Letting weeds grow wild isn’t a proper way to natural grassland
McKinney Park better served by planting useful, native shrubs, writes P.J. Cotterill.
On Aug. 2, media interviewed an Edmonton resident who was complaining about shoulderhigh weeds in Louise McKinney Park. He said they were unsightly and gave the city a bad image.
City of Edmonton staff responded that the weeds were being left intentionally; the goal was to achieve a “pristine, natural grassland” and citizens only need patience for this to happen.
I’m afraid this is an extreme case of wishful thinking. Where is the evidence that weeds will metamorphose into a natural grassland, let alone a pristine one? Are there any local examples, or indeed any examples, anywhere? Is there anything in the relevant scientific literature? Some tall-grass prairies of southern Manitoba have been restored, but only with tremendous effort on an agricultural scale, involving seed harvesting, cultivation for seed production, ground preparation, sowing and further management.
It is true that the tall annual weeds present in the park are the first stage in a natural succession from bare ground to an area covered in vegetation. But in our urban situations these annual weeds will be followed by perennial weeds, which can persist indefinitely. In Edmonton, smooth brome grass, Kentucky bluegrass, common dandelion, Canada thistle and common tansy come readily to mind.
Edmonton has already conducted two experiments with succession in its parks. Ground has been cleared, weeds have grown, and in neither case has natural grassland resulted.
In the mid-1980s MacKinnon Ravine was stripped bare during a landscaping makeover. For a couple of years it sported a crop of weeds so lush anyone wanting to study plant growth habit or weed ecology could have had a field day.
Today, the ground cover is predominantly smooth brome with liberal amounts of alfalfa — a hay field any farmer would be proud of. Native plant communities are represented only by woods on the north-facing slopes, and even these are heavily infested with non-natives: European mountain ash, Manitoba maple, caragana and cotoneaster.
Terwillegar Park was disturbed for gravel extraction prior to the 1960s. About one-third of the approximately 114 plant species reported for the Terwillegar floodplain area are non-native.
However, they occupy so much ground that the overall effect is a field of weeds, dominated by field scabious and smooth brome.
Even the copses of native trees (aspen and balsam poplar) have an understory of smooth brome.
Even if the city could reconstruct natural grasslands, albeit at enormous expense, the question arises, should it? Native grasses in Edmonton are restricted to steep, well-drained slopes, sandy soils, solonetzic soils, mature woodlands and wetlands.
Plains rough fescue, the major grass occupying the black earth and clay soils of pre-settlement grasslands, has been almost entirely replaced by smooth brome and Kentucky bluegrass.
So where are the native species to come from? Further, Edmonton lies close to the dry mixed-wood region of the boreal forest natural region, so open ground tends to colonize naturally with poplars and white spruce, or with shrubs, not grass. And if a natural grassland is artificially achieved, is the city really prepared to maintain it by the necessary tree-busting agencies: fire and grazing?
I suggest that Louise McKinney Park be planted with native shrubs, already available from nurseries: beaked hazelnut, red-osier dogwood, pin-cherry, chokecherry, saskatoon, wolf willow, high-bush cranberry and Canada buffaloberry.
These mimic the river valley’s natural shrub communities and are useful to wildlife.
If the city is really interested in naturalness, the first thing it can do is protect and manage its existing natural communities for sustainability. This includes adaptive management plans for natural areas, better weed control, restricting rather than encouraging access to wilder parts of the river valley, and not disturbing land unnecessarily, for example, by planting fruit trees in the valley.
Then, it should apply knowledge from ecology and restoration science to develop natural or hybrid ecosystems that are both healthy and visually appealing.