Ontario Weeds: Pale smartweed

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Excerpt from Publication 505, Ontario Weeds, Order this publication

Table of Contents

1. Name
2. Other Names
3. Family
4. General Description
   • Photos and Pictures
5. Stems
6. Leaves
7. Flowers and Fruit
8. Habitat
9. Similar Species
10. Related Links

Name: Pale smartweed, Polygonum lapathifoliurn L.,

Other Names: persicaire pâle, Pale persicaria, White smartweed, persicaire à feuilles de patience, persicaire élevée

Family: Buckwheat or Smartweed Family (Polygonaceae)

General Description: Closely resembling Lady's-thumb but more variable in height, occasionally exceeding 2m (6½ft) in swampy areas, usually only 45-90cm (11/2-3ft) in cultivated fields, or nearly prostrate in open areas.

Photos and Pictures

Lady's Thumb. A. Plant. B. Portion of stem with leafstalk and ocrea. Pale smartweed. C. Portion of stem with lower surface of leaf blade, leafstalk and ocrea. Pennsylvania smartweed. D. Portion of lower stem with leafstalk and ocrea. E. Portion of upper stem with leaf stalk and ocrea showing stalked glands.

Stems: Stems somewhat swollen at or just above the nodes, particularly on large plants; ocrea smooth and papery in texture, lacks surface hair, and is either without a fringe on the margin or has such a short fringe of hair it cannot be seen by the naked eye; lower stems smooth and hairless but upper stems and branches near and in the inflorescence somewhat rough or gummy due to tiny, yellowish to amber, unstalked glands mostly less than 0.2mm (1/100in.) high, resembling but much shorter than.

Leaves: Leaves about the same size and shape as in Lady's-thumb but the undersurface of the first 5 to 7 true leaves above the cotyledons (seed leaves) with a short, dense, felt-like mat of whitish hair, then abruptly changing with the next leaf (6th to 8th leaf above cotyledons); that leaf and all others above it lacking the whitish hair, but instead having numerous, extremely tiny, yellowish to amber glands, slightly imbedded in the undersurface of the leaf; these glands visible as glints of reflected light when the leaf is rolled backwards over the finger or their presence indicated by a yellowish smudge if a fresh leaf is rubbed against white paper or cloth.

Flowers & Fruit: Flowering spikes whitish, or rarely pinkish, long and slender, 7-10cm (3-4in.) long and often drooping, those on side branches usually shorter and rarely drooping; lateral spikes usually with at least 1cm (2/5in.) of bare stem below the lowest flower; sepals of flowers longer than the seed, their tips folded together into a "beak" above the fruit ("seed"); "seed" shiny, black, roundish in outline but flattened in cross-section, the flattened sides slightly hollowed or dented in the middle, usually less than 2mm (1/12in.) in diameter. Flowers from June to August.

Habitat: Pale smartweed is a common native plant around streams, lakes and wet depressions throughout Ontario, often persisting as a weed in those locations whenever they come under cultivation. It also occurs on moderately well-drained upland soils and is a frequent weed in cultivated fields, pastures, roadsides and gardens.

Similar Species: It is distinguished from Lady's-thumb by the matted white-hairy undersides of its lowermost 5 to 7 leaves, the tiny, yellow to amber glands on the under-sides of its upper leaves, the lack of hair on its ocrea, its upper stems glandular-roughened, and its "seeds" never rounded-triangular but always flattened and with the sides slightly depressed.

Related Links

... on general Weed topics
... on weed identification, order OMAFRA Publication 505: Ontario Weeds
... on weed control, order OMAFRA Publication 75: Guide To Weed Control

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