Mullein survey and removal efforts on Mauna Loa in Hawai'i Volcanoes National Park

Abstract

Common mullein (Verbascum thapsus L.) is a biennial herb naturalized in mostly temperate areas on the Island of Hawai’i. Mullein was first observed growing on Mauna Loa in Hawai’i Volcanoes National Park in the 1970’s. Localized efforts to eradicate mullein along roadsides in the Park began in 1989. This report documents monitoring and control efforts of mullein in the Park between 1994-1 999. Mullein occupies 1,003 hectares of open lava flows located between 1,500 and 2,100 m elevation in the Park. Range expansion is expected to continue at both lower and higher elevations. Densities and recruitment are highest (>1,400 ind/ha) on weathered ‘a’a flows that edge vegetated kipukas. Lowest densities (<1 ind/ha) and recruitment occur on open ‘a’a with little or no soil development. Recruitment of new individuals is highly variably between years and is likely influenced by fluctuations in seasonal rainfall. Manual uprooting mullein effectively reduces populations by the third or fourth year of annual removal treatments. Successful control requires consistent and thorough follow-up treatment. Workloads vary between 5-10 ha/worker days in moderate to low density infested areas. An estimated 200 worker days will be required annually to remove mullein from existing populations, other than the most densely infested areas in the Park. Based on these findings, management should emphasize containment and range reduction of the current distribution. Cooperative agreements with private and state landowners should be made to control mullein in areas adjacent to the Park in order to reduce re-invasion from outside areas into the Park. Use of herbicides and biological control agents as alternatives to uprooting mullein should be investigated. Long term studies of the impact of mullein on native plant communities will help managers prioritize mullein control efforts in the Park.