Frequent mowing in pursuit of a tidy lawn or garden can create stronger, more resilient toxic superweeds
Keeping a lawn or garden tidy and free of weeds often involves regular mowing, but a new study suggests this common practice might be causing more problems than it solves with regards to one particular weed. A new study reports that frequent mowing of a common backyard weed, Silverleaf Nightshade, Solanum elaeagnifolium, can transform it into a resilient “superweed” that is more difficult to control.
Silverleaf Nightshade is easily identified from its purple flowers, which sometimes are white or violet, and its prickly spines. It is a New World plant that may have originated in North America before being accidentally introduced to South America or in the opposite direction. It is found from Kansas to Louisiana, down through Mexico, and further south into Uruguay, Argentina and Chile. It is invasive, also being found in South Africa and in Greece where it infests fields and slurps up valuable nutrients intended to support the growth of cash crops. In return for the windfall, this plant grows poisonous berries and flowers, and prickly spines.
Silverleaf Nightshade has a number of close relatives that also produce toxic berries but some of its relatives include friendly plants such as tomatoes, peppers, potatoes and eggplants.
Considering the toxic nature of Silverleaf Nightshade, it would seem prudent to control it from growing and spreading.
To that end, a team of researchers from the University of Arkansas and the University of Texas Rio Grande Valley investigated how Silverleaf Nightshade reacts to different mowing frequencies. They observed mowed, unmowed, and frequently mowed patches of these plants over several years in and around Edinburg, Texas (Figure 1).
Senior author of the study, Rupesh Kariyat, who now is an associate professor of entomology and plant pathology at University of Arkansas and director of the Arkansas Agricultural Experiment Station, began the study whilst at the University of Texas Rio Grande Valley, when he and his graduate student, Alejandro Vasquez, designed a five-year, two-part study to observe the effects of frequently mowed Silverleaf Nightshade.
“Alejandro’s questions were, ‘How do these flowers differ between mowed and unmowed plants?’” reported Professor Kariyat. “‘And does that have consequences for the insects that actually feed on them?’”
After several years of study, Professor Karyiat, Mr Vasquez and collaborators found that Silverleaf Nightshade responded to mowing frequency in several ways. First, mowed Silverleaf Nightshade plants produced larger flowers, although these weighed less than those grown by unmowed plants. But perhaps surprisingly, further increasing the frequency of mowing resulted in bigger and heavier flowers, suggesting that the plant compensated by putting more resources into reproduction when mowing threats increased. Further, the flowers became more toxic to caterpillars, leading to less pressure from herbivores.
A second response to increased mowing frequency was increasing the numbers of spines that the plants grew, probably as a defense mechanism against herbivores like caterpillars. Indeed, Professor Karyiat, Mr Vasquez and collaborators found these spines were effective against caterpillars that fed on mowed Silverleaf Nightshade leaves because they grew significantly more slowly than those that fed on unmowed leaves.
A third response that the team observed revealed that mowed plants grew deeper taproots — nearly 5 feet deep in the first generation of mowed plants — thereby allowing them to obtain water and nutrients more efficiently from deeper underground. Additionally, tilling Silverleaf Nightshade patches spreads the plants around because its roots are rhizomic, and thus, like many weeds, it can propagate asexually over multiple years and growing seasons using these broken roots.
Another adaptation to mowing was that the plants produced some groups of seeds that germinated faster whilst others were delayed. This “staggered” germination was the plant’s way to ensure survival over the long haul, where some seeds germinate quickly whilst others could lie dormant for longer periods. This strategy protects the plant from frequent mowing so if some seedlings are destroyed, others will emerge later, guaranteeing the plant’s survival.
“You are trying to mow these plants so that the plants are getting eliminated,” noted Professor Kariyat. “But what you are actually doing here, you are making them much worse, much stronger.”
These concerning findings — rather like a plant version of a horror film — raise important questions about current weed management practices: how can a gardener or landscaper cope without creating an army of superweeds?
Well first, don’t panic. Because this study focused exclusively on Silverleaf Nightshade, other weeds — even the plant’s closest relatives — probably won’t react the same way to frequent mowing. (Indeed, my personal experience with tomatoes and peppers suggests they do not.) However, this study does provide deeper insights into the defensive capabilities of plants when humans declare war on weeds.
“This should be something that we consider when we make management plans,” Professor Kariyat explained. “Management practices need to be better understood using the ecology and biology of the species and the other species which interact with them.”
Additionally, the researchers think their findings can apply to anyone who deals with weeds, not just amateur gardeners and professional landscapers.
“As scientists, we want our research to be accessible and applicable to anyone,” concluded Mr Vasquez, “and mowing is a concept the world at large can understand.”
Source:
Alejandro Vasquez, Alexa Alaniz, Robert Dearth & Rupesh Kariyat (2024). Continuous mowing differentially affects floral defenses in the noxious and invasive weed Solanum elaeagnifolium in its native range, Scientific Reports 14: 8133 | doi:10.1038/s41598-024-58672-w
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