Photo worth seeing: Frequent mowing creates superweeds

Natural predators, such as the tobacco hawk moth, eat the flowers of the silverleaf nightshade. © Courtesy Alejandro Vasquez

The silverleaf nightshade (Solanum elaeagnifolium) is native to North and South America, but can also be found in parts of Africa and Europe. The plant owes its name to its leaves and stems, which look silvery to greyish due to a layer of hairs. The flowers, in turn, are usually violet or violet-white. There are thorns a few millimetres long along the stem. The silverleaf nightshade is a weed because it also spreads in fields and thus “takes away” nutrients from crops. The plant is even harmful to humans and livestock: the berries are highly poisonous and the thorns can cause skin irritation, known as weed dermatitis.

Anyone who finds such a plant in their garden or field will probably want to get rid of it. However, mowing it down could have exactly the opposite effect. This is the conclusion reached by Rupesh Kariyat of the Arkansas Agricultural Experiment Station and his colleagues. The silverleaf nightshade has been studied there for more than a decade. As part of a five-year study, Kariyat and his colleague Alejandro Vasquez investigated the effects of frequent mowing on the nightshade.

The study found that the more often the silverleaf nightshade was mown, the more defense mechanisms it developed. For example, more spines protruded from the stem and the plant’s flowers became more poisonous. This prevents natural predators such as the tobacco hawk moth from eating the flowers. One such moth can be seen in the photo sitting on a nightshade flower. The increased mowing makes the weed more effective at keeping caterpillars of this species away. Mowing also causes the weed’s roots to lengthen. This allows the plants to reproduce asexually for several years and thus survive, the team explains.

When it comes to harmful weeds like silverleaf nightshade, mowing less is more. However, the research team cannot yet say whether this also applies to related plant species. These may or may not respond with similar measures. However, the results of the study at least help to find the right way to deal with silverleaf nightshade and provide insights into the defense capabilities of plants.

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