Urban ragweed populations in vacant lots: An ecological perspective on management

Abstract

Ragweed (Ambrosia artemisiifolia L.) is one of the most prolific producers of allergenic pollen in North America, and negatively impacts tens of millions of people each year. Recent work shows that local plant populations can be the most important source of allergenic pollen in the urban environment. This research emphasizes that management choices that influence species abundance can determine the burden of allergenic pollen for people living near these plant populations. In Detroit, MI, USA, ragweed populations are predominantly found in vacant lots; the management of these lots could therefore have large impacts on allergenic pollen burdens. The main form of management in these vacant lots is mowing, which occurs at frequencies ranging from monthly mowing to no mowing. We hypothesized that annual or biennial mowing would result in conditions where ragweed populations could thrive. To test this, we conducted a vegetation survey of vacant lots in Detroit, in which we quantified the mowing regime, characteristics of the vegetation, and ragweed presence and stem density. We found that ragweed was significantly more likely to be present in lots that were mowed annually or biennially; unfortunately these are the most common management types, accounting for 51% of vacant lots in Detroit. Ragweed's association with this disturbance regime fits with its early successional status, as it is most competitive in recently disturbed soils where there is reduced competition for resources such as light. We therefore recommend one of two alternative management regimes for reducing ragweed in vacant lots: either mow them frequently (multiple times a growing season) or do not mow them at all. Both approaches will reduce ragweed prevalence in vacant lots and reduce allergenic pollen exposure for people who live near vacant lots.