Invasive plant spreads across Portugal and lengthens allergy season

Known for its showy plumes and used as an ornamental for decades, pampas grass, also known as Cortaderia selloana, is spreading, threatening biodiversity and raising vegetation management costs. Experts call for urgent action to halt its advance and reduce pollen exposure. For decades, pampas grass, also known as Cortaderia selloana, was seen mainly as an ornamental plant. Its large feathery plumes made it a familiar sight in gardens and, more recently, in decorative arrangements indoors. But what many still regard as a showy plant is, in Portugal, an invasive alien species that can spread rapidly and has potential consequences for health. This invasive plant has already been identified in at least 208 municipalities in mainland Portugal, according to a study published in 2025 (source in Portuguese). The real number may, however, be higher, as existing maps depend on records submitted by people on the ground. The species’ spread is particularly visible along the coast. Areas where there used to be isolated specimens have been turning into zones densely occupied by the species, according to biologist Hélia Marchante, a lecturer at the School of Agriculture of the Polytechnic University of Coimbra and a specialist in invasive plants. "It is a very, very fast-spreading species", she told Euronews. Unlike other invasive plants that can benefit from specific triggers such as wildfires, the plant also known as cortaderia has an especially effective advantage for spreading: "It only needs the wind." A plant that has learnt to escape the garden The history of the invasion helps explain how serious the problem has now become. Native to South America, cortaderia was introduced in different parts of the world as an ornamental plant. For decades, the fact that only female plants were present in gardens is thought to have limited its ability to reproduce. According to Marchante, that changed at the end of the last century, when hermaphrodite plants were introduced, 'almost certainly by accident', as "pollen donors". Fertilisation of the female plants then allowed the production of large quantities of seed. "From then on it only needs the wind," the researcher said. The process repeats itself every year. During flowering, the plants that produce pollen release it into the atmosphere. In female plants, once fertilisation has taken place, each flower can give rise to a seed. The plumes end up acting as a kind of transport system: thousands of tiny seeds can be carried by the wind and colonise new sites. Spread does not depend solely on natural phenomena, however. People who move the plumes to use them as decoration, as well as vehicles, clothing or equipment, can also inadvertently help to carry seeds to other places. "It is the movement of the plant itself that has the potential to spread it", warns Marchante, also pointing to cases where plumes are taken indoors. Pampas grass is no longer just an ornamental feature of the landscape. It is also an invasive species capable of altering the ecosystems where it becomes established. Once the species moves from isolated specimens to large stands, it becomes dense enough to exclude other plants, which can no longer survive in those areas. The impact extends to fauna as well, since the plant’s sharp leaves and the high density of the stands can make it harder for animals to move through. There are also less visible effects. According to Marchante, some studies show that the species can also alter nutrient cycles and modify the soil and the microorganisms that live in it. The problem goes beyond the environment The environmental dimension of the invasion is only part of the story. Cortaderia also has consequences for human health. Mário Morais de Almeida, an immunoallergology specialist and president of the World Allergy Organisation, explains that pampas grass belongs to the grass family and therefore releases pollen that can trigger symptoms in sensitised people. "As this is a grass species, it will indeed release its pollen into the atmosphere, again carried by the wind, and it can cause significant allergic symptoms." The problem is particularly serious because of the time of year when cortaderia flowers. The grasses most familiar to Portuguese people are prominent in spring and early summer. Cortaderia can extend that exposure into late summer and autumn. For people with allergic rhinitis or conjunctivitis, exposure can lead to sneezing, itching, runny nose, congestion, red and watery eyes. In those with asthma, exposure can contribute to respiratory symptoms and more severe attacks. "This grass, which can indeed release very, very large amounts of pollen, manages to do so throughout the summer, and even into autumn," says Morais de Almeida. However, the doctor stresses that there is currently no specific epidemiological evidence that would allow a given number of allergy cases in Portugal to be attributed to pampas grass. What exists is knowledge about grass allergies, clinical experience and evidence of the allergenic capacity of this species’ pollen. "At the moment we do not have evidence that is directly linked to pampas grass," he notes, adding that there is nonetheless a perception that allergic complaints are lasting longer. According to the specialist, the combination of climate change, different types of pollen and other agents present in the atmosphere makes the picture more complex. Because the seeds are carried by the wind, the impact is not necessarily confined to the immediate vicinity of the plant. Marchante explains that the seeds can travel several kilometres, especially in conditions of strong wind, while the pollen can also be transported over long distances. There is another issue: people can unwittingly help to move the pollen around. "People pick the plumes because they are very beautiful, they paint them, they take them home," the biologist explains. In addition to contributing to the spread of seeds, that behaviour can bring potentially allergenic material into homes. Morais de Almeida therefore offers an important piece of advice: do not take the plumes home and avoid handling the plant. On top of its