Understanding Cattail (Typha spp.) Invasion and Persistence in Forested Wetlands Created by the Virginia Department of Transportation

About the report:

Common cattail (Typha latifolia) is a native plant species listed as an invasive weed by some regulatory agencies.  While it is not listed as a noxious weed by the Commonwealth of Virginia, control of cattail populations in created forested wetlands is still mandated by some regulatory permit requirements and requires either mechanical removal, treatment with herbicide, or both.  Permitting agencies reason that cattail removal will prevent interspecies competition between cattails and desirable planted tree species.  This regulatory requirement has added significantly to the Virginia Department of Transportation’s (VDOT) cost of wetland compensation and mitigation.  However, some researchers have questioned spending resources on cattail removal since the data do not support the hypothesis that cattails retard or inhibit planted tree growth.  Rather, created wetland vegetation communities demonstrate more complex relationships than can be explained by simple interspecies competition.  A site’s hydrology (generally, water that is too deep) appears to be the driving force in an increase in planted tree morbidity and/or a decrease in woody growth.
 
Because of conflicting information in the literature, this study was undertaken to help clarify the environmental conditions (biological, chemical, and physical) that enable Typha spp. to rapidly colonize primary succession (created mitigation) forested wetlands.  The vegetation of 20 VDOT-created forested wetlands was collected and analyzed during the 2006-2007 growing seasons.  Further, the hydrology, nutrient dynamics, and vegetation were studied at four of those sites.

The data showed that cattails, while present in young sites, were rare in mature created hardwood wetlands.  Woody vegetation, particularly specific volunteer species, became prominent after 10 to 15 years.  In addition, species such as bald cypress, black willow, and red maple trees were common dominants in the older sites.  Preliminary data showed that the growth of planted bald cypress trees in cattail-dominated areas was not significantly different than in those surrounded by non-cattail herbaceous perennials.  Environmental data indicated a weak relationship (trend) in cattail dominance and water depth, but there was no significant difference in environmental parameters.

The study concludes that the resources used to eradicate cattails, except where the hydrology has been compromised, are better spent on seeding and/or planting woody species such as bald cypress, black willow, river birch, and red maple that are better adapted to the draw-down, plant, and then flood conditions that are frequently encountered on these sites.  Additional research on the direct effect of perennial herbaceous plants on planted woody species growth is recommended. 

Authors