Koenigia × fennica

Koenigia × fennica, known as Finnish knotweed,[2] is a hybrid between two species of Koenigia, K. alpina and K. weyrichii, members of the family Polygonaceae, the knotweed family. It generally only known as a cultivated garden plant, but plants have been recorded a few times surviving in abandoned areas in northern Europe, especially in Finland. The cultivar 'Johanniswolke' is considered an attractive ornamental perennial plant.

Koenigia × fennica
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Clade:	Angiosperms
Clade:	Eudicots
Order:	Caryophyllales
Family:	Polygonaceae
Subfamily:	Polygonoideae
Genus:	Koenigia
Species:	K. × fennica
Binomial name
Koenigia × fennica (Reiersen) T.M.Schust. & Reveal[1]
Synonyms[1]
Aconogonon × fennicum Reiersen Persicaria × fennica (Reiersen) Stace Polygonum × fennicum (Reiersen) S.R.Majorov

Description

Like the other species of the former genus Aconogonon (all now classified in Koenigia), the hybrid is a perennial, growing from short rhizomes. It grows to about 1.5 m (5 ft), with leaves up at least 10 cm (4 in) long and about 4.5 cm (1.8 in) wide.[2] The hybrid is intermediate in appearance between its parents. It differs from Koenigia weyrichii in having less persistent ochreas (sheaths surrounding the stems), less pointed leaves with fewer hairs on the underside, pedicels that are jointed in the upper part, and larger tepals. It has broader leaves than Koenigia alpina, with a width at least 40% of the length. It usually does not produce fertile seed.[3]

Inflorescence

The hybrid may have arisen a number of times in Europe. Reiersen, who had studied the taxon since the late 1980s, noted in 2000 that Danish plants had hairier leaves than the other Scandinavian plants, and that Danish plants produced fertile pollen and differed in other small aspects of the floral morphology. There were three plants known in Britain until the 2000s, two of them cultivated in botanical gardens, these differed from each other, and from the Scandinavian collections, with the plant from Yorkshire producing fertile seed.[4][5]

Taxonomy

Scandinavian plants of Koenigia × fennica had originally been identified as K. mollis in Nordic floras,[3][6] but in 1999 Reiersen described them as a new hybrid, Aconogonon × fennicum, both purported parents (now Koenigia alpina and K. weyrichii) then being placed in the genus Aconogonon.[3][7] Clive A. Stace, who in the 1997 edition of the New Flora of the British Isles had classified the three known British plants as a variant of Persicaria alpina, promptly moved the taxon to P. × fennica in 2002, believing Aconogonon to be invalid as a genus.[4] Aconogonon has since been subsumed into Koenigia. The genus Koenigia is placed in the tribe Persicarieae, whose taxonomic history has been described as "exceptionally convoluted, even by Polygonaceae standards".[8] The generic boundaries within the tribe have varied considerably; Koenigia × fennica has also been placed in Persicaria and Polygonum.[1]

The orthographical variant Aconogonum is used by some sources.[6][9]

Distribution

It was widespread in cultivation in Nordic countries, especially Finland, when first described in 1999.[3] The first plants appeared in the 1970s in Finland.[10] Reiersen in 2000 recorded over 100 instances of it being found in Finland, a dozen or so each in Norway and Denmark, and twice in Sweden.[4] By 2018 it had been found 514 times in Finland.[10] It is recorded as a "new resident alien" in the 2019 Checklist of the Vascular Plants of Finland, and "spreading in the wild".[11] It has only been found once as a garden escape in Britain, a plant found in 1981 in Yorkshire.[1][2][4] In Belgium it has been sighted twice as a garden escape on abandoned grounds, both in 2014.[6]

Horticulture

The cultivar 'Johanniswolke' first appeared in Germany in the early 2000s as Aconogonon 'Johanniswolke', supposedly as a collection from Asia.[9][12] It is often classified as "A. speciosum" in horticulture.[13][14] Around the same time the hybrid was also being sold elsewhere in Europe under the name "Polygonum polymorphum" or "Persicaria polymorpha",[12][15][16] and different other combinations of these names now exist in horticulture.[17][18][19] Neither P. polymorpha nor Aconogonon speciosum exist as valid taxonomic names, these are horticultural inventions,[20][21] whereas Polygonum polymorphum is a synonym of the related Koenigia alpina.[22] The plant has been called "white dragon knotweed" in the USA.[12]

This plant is grown as an ornamental perennial. It grows to a height of 150–200 cm (4 ft 11 in–6 ft 7 in). Flowering in June,[12][13] or July to August in far northern latitudes,[23] it has creamy white flowers which turn pink as they age. Although shrub-like in appearance, it is herbaceous, dying back to the ground over winter.[12][17] It has been described as "very good value for almost instant impact"[12] and an "eye-catcher",[9] although some people find the flower scent to be unpleasant.[9][12][23]

In Finland the hybrid can even be grown in gardens in the north of the country. It is recommended for use as a hedge in climates where the weight of excessive snowfall destroys hedges made of shrubs.[23]

