Great Water
Dock
Rumex
britannica
Knotweed family
(Polygonaceae)
Description:
This perennial herbaceous plant is 2½–6' tall; it is unbranched below
and branched above. The lateral stems arch upward, becoming more or
less erect, like the central stem. The stems are light green to reddish
green, terete, longitudinally furrowed, hairless, and rather stout
(especially the central stem). Alternate leaves occur along the entire
length of these stems below their inflorescences; the leaves are
arching and widely spreading. Lower leaves are up to 12" long and 4"
across, while upper leaves are up to 6" long and 1½" across; they
become gradually smaller in size as they ascend up the stems. The
leaves are lanceolate-oblong to elliptic-oblong in shape; their
margins are flat to undulate (up-and-down) and they are usually finely
toothed (denticulate), rather than toothless. The leaf bases are
rounded to narrowly cuneate (wedge-shaped), while their tips are
bluntly acute to acute.
The upper leaf surface is medium green, hairless, and rather glossy,
while the lower leaf surface light green or whitish green, hairless,
and often glaucous. The petioles are light green or light reddish
green, hairless, and relatively short (up to 2" in length) for the
middle to upper leaves; lower leaves have longer petioles up to 6" in
length. Leaf venation is pinnate. The lateral veins of the leaves form
about an 80º angle with their central veins. The petioles are concave
above, while below they are convex and sometimes furrowed. Membranous
sheaths up to 1½" long wrap around the stems where the petioles of the
leaves occur. As they age, the sheaths become discolored (often red)
before finally turning brown; they also become shredded or ragged along
their upper margins.
The central stem and lateral stems terminate in
panicles of flowers about 4-16" long. Along the stalks (rachises)
of panicles, there occurs whorls of 5-25 flowers on drooping
pedicels. These whorls of flowers are densely distributed to slightly
interrupted along the stalks of mature panicles. The floral stalks have
similar characteristics to the stems below, except the former tend to
be more narrow in diameter. Each flower is perfect and about 3-4 mm.
across, consisting of 3 outer tepals, 3 inner tepals, 6 stamens, and an
ovary with a tripartite style. Except for the anthers of the stamens,
all parts of the flower are light green to reddish green; they are also
hairless. The outer tepals are narrowly oblong in shape, while the
inner tepals are more upright and oblong-oblanceolate in shape.
The slender pedicels of mature flowers are 5-10 mm. in length, light green, and hairless;
they have inconspicuous joints near their bases. The blooming period
occurs from mid-summer to early autumn, lasting about 2-4 weeks for a
colony of plants. The flowers are cross-pollinated by the wind.
Afterwards, the 3 inner tepals of each flower develop into the valves
of the fruit, while an achene is contained within a narrowly lanceoloid
tubercle along the upper-center of each valve (3 achenes per fruit). The
valves of the fruit are light green to reddish green (while immature),
orbicular-cordate in shape, and minutely toothed
(crenulate-denticulate) along their margins. A network of fine veins
occurs across the outer surface of each valve. At maturity, the fruits
turn brown and their valves become 4-7 mm. long and a little less
across. The achenes of the mature fruits are 3–4.5 mm. long and 1.5–2.5
mm. across; they are reddish brown to brown with a 3-angled ovoid
shape. The root system consists of a stout taproot. This plant
reproduces by reseeding itself.
Cultivation:
The
preference is
full to partial sun, wet to moist conditions (including very shallow
water), and calcareous soil containing considerable organic matter.
Range
& Habitat: The native Great Water Dock (Rumex britannica)
is occasional in NE
Illinois, uncommon in central Illinois, and either rare or absent in
the rest of the state (see Distribution
Map). This plant occurs
primarily in northeastern USA, the Great Lakes region of the upper
Midwest, and adjacent areas of Canada. Illinois lies along the southern
range limit of this species. Habitats consist of various wetlands,
including swamps, cattail marshes, fens, calcareous seeps, low areas
along springs, and roadside ditches. Great Water Dock usually occurs in
higher quality natural areas.
Faunal
Associations:
Many kinds of insects feed on the foliage, seeds, roots, etc., of dock
species (Rumex spp.).
Insects that occur in wetlands are especially
likely to feed on Great Water Dock (Rumex
britannica). Examples include
the larvae of Lycaena
hyllus (Bronze Copper) and other Copper
butterflies, flea beetles (Mantura
spp., Chaetocnema
concinna), Lixus
concavus (Rhubarb Weevil) and other weevils, maggots of Contarinia
rumicis (Dock Seed Midge), Poecilocapsus lineatus (Four-lined
Plant
Bug) and other polyphagous plant bugs, Dysaphis radicola (Apple-dock
Aphid) and other aphids, larvae of Ametastegia
glabrata (Dock Sawfly)
and other Ametastegia
spp., larvae of Phragmatobia
fuliginosa (Ruby
Tiger Moth) and other moths, and leaf-mining maggots of Pegomya bicolor
(an Anthomyiid fly); see Opler & Krizek (1984),
Harms &
Grodowitz (2009), Vestal (1913), Felt (1917), Knight (1941), Blackman
& Eastop (2013), Smith (2006), Covell (1984/2005), and Needham
et al. (1928). The seeds of dock species are eaten by many granivorous
songbirds, including the Swamp Sparrow, Song Sparrow, Bobolink, and
Red-winged Blackbird. These plants are also a source of food for the
Meadow Vole and Muskrat (Martin et la., 1951/1961; Hamerstrom &
Blake, 1939).
Photographic
Location:
A fen at Cowle's Bog in NW Indiana.
Comments:
Great Water Dock (Rumex
britannica) is one of the larger dock species
(Rumex spp.)
in Illinois. It is non-weedy and occurs in wetlands. In
general, Great Water Dock can be distinguished from other dock species
by its large size, strong preference for higher quality wetland
habitats (at least in Illinois), the large size of its lower leaves (up
to 12" long and 4" across), the finely toothed (denticulate) and
crisped margins of its leaves, the nearly perpendicular lateral veins
of its leaves (relative to the central veins), and more technical
characteristics of its 3-valved fruits. The valves of its fruits are
minutely toothed along their margins, rather than spiked or toothless.
In addition, the valves of its fruits are more orbicular in shape than
those of many other dock species. Both valves of its fruits and its
achenes tend to be a little larger in size than most dock species in
Illinois, and Great Water Dock has fruits with 3 achenes each, rather
than a single achene per fruit. A scientific synonym of this plant is
Rumex orbiculatus.