Knotweeds & Knotgrasses

Common Buckwheat Equal-leaved Knotgrass Pale Persicaria Redshank

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. 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.

Common Knotgrass Common Knotgrass Common Knotgrass
Habit
Flower
Nutlet


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.

Common Knotgrass Common Knotgrass Common Knotgrass Common Knotgrass
Habit
Flowers
Leaves
Nutlet


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.

Cornfield Knotgrass Cornfield Knotgrass Cornfield Knotgrass Cornfield Knotgrass
Habit
Habit
Flowers
Leaf


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.

Redshank Redshank Redshank Redshank
Habit
Flowers
Leaf
Ocrea


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 similar species by the yellowish glands at the base of the flower spike which have the glandular heads larger than their stalks.

Pale Persicaria Pale Persicaria Pale Persicaria Pale Persicaria
Habit
Flowers
Leaf
Stalkless glands
Pale Persicaria Pale Persicaria Pale Persicaria
Habit
Flowers
Stalkless glands


Pennsylvania Smartweed      Persicaria pensylvanica

Introduced. 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.

Pennsylvania Smartweed Pennsylvania Smartweed Pennsylvania Smartweed Pennsylvania Smartweed
Flowers
Leaves
Ocrea
Stalked glands


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 gracefully arching flower spikes; leaves have a strong, peppery bite to them.

Common Water-pepper Common Water-pepper Common Water-pepper
Habit
Flowers
Leaf


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.

Pink-headed Persicaria Pink-headed Persicaria Pink-headed Persicaria Pink-headed Persicaria
Habit
Habit
Flowers
Leaves


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.

Amphibious Bistort Amphibious Bistort Amphibious Bistort Amphibious Bistort
Habit
Flowers
Terrestrial leaves
Leaf hairs


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.

Common Bistort Common Bistort Common Bistort Common Bistort
Habit
Flowers
Leaf
Ocrea


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.

Red Bistort Red Bistort Red Bistort Red Bistort
Habit
Flowers
Leaf
Leaf base


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.

Lesser Knotweed Lesser Knotweed Lesser Knotweed Lesser Knotweed
Habit
Flowers
Leaf
Ocrea


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.

Himalayan Knotweed Himalayan Knotweed Himalayan Knotweed Himalayan Knotweed
Habit
Flowers
Leaves
Ocrea


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.

Common Buckwheat Common Buckwheat Common Buckwheat Common Buckwheat
Habit
Flowers
Leaf
Leaves


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.

Japanese Knotweed Japanese Knotweed Japanese Knotweed Japanese Knotweed
Habit
Flowers
Flowers
Leaves
Japanese Knotweed Japanese Knotweed Japanese Knotweed Japanese Knotweed
Seed capsule
Stem
Spring shoot
Winter stems


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.

Giant Knotweed Giant Knotweed Giant Knotweed Giant Knotweed
Habit
Flowers
Flower and seed capsules
Leaf


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.

Russian Vine Russian Vine Russian Vine Russian Vine
Habit
Flowers
Flowers
Leaves
Russian Vine Russian Vine Russian Vine
Winter habit
Stems
Bud


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.

Black Bindweed Black Bindweed Black Bindweed Black Bindweed
Habit
Flowers
Leaf
Seed capsules


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.

Wireplant Wireplant Wireplant Wireplant
Habit
Female flowers
Leaves
Leaves
Wireplant Wireplant
Black nutlet
Ocrea