Persicaria lapathifolia (L.) Delarbre, Pale Persicaria
Account Summary
Native or possibly introduced, occasional, perhaps only casual. Circumpolar southern-temperate, but also widely naturalised in both hemispheres.
1900; Praeger, R.Ll.; Co Fermanagh.
July to October.
Growth form and preferred habitats
Overall this genetically very variable and phenotypically extremely plastic annual knotweed looks rather similar to the extremely common ruderal weed, P. maculosa (Redshank). It is larger than the latter, sometimes reaching 1.2 m in height, although more often it is less than half this. The stems are usually very pale green, tinged pink, and the flowers typically have a greenish-white, rather anaemic appearance (and thus its English common name, 'Pale Persicaria'). Flower colour is very variable, however, and it can also be a dingy pale or a deeper pink. Also, the peach-like leaves are blotched with black (often), or sometimes not; the yellow glands on the peduncle are densely present and can often be seen with the naked eye (Lousley & Kent 1981). Flower colour, habit and pubescence are all very variable, as Stace (1997) very helpfully points out!
Like many other Persicaria species (and especially like P. maculosa), ecologically it is a weedy pioneer colonist of a wide range of open or recently disturbed habitats, including cultivated land, manure heaps, waste ground and moist soils marginal to water. P. lapathifolia occurs on various types and textures of soil, from river gravel to silt, through sand and clay, but it appears most often, and can sometimes form large, dense stands in mildly acid, nutrient-rich, peaty fen conditions, avoiding strongly acidic and nutrient-poor terrain. The species is a weak competitor and very stress tolerant. The chief habitat requirement therefore is sufficient disturbance to reduce competition from associated, often weedy species (Simmonds 1945; Sinker et al. 1985; Mitich 1998).
Variation
In addition to the above remarks, P. lapathifolia is extremely variable genetically, and is phenotypically very plastic with respect to local environments. Two varieties are recognised in Britain and Ireland by Sell & Murrell (2018), var. lapathifolia, and var. salicifolia, the latter having leaves that are whitish and densely arachnoid-hairy beneath (ie cobweb-like). In Scandinavia, two subspecies are described, subsp. lapathifolia and subsp. pallida (With.) S. Elkman & T. Knutsson, the latter being a crop weed that can be further divided into two varieties, var. incana and var. linicola, the latter being a weed specific to flax crops.
Worldwide, P. lapathifolia s.l. is an extremely complicated and variable complex in which a great number of species, subspecies, varieties and forms have been described. In the boreal belt of the N hemisphere, the variation is grouped around two main types, P. tomentosum Schrank and P. nodosum Pers. However, the two types are not everywhere distinctly separate (eg in Scandinavia), and also their ranges overlap (Hultén 1974, pp. 244 & 390, Map 236).
Hybrids
Hybrids of P. lapathifolia are known with P. foliosa (H. Lindb.) Kitag., P. hydropiper (Water-pepper) and P. maculosa, which underlines the fact that reproduction in this species does not entirely involve selfing (Jonsell et al. 2000).
Reproduction
Being annual, P. lapathifolia reproduces entirely by seed. Decumbent lower branches may root at nodes touching the ground, although there is no suggestion of vegetative reproduction. The plant dies in the autumn.
Flowering occurs around four to six weeks after germination, generally between June and September (Mitich 1998). The flowers are bisexual (perfect) but pollen production is very low (about 30 pollen grains per anther). The low ratio of pollen to ovule production is characteristic of inbreeding, and indeed fertilisation is almost or entirely autogamous (selfing), although small insects (thrips) have been recorded visiting, possibly assisting pollination (Simmonds 1945; Jonsell et al. 2000).
Fruiting takes place from mid-July onwards. The fruit is single-seeded, dry (ie a nut-like achene), and it is contained within the persistent perianth. The achene is lens-shaped (lenticular) or rarely triangular in section (trigonous), brown or black in colour, and either glossy or dull. The number of seed produced is extremely variable, ranging from 10 to about 1,500 per plant (Simmonds 1945). A weed seed trial found an individual plant could yield up to 19,300 seeds when grown without competition (Stevens 1957).
Seed dispersal
There is no specific seed dispersal mechanism, the mature fruit simply fall or are knocked off the parent plant. As with P. maculosa, birds and herbivorous mammals (cattle, horses and deer) may eat the plants and transport seed, depositing it with their faeces (Ridley 1930). There is no information regarding seed viability after passing through the alimentary canal, although this was reported for the closely related P. maculata.
In the past, fruits were known to occur as impurities of agricultural clover seed and livestock feed, and the species has undoubtedly been spread by man in this manner (Simmonds 1945). Seed cleaning processes are very much more efficient nowadays, and dispersal as a crop seed impurity has probably been very greatly curtailed, if not totally eliminated everywhere.
Seed germination and longevity
P. lapathifolia and P. maculosa not only look alike, an experimental study found their seeds also behave in a very similar manner when cultivated and disturbed three times per year. Results showed a relatively high percentage of Pale Persicaria seed germinated in the April and May of the year after production, and in subsequent years seed numbers progressively declined, although a small percentage persisted and remained viable after five years (Roberts & Neilson 1980). This is entirely consistent with the ecology of other weedy members of this family, all of which appear to demonstrate prolonged survival in the soil seed bank.
