Knotweeds & Knotgrasses
What are they?
This is a rather complex group of plants, all of which - along with docks - form the family Polygonaceae. These plants have petalless flowers, but the sepals are enlarged and petal-like and referred to as tepals. The flowers may be open, with well spaced tepals, or may have the tepals carried in a tighter, bell shape. The flowers are carried in clusters, either in loose spikes or in tight heads. This group contains a wide range of garden ornamentals, natives and some rather aggresive alien invaders.
Where are they found?
With such a diverse group, it's perhaps not surprising that these plants are found in a wide range of habitats and the habitat can often help with the identification process. Most native species are either wetland plants or plants of disturbed ground in gardens and on farmland. Garden ornamentals may be found as survivors from dumped garden waste in all kinds of overgrown corners, roadsides and rough ground.
Identification
Because this is a diverse group, it's nice to know that there's a common feature that defines them readily. All of these species have a thin, papery structure at the base of the leaf which wraps around the stem like a sheath. This structure is known as an ocrea (sometimes spelt ochrea) and can be used to recognise members of this family (see also the docks), while some species have an ocrea with features that distinguish the plant from other, rather similar species. Note that many species can vary in having either pink or white flowers. Such variation is due to the amount of red, anthocyanins in the plant and can also affect the colour of the stems and ocrea. Thus, you may find plants that differ in stem, flower or ocrea colour to the photos shown here. Care needs to be taken with many members of this group and details are given in the text for each species. Be prepared to use a hand lens to check for stem glands, flower glands and hair structures - and you may need to have a small ruler handy!
Common Knotgrass Polygonum aviculare
Native. Widespread throughout the region and often an abundant plant, either as a creeping mat on the ground or more upright amongst taller vegetation. On all types of enriched, usually bare, ground. Flowers July to September. Sprawling or trailing annuals with narrow leaves and long, wiry stems. The leaves of the main stems are clearly larger than those of the side stems (though take care when assessing this as the leaves of the main stems tend to drop as the plant continues to grow). Tepals white with a green central stripe, sometimes tinged pale pink. Nutlets pointed, 2.5-3.5mm long, concave on all three sides.
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Equal-leaved Knotgrass Polygonum depressum
Native. Widespread throughout the region and often an abundant plant, typically as a flat, creeping mat on bare ground that is well trodden, especially gravel parking areas and footpaths. Flowers July to September. Sprawling or trailing annuals with ovate leaves and wiry stems; typically less elongate, more compact than Common Knotgrass. The leaves of the main stems are about the same size as those of the side stems. Tepals white with a green central stripe, sometimes tinged pale pink. Nutlets pointed, 1.5-2.5mm long, concave on only one side, convex on the other two sides.
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Cornfield Knotgrass Polygonum rurivagum
An ancient introduction which occurs thinly throughout the region and is probably under-recorded. Typically found in arable fields and most easily found after harvest, when the siry stems scramble over and through the stubble and taller weeds. Flowers August to November. A wiry, very slender annual that is easily overlooked. The leaves are typically less than 3mm wide and the flowers usually have a rich pink tinge to the tepals.
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Ray's Knotgrass Polygonum oxyspermum subsp. raii
Native. A very rare plant of coastal shingle, occurring sporadically in very small quantity around Felixstowe and in north-east Essex. Formerly recorded from Norfolk. Flowers June to September. A prostrate plant with slender stems and grey-green leaves. The nutlets are 3.5-5mm long, very shiny, sharply pointed and protrude beyond the tips of the petals.
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Redshank Persicaria maculosa
Native. Common throughout the region in a wide variety of habitats, especially in damp places and on heavier soils, where small plants can form solid stands. Flowers June to October. A remarkably variable species that may be anything from 20-100cm in height. Leaves typically with a dark blotch in the middle (though not always) and flowers pink or cream. Best told from similar species by the lack of yellowish glands at the base of the flower spike.
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Pale Persicaria Persicaria lapathifolia
Native but probably also some introduced populations from mainland Europe, too. Common throughout the region in a wide variety of habitats, especially in damp places and on heavier soils. Flowers June to October. A very variable species that may be anything from 20-100cm in height and often well branched to form shrubby growths. Leaves typically without a dark blotch in the middle (though not always) and flowers pink or cream. Best told from Redshank by the yellowish glands at the base of the flower spike which have the glandular heads larger than their stalks. Seeds are flattened and disc-shaped. In really difficult individuals it is worth checking the tips of the veins on the tepals, which in Pale Persicaria branch to frorm distinctive, anchor-shaped marks.
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Dense-flowered Smartweed Persicaria glabra
Introduced from North America. Very rare in the UK but recorded in mid-Norfolk in 2019 by the Norfolk Flora Group and perhaps overlooked due to similarity to other species. Flowers June to October. Extremely similar to Pale Persicaria in having yellowish glands below the flowerheads, but the seeds are biconvex or three-sided, while the veins on the tepals do not branch out at the outer tips to form anchor-shaped marks.