The cultivar 'Johanniswolke' is supposed to be somewhat smaller than the nominate form, although it grows just as tall as the plants circulating under the name P. polymorpha. It is furthermore said to be sterile, flowers which become dark pink after anthesis, and to have non-invasive rhizomes which stay short and compact.[16] P. polymorpha is said to sometimes send up a shoot less than a meter from the main clump after many years, have a rhizome which is able to grow 50cm downward to escape a barrier, and to rarely produce black berries.[15]

References

1. "Koenigia × fennica (Reiersen) T.M.Schust. & Reveal", Plants of the World Online, Royal Botanic Gardens, Kew, retrieved 2020-05-21
2. Stace, Clive A. (2019), New Flora of the British Isles (4th ed.), Middlewood Green, Suffolk: C & M Floristics, pp. 465–466, ISBN 978-1-5272-2630-2
3. Jonsell, Beagt (1999), "Additional nomenclatural notes to Flora Nordica (Lycopodiaceae - Polygonaceae)", Nordic Journal of Botany, 19 (2): 385–387, doi:10.1111/j.1756-1051.1999.tb01219.x
4. Stace, Clive A. (2002), "A new name in the British Flora: Persicaria x fennica (Reiersen) Stace" (PDF), Watsonia, 24: 109–110, retrieved 2020-05-24
5. Porter, H.; Bailey, J. (1991), "An alien Polygonum hybrid new to the British Isles" (PDF), B.S.B.I. News, Treforest: Botanical Society of the British Isles, vol. 57, p. 54, ISSN 0309-930X, retrieved 2020-05-25
6. Verloove, Filip (25 July 2014), "Aconogonum x fennicum", Manual of the Alien Plants of Belgium, Scratchpads - biodiversity online, retrieved 2020-05-24
7. "Aconogonon × fennicum Reiersen", The International Plant Names Index, retrieved 2020-05-21
8. Schuster, Tanja M.; Reveal, James L.; Bayly, Michael J. & Kron, Kathleen A. (2015), "An updated molecular phylogeny of Polygonoideae (Polygonaceae): Relationships of Oxygonum, Pteroxygonum, and Rumex, and a new circumscription of Koenigia", Taxon, 64 (6): 1188–1208, doi:10.12705/646.5
9. Salomon, Christine (11 January 2020), "Aconogonon 'Johanniswolke' – (Garten-)Bergknöterich, Buschknöterich, Staudenflieder", Gartenfreud – Gartenleid (in German), Fritz Enßner, retrieved 2020-05-25
10. Lampinen, R.; Lahti, T. (2019), "Suomenröyhytattaren (Aconogonon x fennicum) levinneisyys Suomessa", Kasviatlas 2018 (in Finnish), Helsingin Yliopisto, Luonnontieteellinen keskusmuseo, Helsinki, retrieved 2020-05-26
11. Kurtto, Arto; Lampinen, Raino; Piirainen, Mikko; Uotila, Pertti (2019), "Checklist of the vascular plants of Finland - Suomen putkilokasvien luettelo" (PDF), Norrlinia, 34: 80, ISBN 978-951-51-5364-7, ISSN 0780-3214, retrieved 2020-05-24
12. Kingsbury, Noel (2014), Gardening with Perennials: Lessons from Chicago's Lurie Garden (PDF), Chicago; London: The University of Chicago Press, p. 88, ISBN 978-0-226-43745-3, archived from the original (PDF) on 2020-05-21, retrieved 2020-05-21
13. "Aconogonon speciosum 'Johanniswolke' Perennials that flower in June", Lubera UK, Lubera AG, 26 May 2020, retrieved 2020-05-26
14. "Bergknöterich 'Johanniswolke' - Expertenwissen zu dieser Staude", Baumschule Horstmann Pflanzenversand (in German), Baumschule Horstmann GmbH & Co. KG, 2020, retrieved 2020-05-26
15. Ghyselen, Chris (2020), Persicaria planten Chris Ghyselen - tuinarchitect (in Dutch), Chris Ghyselen, retrieved 2020-05-26
16. Schepers, Arjan (2020), Arjan Schepers Vaste Planten - Aconitum lamarckii (in Dutch), Arjan Schepers, retrieved 2020-05-26
17. Polygonum polymorphum 'Johanniswolke' – Busch-Knöterich, Staudenflieder (in German), Staudengärtnerei Gaißmayer, 2020, archived from the original on 2020-05-21, retrieved 2020-05-21
18. "Persicaria × fennica 'Johanniswolke'", RHS Gardening, Royal Horticultural Society, May 2020, retrieved 2020-05-26
19. "Persicaria polymorpha 'Johanniswolke'", vasteplant.be (in Dutch), Jan Spruyt - Van der Jeugd BV, 2020, retrieved 2020-05-26
20. "Aconogonon speciosum", The International Plant Names Index, retrieved 2020-05-26
21. "Persicaria polymorpha", The International Plant Names Index, retrieved 2020-05-26
22. "Polygonum polymorphum Ledeb.", Plants of the World Online, Royal Botanic Gardens, Kew, retrieved 2020-05-26
23. Aconogonon Arboretum Mustila (in Finnish), Arboretum Mustila, 25 May 2020, retrieved 2020-05-26