Study points out certain weeds' biodiversity role

Many of the species parks departments consider "weeds" are important for biodiversity and could be left alone, particularly in areas of low residential use, according to those involved in a recent study into urban meadows (HW, 8 July).

Study: surveys in Edinburgh, Bristol, Leeds and Reading - image: University of Edinburgh
Study: surveys in Edinburgh, Bristol, Leeds and Reading - image: University of Edinburgh

The trend of relaxed mowing regimes and planting of wild flower meadows is helping to create restaurants for pollinators, even when the meadows contain flowers traditionally thought of as weeds. Damien Hicks, from the Institute of Evolutionary Biology at the University of Edinburgh, found that perennials produce up to 20 times as much nectar and six times as much pollen as annuals, after studying more than two million plants in wild flower meadows.

The authors believe the study could have implications for how green-space professionals manage urban meadows, though Hicks said he is not qualified to say how they could change parks' practices.

"But (I) can say recent practices of relaxed mowing are of interest to us. They are sometimes contentious and can take years to deliver wildlife and social benefits. But last weekend I saw road verges in south England, which had been 'ignored' for several years, now looking similar to sown perennial seed mixes."

The University of Edinburgh surveyed 60 large meadows in the Scottish capital, Bristol, Leeds and Reading, working with city parks teams and local schools, which planted and looked after each meadow. The researchers compared Rigby Taylor's "Rainbow" mix, which contains 14 annual native and non-native species, and the perennial "Special Pollen & Nectar Wildflower" mix sold by Emorsgate Seeds, which contains 23 native species. Results showed wild flower meadows made up of common perennial species provided more food for pollinators that those made up of annual species.

City of Edinburgh Council head of parks, green space and cemeteries David Jamieson said his department provides land for creating meadow areas and was keen to be involved because the study matches what they are already doing - naturalising extensive areas of open spaces.

"Pollinators need a variety of flowering plants throughout the year but particularly the beginning of the growing season is a critical time," he added. "What this means for urban meadow planting is that if you want biodiversity in the urban environment you need areas that are not necessarily unmaintained but left in a more natural state. A lot of species considered weeds by parks departments and others are in fact important for biodiversity."

Jamieson said local authorities could echo Edinburgh in identifying areas of low residential use such as banks, verges and areas under trees, and naturalising them. For example, his parks team has increased the amount of council green spaces that were more naturally managed by 10 per cent. This includes reducing mowing frequencies from 10-15 times a year to once or twice.

"We've been taking this approach for a couple of years and have already found new species appearing, such as the northern marsh orchid," he added. "In a couple of years we will see more diversity of species and the area will look better as well."

The study found flower species determine both what is on the menu and also when that food will be available, according to Hicks. Perennial meadows contained more pollen and nectar but also flowered earlier in the year, making them particularly good for queen bumblebees, wild bees and butterflies coming out of hibernation.

Research also showed a nectar and pollen contribution early in the year came from weeds such as dandelions and buttercups, suggesting leaving some to flower would help early-spring pollinators. High-performing plants included rough hawkbit, wild carrot, common poppy, black knapweed, corn marigold and dandelions.

Ultimately, the study's authors said: "For green-space managers, key considerations will include public attitudes towards change in land use, cost of establishment and maintenance of the meadows. The latter will be dependent upon the type of mix used and aim of the planting, careful matching of seed mixes to local site conditions, and management requirements," such as weeding frequency and litter removal.

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