Black nightshade causing major problems for Norwegian vegetable production

Black nightshade (Solanum nigrum L.) is an alien species in Norway. Until around 1970, it was not considered a serious weed problem. However, over the past 50 years, the species has become an increasingly common weed in vegetable and potato production, particularly in root crops such as carrots.

Since the turn of the millennium, problems caused by black nightshade—and later also by other nightshade species—have continued to increase. Today, the greatest challenges are associated with black nightshade (Solanum nigrum L.) and hairy nightshade (Solanum physalifolium Rusby).

Why nightshade weeds are difficult to control

Nightshade species produce berries containing large numbers of seeds. The seeds survive passage through birds' digestive systems and therefore spread easily to new areas. Once established in a field, the seeds remain viable in the soil for many years. This means that the nightshade problem does not disappear on its own.

According to the Norwegian Agricultural Extension Service (NLR), between 67 and 87 per cent of vegetable growers in southeastern Norway now have nightshade infestations in their fields.

"One reason nightshade species are particularly difficult to control is that they germinate late in the season," explains NIBIO researcher Kirsten Tørresen.

"Early weed control, which is effective against many other weeds, therefore has limited impact. In addition, several herbicides that previously provided good control of nightshade are no longer approved."

New project aims to find solutions

Tørresen is project leader for a new research project that started in February and will run for four years.

The SOLWeeds project aims to develop effective solutions based on both direct and preventive measures. Field trials will investigate strategies such as crop rotation, stale seedbeds, cover crops, non-chemical control methods and herbicide use.

In vegetable crops, the project will also test a robot that applies herbicides only to the weeds leaves.

"Our goal is to develop methods that make nightshade weeds easier to control. This will increase yields and improve profitability for growers," says Tørresen.

Field trials have already started in Østfold County (potatoes, carrots and parsnips) and in Agder County (potatoes).

– I tillegg har vi ansatt en stipendiat, Lovisa Ek Hansson, som de neste tre årene skal fordype seg i søtvierartenes biologi og utvikling på norske jorder.

Rådgivere i Norsk Landbruksrådgiving forteller at flere produsenter allerede har valgt bort arealer eller gitt opp leiejord som er sterkt infisert med søtvier. Lukekostnadene har blitt så høye at produksjonen ikke er lønnsom. Tidlig gulrot og potet under plast er særlig utsatt.

– Søtvier er derfor ikke bare et agronomisk problem. Det er også et driftsøkonomisk og strukturelt problem for norsk matproduksjon, avslutter Tørresen.