Seed germinability and its seasonal onset of Japanese knotweed (Polygonum cuspidatum)

Abstract

Sexual reproduction is known to be an important means of propagation for native Japanese knotweed populations in Asia. For naturalized populations in the United States, however, its relative importance compared with propagation by rhizome and stem fragments has not been established. This article presents two related studies that address two basic questions regarding the potential for propagation by seed: (1) are seeds that are produced by Japanese knotweed in areas of Philadelphia, PA, capable of germinating in the field? If so, (2) when and how rapidly during the year do these seeds acquire germinability? Field germination experiments assessed germination of planted seeds at intervals during the growing season at two different study sites (one in 2000 and the other in 2002) and showed that germination was common at both sites. The 2002 experiment also demonstrated that germination occurred both within existing stands of Japanese knotweed and in areas well removed from existing stands, and that both planted and naturally occurring seeds germinated. Experiments on seasonal acquisition of seed germinability used batches of seeds collected weekly between early September and November 2000 from three different study sites. Results showed that at two of the study sites, a rapid increase in germinability from less than 10% to greater than 90% occurred between collection dates in mid-September and mid-October 2000. A smaller and more gradual change occurred at the third site, where maximum germinability was roughly 50%. Regarding management of Japanese knotweed, the field germination experiments suggest that spread of this plant by seed is a realistic possibility in the United States. Because of the rapidity with which germinability is acquired, the high germinability attained, and the large number of seeds produced, we recommend that female (male-sterile) plants or their inflorescences be removed from seed-producing populations before the formation of fruits to minimize spread by seed.

The Japanese knotweed invasion viewed as a vast unintentional hybridisation experiment

“…and also created some most unexpected hybrids between highly divergent parents. In terms of threats posed, F. x bohemica and its backcrosses are of major importance, particularly in North America (Bram and McNair, 2004), where recruitment from seed is much more common than in Europe. F. x conollyana, inspite of its frequent production as seed, rarely germinates in the wild and even when it does, is not a very strong competitor.…”

The Biology of Invasive Alien Plants in Canada. 5. Polygonum cuspidatum Sieb. & Zucc. [= Fallopia japonica (Houtt.) Ronse Decr.]

“…Maximum germination was generally achieved after lateSeptember, with seeds from some sites reaching maximal germination in late October (Bram and McNair 2004). However, maximum percentage germination varied drastically (40->90%), which may be due to either intraspecific variation in seed germinability or sites containing different Polygonum species, since identification to species was not confirmed.…”

“…(c) Seed Banks, seed Viability and Germination - Bram and McNair (2004) showed that a moist chilling (4°C) of 30 d immediately after mid-September through mid-October seed collection generally increased maximum percent germination compared with the same stratification treatment given the following year in Philadelphia, PA populations. Maximum germination was generally achieved after lateSeptember, with seeds from some sites reaching maximal germination in late October (Bram and McNair 2004).…”

“…(a) Floral Biology -As with other features of this species, its floral biology in North American populations is paradoxical, with reports of populations being completely male sterile, male only, gynodioecious, or subdioecious (Locandro 1973;Forman and Kesseli 2003;Bram and McNair 2004). This variation in floral biology is likely a product of either misidentification of P. cuspidatum with congeners, or the result of inherent variation in the native genome established via multiple introductions.…”

“…This suggests a very low percentage of the male sterile flowers being fertilized, likely due to a lack of nearby compatible congeneric pollen donors. A similar estimate of potential seed production in a North American population (Philadelphia, PA) approaches 127 000 per stem, assuming all flowers are successfully pollinated (Bram and McNair 2004).…”