allergenic potential, the leaves themselves are sharp enough to cause injuries. "The pleasant appearance the plant may have" should not make people forget the risks associated with exposure to it, the doctor warns. The danger of letting another year pass For Marchante, one of the main challenges is precisely time. The more a plant grows and produces seed, the greater the likelihood that new individuals will emerge nearby and that the invaded area will expand. "Each one that is left on its own in a more isolated area is another that will produce thousands of seeds, which will germinate and give rise to new plants next year," she explains, stressing that "if we fail to act now, next year it will be much, much more difficult, much more expensive and much worse to intervene". The strategy does not necessarily have to focus on trying to eliminate all the large stands that are already established in one go. The biologist argues for the identification and priority removal of isolated specimens and small clusters on the edges of the areas currently occupied. The aim is to prevent these hotspots from acting as new starting points for invasion of fresh territory. In some of the areas where the species has been established for longer, complete eradication will be much more difficult, if not practically impossible, the specialist believes. The alternative is control and ongoing intervention. "What I think is that we still have a huge amount of work we can do, because there are so many places with small nuclei of individual plants," she says. The timing of intervention is equally important. Removing only the plumes after the seeds have already been shed can mean working without preventing the spread that has already occurred. "To be sure of eliminating this plant, there has to be a window of time before May; it is during that period that it should be pulled out," she explains. And she has a clear message: "In fact, I say pull it up, pull it up by the roots, pull everything up, always pull it up; that is what we want." However, during the period when the seeds are being released, any intervention must be carried out in such a way as to avoid the control measures themselves contributing to their dispersal. Prohibited species still widespread Cortaderia has been on the Portuguese national list of invasive species (source in Portuguese) since 2019. The legislation bans, among other actions, its possession, planting, sale and introduction into the wild. In practice, however, it can still be found in private gardens, municipal spaces, industrial areas, roadsides and other locations. Marchante points to a lack of awareness of the law and a lack of enforcement as some of the obstacles to controlling the species. There is also a coordination problem. A municipality may remove plants from a garden under its management, only to find specimens in private plots or in infrastructure managed by other bodies. The invasion also has a socioeconomic dimension, as the presence of large cortaderia plants makes vegetation management more difficult and more expensive, particularly in areas where vegetation has to be cleared for wildfire prevention. There are already projects such as LIFE COOP Cortaderia (source in Portuguese), which brings together institutions in Portugal, Spain and France to fight this invasive species, and municipalities involved in control measures, but the researcher calls for a coordinated strategy. The Institute for Nature Conservation and Forests (ICNF) has drawn up a proposal for a National Action Plan for the Control of Cortaderia under Decree-Law No. 92/2019 (source in Portuguese), although its official publication and implementation have been delayed. The biologist hopes that the forthcoming National Action Plan against invasive alien species will set out priorities, responsibilities and ways of intervening. To make those measures effective, she argues that the plan should be accompanied by funding mechanisms enabling municipalities, non-governmental organisations, parish councils, road management bodies and even private citizens to cover the costs of removal. "To remove one of the big cortaderia plants, you often need a mini excavator," she notes, stressing that the size of some specimens makes control operations costly, and not everyone can afford them. What can you do if you come across cortaderia? For citizens, the first recommendation is not to plant or move the species. Anyone who has cortaderia in their garden should seek to remove it, ideally with technical support if it is a large specimen. People who spot specimens in public spaces can also help to build knowledge about the species’ distribution. Hélia Marchante recommends using the iNaturalist (source in Portuguese) platform, where it is possible to photograph a plant and record its location. These data help build maps that can then be used to set priorities for action. "That is very important when strategies are being drawn up to eliminate it," she stresses. While the plant has not yet been removed, cutting the plumes before the seeds are released can help to curb dispersal. But that is only a temporary measure. "Removing the plumes does not solve the problem," the biologist stresses. "The aim must always be to remove the whole plant." For people with respiratory allergies, Morais de Almeida also advises reducing exposure during periods of higher pollen concentration. In certain circumstances, wearing a mask and eye protection may be useful, along with sticking to medication prescribed for diagnosed allergic disease. "We know that early in the morning and later in the afternoon are the times when pollen levels are highest, so at those times we should avoid airing or ventilating our homes; otherwise we may be contaminating the indoor environment. If we are travelling by car, we should probably keep the windows closed; if we are on a motorbike, we should opt for a full-face helmet and wear goggles," the immunoallergology specialist recommends. The doctor also draws attention to specific weather conditions. During storms, humidity can contribute to the fragmentation of pollen grains, making them easier to inhale, which can worsen respiratory symptoms in sensitised individuals.