A study published last year by UC Davis alumni Marit Wilkerson has shown that although hedgerows contain weedy species, they don’t cause those weeds to invade agricultural fields. Wilkerson suggests that instead, hedgerows may be functioning as filter fences, trapping the weeds. However, in hedgerows with larger plants that cast more shade, the amount of weedy species dropped dramatically. She suggests that if it were possible to create conditions in hedgerows to allow woody plant species to thrive, then smaller herbaceous weeds would likely decline. These findings provide an interesting goal for growers putting in hedgerows- to figure out ways to ensure the best growth of their plant species. This might be hard in California, where drought is likely making establishment of new hedgerows without irrigation a challenge.
Another interesting finding to come out of this study was that what was planted next to a hedgerow affected the weed composition and density. Hedgerows next to row crops and vineyards had fewer weeds than those next to sloughs or grasslands. These more natural habitat types are not only less-managed, but also provide more habitat for species likely to disperse weed seeds, such as birds. However, other studies have shown that proximity to natural habitats is highly beneficial for pollinators, leading to increased yields in crop fields. Therefore a balance between biodiversity and ecosystem service benefits and weed suppression needs to be struck. Fortunately, taking actions to reduce weed populations before planting a hedgerow can create a “legacy effect,” limiting weeds in the hedgerow to volunteers from the landscape.
Knowing the history of your hedgerow and the landscape context (whether it is likely to be invaded by weeds), and managing it so that woody native plants grow large enough to shade out unwanted herbaceous weeds seems like a good recipe for making the planting more successful at limiting ingress of weeds into agricultural fields.