Skaneateles Conservation Area/Invasive species/Reynoutria

Note that even though Reynoutria japonica was first described by that name by Maarten Houttuyn in 1777, fairly recent sources have placed it and other Asian knotweeds either in Polygonum or Fallopia. However, the latest phylogenetic studies seem to show that a separate Reynoutria genus is waranted.

The synonymy for the knotweed species covered here is:

• Japanese knotweed: Reynoutria japonica ≡ Polygonum cuspidatum ≡ Fallopia japonica
• giant knotweed: Reynoutria sachalinensis ≡ Polygonum sachalinense ≡ Fallopia sachalinensis
• Bohemian knotweed: Reynoutria × bohemica ≡ Polygonum × bohemicum ≡ Fallopia × bohemica

Reynoutria spp. (knotweeds)

Most Asian knotweeds (Reynoutria spp.) observed in Onondaga County have been classified as Reynoutria japonica var. japonica (Japanese knotweed), which is very common in the vicinity of the Skaneateles Conservation Area (SCA), but has not been identified within its actual boundaries (yet). It is present along Old Seneca Turnpike, a few houses to the east of the SCA, and there is a large stand along the Charlie Major Nature Trail in Mottville, a couple miles to the west of the SCA.

Reynoutria sachalinensis (Giant knotweed) is very similar to Japanese knotweed, and past observers may have confused them or their hybrid, Reynoutria × bohemica (Bohemian knotweed).

Although it only lists Tompkins County for vouchered Reynoutria × bohemica specimens, the New York Flora Atlas states that the hybrid often forms dense large monospecific stands, is widespread and often mixed with Reynoutria japonica var. japonica,

Invasiveness ranking for Reynoutria spp. (knotweeds)

The likely Reynoutria (knotweed) species in the vicinity of the Skaneateles Conservation are:

• Reynoutria japonica var. japonica (Japanese knotweed), the most likely at the SCA;
• Reynoutria sachalinensis (giant knotweed), less likely at or near the SCA;
• Reynoutria × bohemica (Bohemian knotweed), which is a hybrid of previous two.

They are all considered to be very highly invasive in New York State, with a relative maximum score of 95/97 = 97.94%, and they are prohibited by New York State law. Because they hybridize extensively they were treated together for evaluating invasiveness.

1. Ecological impact (40/40)

1.1. Impact on Natural Ecosystem Processes and System-Wide Parameters (Major, possibly irreversible, alteration or disruption 10/10)

• Slow to decompose creating an extensive organic layer (O horizon) above the soil.

• Can limit access for fishing and wildlife, cause flooding by decreasing water flow through rivers and channels, and become a fire hazard during the dormant season.

1.2. Impact on Natural Community Structure (Major alteration 10/10)

• Early emergence, rapid growth and persistence of dead material from previous year's growth can eradicate all layers below.

1.3. Impact on Natural Community Composition (Major alteration 10/10)

• Forms monocultures.

1.4. Impact on other species or species groups (Severe impact 10/10)

• Clogs waterways and lowers the quality of habitat for wildlife and fish, reducing the food supply for juvenile salmon in the spring.

2. Biological characteristics and dispersal ability (25/25)

2.1. Abundant reproduction (4/4)

• Spread through rhizomes, root fragments, stolons, and stem fragments that can root and form new plants..

• The extensive rhizome system can reach over 50 feet in length, allowing the plant to emerge early, shade out other vegetation, and reduce native species diversity.

2.2. Numerous opportunities for long-distance dispersal (4/4)

• Most reproduction in Japanese knotweed is believed to be vegetative, although the role of sexual reproduction is receiving increasing attention.

• Fruits are thin and wind dispersed; plant fragments can be transported by water, with marine dispersal reported

• Seeds do not appear to be an important means of reproduction in the United States

Observations of Reynoutria spp. (knotweeds) at the SCA

The following photographs and corresponding iNaturalist observations of Reynoutria were made at or very near the Skaneateles Conservation Area. Click on images to enlarge and read details on Wikimedia Commons or on the "iNat obs" links to view the corresponding observations at iNaturalist.

• Reynoutria japonica on Charlie Major Nature Trail, 8 Sep 2018. iNat obs
• 
  Placeholder image. See iNat obs
  for actual photos at this location.
  Please use CC-BY, CC-BY-SA, or CC0
  (not CC-BY-NC) for iNat image licenses.