Dog-strangling Vine Control in High Park

Protecting the Oak Savannah

[This article first appeared in High Park News, Summer 2005]

Visitors to High Park may encounter pesticide warning sign postings in the park and wonder why there is a continued need for pesticide use in the Park. Limited use of pesticide is targeted for specific invasive plant species that present a serious threat to the natural ecosystems in the Park. During the early summer to early fall, Toronto Urban Forestry field staff are involved in the restoration of the oak savannahs in the Park, fighting to protect our native vegetation from invasive plants such as Dog-strangling vine.

An intensive management program was initiated in 2000 to start controlling Dog-strangling vine with the registered herbicide, “Roundup”, after previous attempts to control this plant with non-chemical methods including pulling, cutting, burning and smothering proved unsuccessful. City staff apply herbicide (herbicides are a type of pesticide; they are a chemicals used to control weeds) directly to the plant by wiping or spot spraying the targeted plants to minimize damage to the environment and surrounding native vegetation. Some infestations may require more than one treatment per season since the plants continue to flower throughout the season instead of having one flowering period. Repeated treatments over several years are required for large aggressive colonies, to target individual plants that were missed during previous treatments or to find new plants that have sprouted from seed.

Each area in the Park that is proposed for treatment is marked with pesticide warning signs 24 hours in advance of treatment; signs must be left up for 48 hours after treatment. The date listed under date sprayed is the actual treatment date so Park users can monitor the stage of postings. Due to these posting requirements, more than one area may be signed at one time since the crew has moved on to another area. It is not recommended that Park users enter the treatment areas while they are posted. If necessary, however, staying on the designated pathways will reduce exposure to pesticide.

Dog-strangling vine is a serious pest in Toronto’s natural areas since it spreads by root and by abundant seed that can be carried by wind. It can invade sites in both sun and shade. However, it is much more aggressive in full-sun, which makes the open habitat of the oak savannah particularly vulnerable to invasion. The plants twine onto neighbouring plants and outcompete them for sun and other resources. The plant was introduced from Russia and Ukraine and it is therefore able to grow without natural controls. Dog-strangling vine can easily be identified by common traits in the milkweed family including star shaped flowers, green oval leaves, seed pods and fluffy seeds.

Landowners in the High Park area can contribute to the control efforts in the park by controlling Dog-strangling vine on their own property so that the seed is not spread into the Park. For small infestations, the plants can be dug out of the ground or, at least, the seed pods can be cut off the plant to prevent spread.

For further information please refer to the City of Toronto Urban Forestry Services web site. Look for information about how to control invasive plants, appropriate native plant lists for Toronto and where to obtain native plants in Toronto.

More facts

• Generally seed pods develop mid-July and open mid-late August.

• Two species are found in Toronto and within High Park. Pale Dog-strangling vine '(Vincetoxicum rossicum') is much more widespread than Black Dog-strangling vine '(Vincetoxicum nigrum)'. Both species flower from approximately mid-June to mid-July.

Author: Cara Webster, Restoration Specialist, Urban Forestry Services, Toronto

Reference: Stephen Darbyshire, Trail & Landscape, Vol. 37, No. 4 (2003).

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