Fossil record
All records are for achenes which have been found in small numbers from the Cromer Forest Beds onwards in both glacial and interglacial stages, proving ancient native status and suggesting possible periglacial survival. In the current Flandrian, records are numerous and extend from the Bronze Age to the Mediaeval period in archaeological sites. The evidence suggests the fruits were previously part of the prehistoric human diet (Godwin 1975).
Fermanagh occurrence
P. lapathifolia is only occasional in Fermanagh, but it is widespread mainly around Lough Erne and in the lower-lying, rather more fertile, less wet farmland of the east of the county. Prior to the current flora survey (Forbes & Northridge 2012) there were just four records. The original Praeger record of 1900 was completely unspecific with respect to site and there were three additional records made by Meikle and co-workers in the 1946-54 period. A total of 34 subsequent records were made from 1986 onwards by several recorders, although the majority were by RHN. The records of this annual are now spread across 32 Fermanagh tetrads, 6.1% of those in the VC. It is mostly found around Lower Lough Erne, but is also scattered thinly to the east of the county.
It occurs on cultivated and disturbed ground, including waterside habitats, roadsides and in old quarries.
Status in Ireland
In the Connemara and the Burren regions of western Ireland, Webb & Scannell (1983) regarded P. lapathifolia as being, "perhaps little more than casual" in its appearances. Furthermore, the Irish Census Catalogue regarded its status as, "possibly introduced" (Scannell & Synnott (1987).
British and Irish occurrence
While P. lapathifolia is physically, biologically and ecologically most similar to P. maculosa, and the two species often occur together, it is nothing like as common as the latter in Britain and Ireland. The difference in presence is no doubt due to Redshank possessing wider ecological tolerances, rather than it being more competitive.
The New Atlas hectad map shows P. lapathifolia is not quite as widespread and omnipresent in lowland England and Wales as Redshank. In Scotland, it becomes much less prevalent than further south in England, being scattered and increasingly eastern and coastal northwards. The species map for Ireland likewise shows that P. lapathifolia is very much less common and widespread than it is in England and Wales. The Irish distribution of P. lapathifolia is also very much more eastern than the almost ubiquitous P. maculosa (Preston et al. 2002).
Although the New Atlas species account suggests that the distribution of P. lapathifolia has not changed appreciably in the last 40 years (calculated Change Index = -0.04), other work suggests the species has been, or is increasing in Britain and Ireland, both in the longer term and also more recently. The authors of the The Flora of County Dublin commented on the positive change the presence of Pale Persicaria has undergone between 1904 and their late 20th century survey: "Very much commoner now than in Colgan's time." (Doogue et al. 1998). The BSBI 'Local Change' project which repeated the 1985-6 'Monitoring Scheme' survey in 811 pre-selected tetrad squares in Britain, also suggests that P. lapathifolia is increasing in Britain (Braithwaite et al. 2006).
Neither of these reports proposes reasons why P. lapathifolia may be experiencing increase, but it may be due to a combination of nitrogen deposition enriching the soil, increased levels of disturbance, and better taxonomic treatment in published Floras, giving rise to more confident field recording.
European and world occurrence
The taxonomy and nomenclature of this taxon has been so confused in the past that Jalas & Suominen (1979) list twelve synonyms. The European range extends from the far north of Scandinavia at 70°N, to the Mediterranean and Macronesian islands in the south (Jalas & Suominen 1979, Map 407). In northern Scandinavia, it becomes rare, casual or ephemeral, and is considered a recent incomer (clearly of crop seed weed origin) (Jonsell et al. 2000). However, taking this polymorphic species in the broad, sensu lato sense, it has spread with agricultural crop seed from a widespread European boreo-temperate origin to become a circumpolar, near-cosmopolitan southern-temperate naturalised weed of arable and disturbed soils. It now extends SE to Iraq and E to the Himalaya region, C China, S Japan and SE Asia, Australia, N America, Mexico, Chile, Argentina, tropical and S Africa. It reached New Zealand as long ago as 1904 (Hultén & Fries 1986, Map 651; Webb et al. 1988).
Names
The genus name 'Persicaria' is from the Latin 'persicum' meaning peach, and translates as either 'peach-leaved' (Gilbert-Carter 1964), or 'peach-like' (Gledhill 1985). The previous generic name 'Polygonum', in use until recently and occasionally revived, is from the Greek 'poly' meaning 'many', and 'gony' meaning 'knee', a reference to the characteristic swollen stem nodes found throughout the genus. The Latin specific epithet 'lapathifolia' means 'leaf like a sorrel', or 'leaf dock-like' (Gilbert-Carter 1964).
The English common name 'Pale Persicaria' is a mere book-name, but at least it indicates the difference in colour of flowers and stems between this species and Redshank (Persicaria maculosa). Alternative common names include 'Pale Smartweed' and 'Pale Knotweed' (Mitich 1998).
Threats
None.