepals that sometimes are shed and they are thus well adapted for wind pollination. The bisexual (perfect) flowers have all their parts present and usually occur between the male and female flowers (Sell & Murrell 2009). Powdery pollen is released on the breeze from anthers on long filaments (Cook 1988). Flowering takes place in July and August. The fruit is a sub-globose schizocarp 2 mm across, that splits into four 1-seeded mericarps (Sell & Murrell 2009). Very little is known about the breeding systems of aquatic species, so it really is not possible to generalize on the question of in- or out-breeding in relation to the mode of pollination of these plants (Cook 1988).

Fermanagh occurrence

With the exception of one site in Co Down (H38) and a few scattered up along the Co Antrim (H39) coast, all recent records of this aquatic for NI are from the Lough Erne and Lough Melvin basins in Fermanagh. This species declined from rarity to local extinction at sites around Lough Neagh, possibly sometime after the late 1940s (Harron 1986) or perhaps even earlier in the 1920s (Hackney et al. 1992).

As the tetrad distribution map demonstrates, M. verticillatum remains widespread, but is still only occasional in Fermanagh, confined to the larger lowland lakes, and scattered in the S and W of the county. Locally it has been recorded in a total of 27 tetrads (5.1%), 23 of which have post-1975 records. Almost all of its sites are in drains, or on fen-fringed, or muddy lakeshores, washed by lime-rich waters around the basins of the larger lakes in the VC.

M. verticillatum is the least frequent of the three species of Myriophyllum found in Fermanagh. As an indication of scarcity, M. spicatum (Spiked Water-milfoil) is over nine times more likely to be found than M. verticillatum, while M. alterniflorum is also over three times more frequent than it in the VC.

Irish occurrence

In Ireland as a whole, Whorled Water-milfoil is most frequent in lakes, ponds, ditches and canals on the Central Plain in Leinster and Connaught. Here, Carboniferous limestone underlies inorganic glacial tills and more recent accumulations of raised and blanket bog peat, but drainage waters in the region are still essentially lime-rich. During the last 50 years, with the advent of mechanised peat extraction and peat-burning power stations, many areas of very extensive, deep peat deposits have been almost entirely removed in this region of Ireland, further exposing the underlying limestone geology.

Both editions of the Flora of the County Dublin (Colgan 1904; Doogue et al. 1998) and the intervening Supplement (Brunker et al. 1961), indicate the highly significant role the Royal and Grand Canals linking Dublin with the great River Shannon and its lakes and tributaries further west have played in the spread and persistence of this always rare aquatic species.

Changes in British and Irish occurrence

There are huge differences in the extent of the recording of this species between the two BSBI Atlas surveys produced in 1962 and 2000. The large number of older, pre-1970 symbols on the New Atlas map indicate that the species decline observed in the earlier BSBI Atlas survey in England has continued and the plant is now regarded as scarce there (Walters & Perring 1962; Stewart et al. 1994). Factors such as widespread eutrophication from agricultural run-off and atmospheric nitrogen pollution, or the deepening of water channels during drain clearance, very readily destroy suitable habitats for this species.

Meanwhile in Ireland, an increased recording effort, assisted by better knowledge of aquatic species in general, means that C Ireland now maintains the best M. verticillatum populations anywhere in these islands (C.D. Preston, in: Preston et al. 2002). Although M. verticillatum does extremely well in the Irish canal system and can grow to nuisance proportions, under-recording is still a possibility here and it may well be that some of the local populations are small and not easily noticed. They certainly are very scattered, reaching even a few isolated hectads on the N Antrim coast (H39).

European and world occurrence

M. verticillatum is widely distributed in temperate parts of Eurasia and N America and is circumpolar. Records from Argentina and Chile, indicated by Hultén & Fries (1986, Map 1373), are errors (Preston & Croft 1997).

Threats

Factors such as widespread eutrophication from agricultural run-off and atmospheric nitrogen pollution, or the deepening of water channels during drain clearance, may very easily destroy suitable habitats.