Poisonous ‘super weed’ created by frequent mowing

Mowing might seem like a pretty sensible way to keep weeds at bay. But in the case of the silverleaf nightshade, the activity puts the plant into survival mode causing it to fight back with some pretty powerful defense mechanisms, says a new study.

Rupesh Kariyat, an associate professor in the department of entomology and plant pathology at the University of Arkansas has been studying silverleaf nightshade (Solanum elaeagnifolium) for more than a decade. His initial work on the plant – which is part of a family of plants that also includes tomatoes, tobacco, peppers, and eggplants – began when he traveled to Greece to work with a collaborator. It was there, from the vantage point of his seat on the plane, that he saw just how widespread the plant was in the country because it grows in the understory of grapevines where it sucks nutrients from the soil and kills the vines.

Fast forward about 10 years and Kariyat found himself studying silverleaf nightshade in southern Texas where he noticed a difference in the plants that were mowed regularly and those that weren’t. In 2018 he and his collaborators began collecting data on the plants and the mowing schedules that affected them. The team found that mowed silverleaf nightshades seemed to thrive underneath the abuse of the blades.

Enter the “super weed”

In a study recently published in Scientific Reports, Kariyat and his team showed that the plants that were mowed morphed to be stronger, and more poisonous, turning into a kind of “super weed.”

Specifically, regularly mowed silverleaf nightshade plants sent their roots much deeper than undisturbed plants, reaching depths of about five feet (about 1.5 m). Also, when the plants grew back after being chopped by the mower, they had more spikes and produced flowers that were more toxic – both adaptations that allowed them to resist predation by caterpillars. This was borne out by the fact that the study revealed that caterpillars feasting on unmowed silverleaf nightshades were heavier than those feeding on plants that were regularly mowed, meaning that they were able to eat more of the unmowed weeds.

Finally, the plants began to develop different seeds: some that germinated quickly and others that took longer to sprout. This created a staggered germination method that ensured higher survivability for the species.

“You are trying to mow these plants so that the plants are getting eliminated,” Kariyat said. “But what you are actually doing here, you are making them much worse, much stronger.”

While Kariyat isn’t quite sure how to handle the conundrum of mowing increasing the strength of silverleaf nightshade, he does say that the findings should influence weed management. Less might truly be more when it comes to dealing with the weed, for example.

“This should be something that we consider when we make management plans,” Kariyat said of the plant’s defenses. “Management practices need to be better understood using the ecology and biology of the species and the other species which interact with them.”

He also says that further research would be warranted to explore the ways other plants in the Solanaceae family respond to human-generated mechanical stressors.

Source: University of Arkansas

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