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Pennsylvania Smartweed Persicaria pensylvanica
Introduced from North America. Old records exist for Norfolk and Cambridgeshire but the species remains rare and seems not to have become established. Flowers June to October. Very similar to some forms of Pale Persicaria but the leaves typically average a little longer, as do the hairless ocreas. Best identified by the yellow glands on the stem below the flower heads, which are carried on slender stalks.
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Common Water-pepper Persicaria hydropiper
Native. Widespread in wet and muddy places but largely absent from chalky or heavy clay soils. Common in Broadland, the Suffolk Sandlings and south Suffolk. Flowers July to November. A slender species with open, gracefully arching flower spikes; leaves have a strong, peppery bite to them and usually clearly undulate margins.
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Small Water-pepper Persicaria minor
Native but rare in wet, muddy places, with a handful of records mostly from Norfolk and Cambridgeshire. Flowers July to September. A tiny species with few-flowered heads and small leaves that are linear in outline and not peppery tasting.
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Pink-headed Persicaria Persicaria capitata
Introduced as a garden ornamental and may rarely be found as a short-lived garden escape or where seeding into pavement cracks in urban areas. Flowers July to November or until the first frosts. A distinctive, small species with strongly blotched leaves and rounded flower spikes.
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Amphibious Bistort Persicaria amphibia
Widespread and common on damp soils along river valleys throughout the region Also occasionally as a weed of cultivation on heavier soils. Flowers July to September. A perennial species that can form extensive patches of vegetation. This species produces rather narrow leaves when growing on land, but also grows in water and then produces wider leaves that float at the surface. Leaves told from other species by the rounded, heart-shaped bases and by the stiff white hairs that often arrange themselves in lines on the leaf. Flowers deep pink.
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Common Bistort Bistorta officinalis
Possibly native at one or two sites but generally thought to be largely an introduction to East Anglia, initially as a culinary plant and latterly as an ornamental; rare in damp meadows, more frequent as a garden throw-out. Flowers June to August. Leaves broad and dock-like. Flowers in tight spikes at the top of long, bare stalks.
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Red Bistort Bistorta amplexicaulis
Introduced from the Himalayas as a garden ornamental and may rarely be found as a garden cast-away. Flowers August to October. A very distinctive species with narrow spikes of deep red flowers and leaves that wrap around the stem at the base.
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Lesser Knotweed Koenigia campanulata
Introduced from the Himalayas as an ornamental and recorded as a rare garden escape in Norfolk. Flowers July to September. A perennial species that can form extensive colonies of stems to 150cm in height. Flowers narrowly bell-shaped and leaves relatively short, ovate, with very distinctly indented veins. Ocrea short.
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Himalayan Knotweed Koenigia polystachya
Introduced from the Himalayas as an ornamental and recorded at a dozen or so sites as a garden escape. Flowers August to September. A perennial species that can form extensive colonies of stems to 200cm in height. Flowers opening widely, not remaining bell-shaped and leaves relatively long, lanceolate. Ocrea long.
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Common Buckwheat Fagopyrum esculentum
Introduced since at least Tudor times as food for both humans and livestock before falling out of favour. More recently becoming widely sown as pheasant cover and occasionally appearing self-sown from birdseed. Flowers July to August. The angular, more or less heart-shaped leaves distinguish this species from other members of the family.
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Japanese Knotweed Reynoutria japonica
Introduced from eastern Asia as a garden ornamental and now one of the most persistent of garden escapes in disturbed ground along railways, roadsides and - especially - along water courses. Flowers August to October. Forms dense stands of bamboo-like stems that die down over winter. Leaves squared off at the base.
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Giant Knotweed Reynoutria sachalinensis
Introduced from eastern Asia as a garden ornamental and persistent where garden waste is dumped. Flowers August to October. Forms dense stands of bamboo-like stems that die down over winter. Leaves heart-shaped at the base. Far less common than Japanese Knotweed but persistent where it occurs.
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Russian Vine Fallopia baldschuanica
Introduced as an ornamental and frequently found spreading into hedgerows or persisting where garden waste is dumped. Flowers September to October. A vigorous, woody vine with tightly twining stems which may grow to 10m or more and smother surrounding vegetation.
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Black Bindweed Fallopia convolvulus
An ancient introduction from mainland Europe and now common throughout the region as a weed of arable and disturbed ground. Flowers July to September. A trailing or twining plant, scrambling over other vegetation. The spear-shaped leaves may be confused with those of the true bindweeds, but the flowers are very different and there is a small ocrea present at the leaf base.
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Wireplant Muehlenbeckia complexa
Introduced from New Zealand as a garden curiosity. Well established and spreading from original plantings on cliffs at Bawdsey, Suffolk. Flowers September to November. A vigorous, climbing or scrambling perennial, capable of smothering other vegetation and forming dense tangles of wiry stems. Male and female flowers appear on separate plants.
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