Efficacy of herbicides for selective control of an invasive liana, old man’s beard (Clematis vitalba)

Abstract

Old man’s beard is a woody liana that has become an invasive weed in many areas of its introduction, through its vigorous spread and negative impacts on the tree hosts it climbs. Control techniques that improve precision and reduce non-target damage are increasingly preferred for weed control yet have not been compared in published research for use against old man’s beard. Field experiments in New Zealand were conducted to: 1) assess targeted herbicide techniques for control of its climbing stems when growing among trees and 2) assess foliar herbicides for control of creeping stems in ruderal sites. For climbing stems, triclopyr in oil was applied around the circumference of woody stems near their base, which was compared with cutting the stems and applying concentrated glyphosate gel (45% ai) to each cut end. Herbicides were applied in autumn directly to individual stem bases of the weed, thereby protecting tree hosts and other non-target vegetation. The basal application of triclopyr to intact stems was highly effective (> 95% mortality) with no damage to nearby trees noted. The glyphosate gel applications to cut stems was less effective (56% mortality by 2 years after treatment). For creeping stems in grass-dominated ruderal sites, selective foliar herbicide sprays had not been previously compared side-by-side for control of old man’s beard. Three selective sprays that do not damage existing grass cover were applied in autumn at their recommended rates: 1) metsulfuron; 2) triclopyr; and 3) a mixture of triclopyr, picloram, and aminopyralid. All herbicide treatments gave effective control, although metsulfuron had a negative effect on grass vigour which might allow new establishment of old man’s beard seedlings by competitive release. These results provide effective options that reduce non-target damage for control of both climbing and creeping old man’s beard stems.

Viability and dormancy of the Clematis vitalba aerial seed bank

“…By June, most C. vitalba stems were still largely bare of leaves, and many seed heads had lost all their achenes, especially at S2. This is consistent with monthly dispersal observations of three other C. vitalba populations at different sites, where 60–85% of all achenes were lost between March and the end of June 2020 (Jarvis‐Lowry 2023). By September, most stems were vigorously producing new terminal and axillary growth.…”

“…Thus, the Dark treatment may have been a simulation of conditions in the soil seed bank for that reason, as well as a lack of light. Indeed, Jarvis‐Lowry (2023) reported that fewer than 40% of C. vitalba seeds sown in field plots emerged as seedlings, which is similar to germination results in the Dark treatment of the current experiment.…”

“…The six S1 C. vitalba collections from November 2019 and 2020, January 2020 and 2021, and March 2020 and 2021 were not representative of the population, but were useful in that they demonstrated germinability of seeds taken directly off the vine throughout the summer. Germinable and/or dormant seeds were present in each collection over the 2‐year period (Jarvis‐Lowry 2023), which confirms that persistent seeds on the mother plant function as a seed bank, however small.…”

“…Achenes were collected every 2 months for 2 years (September 2019–September 2021) and incubated for 28 days using the most effective temperature/light regime found in preliminary germination experiments carried out in 2019 (Jarvis‐Lowry 2023): day/night temperatures of 20°C/30°C with a corresponding 16‐h dark:8‐h light photoperiod. The germination trials either began immediately following collection, or after the achenes were stored dry for up to 5 days at 5°C.…”

“…In early winter, Van Gardingen (1986) estimated mean seed production of C. vitalba as 35,300 m −2 ·year −1 . It is now known that each year, 50% of C. vitalba seeds disperse from the mother plant during autumn dormancy (Jarvis‐Lowry 2023), and thus that estimate may have been only half what it should have been (70,600 m −2 ·year −1 ) because of the seeds missed due to early dispersal. A further 45% of seeds disperse by early spring, showing that the aerial seed bank has a short‐term existence.…”