Fourth-class invaders

• Giant, Canada and flat-topped goldenrods appeared in Europe in the second half of the 18th century
• Scientists are investigating three methods of goldenrod removal: scalping, herbicide spraying and sowing other plants
• The spread of invasive species is inevitable. The most effective way to combat them is rational and sustainable spatial development

"Autumn begins with mimosas, and is golden, fragile and kind..." – sang Czesław Niemen with words from the poem ‘Memories’ by Julian Tuwim. The song went on to become a hit, but both the acclaimed poet and the celebrated composer and singer probably didn't know that they were making a mistake. Mimosas do not grow in Poland, and fields, meadows and roads are enriched with exuberant goldenrod blossoms in the autumn. – They now bloom as early as in August, but that's because the climate is changing. At the time the poem was written goldenrods blossomed in September, providing the appearance of a golden autumn even before the leaves on the trees had discoloured. The poet probably had this plant in mind, but just used the wrong name – says Dr. Magdalena Szymura, Director of the Institute of Agroecology and Plant Production at the UPWr, who wrote about this equally decorative and troublesome plant in the journal 'Land Degradation & Development', in an article titled 'Restoring ecologically valuable grassland on land degraded by invasive goldenrods: lessons from a six-year experiment'. The publication is the result of an experiment she conducted together with Dr. Sebastian Świerszcz from the Polish Academy of Sciences in Warsaw and Krakow, and Dr. Tomasz Szymura, University Professor from the University of Wrocław.

First come, first served

Tuwim's poem has been around for almost a century, while goldenrods have existed in the Polish landscape for much longer. We have our very own European goldenrod, which is a native species and not as expansive. The problem lies with its foreign relatives, which spread uninvited: giant, Canada and flat-topped goldenrod. In the second half of the 18th century, probably enchanted by their beauty, someone brought them from North America to London. Decorative and melliferous, they became an ornamental feature of gardens in Europe, although they found them too cramped and escaped from them, taking up more and more space. They spread in Europe and Asia, and Canada goldenrod also in Australia.

Goldenrods arrived in Poland in the 19th century and are found mainly in south-western Poland, where they first appeared. Flat-topped goldenrod took a liking to Silesia: it remained in a small area where it was brought to a nursery near Opole, with soil contaminated with its seeds. It spread in the vicinity of Opole and Niemodlin, as well as in heaps near Katowice and Bytom, and has, at least so far, not spread further. In contrast, giant and Canada goldenrods spread so widely and have such a negative impact that they have been classified as the highest, fourth class of invasive species. They form dense, single-species communities along roads, railway lines, in wastelands, forests, and roadsides. – Our analyses suggest that it was not habitat preference but the order of arrival in a given area that determines which goldenrod species dominates the community. The 'first come, first served' principle applied. For example, invasive goldenrod species compete not only with native species, but also with each other and with other invasive species, such as rudbeckia and Jerusalem artichoke – says Prof. Magdalena Szymura.

The strength of goldenrods, which can live for 300 years, lies in their ability to adapt to changing conditions, for example to drought by blossoming earlier, or to poorer soil. – Goldenrods are not fussy, and can adapt to anything, as in Bob Geldof's song 'I don't mind at all'. They are only limited by altitude, growing at heights of up to around 800m above sea level, and are not found in higher mountain altitudes – adds Magdalena Szymura.

They spread via light seeds (up to two thousand in one inflorescence) over long distances, and displace other species wherever they appear by competing with them above-ground – overshadowing and taking up living space, as well as underground – spreading through rhizomes. They produce compounds that negatively affect the germination and growth of other plant species. – There are usually 25-30 plant species in a 25 sqm meadow. When goldenrods appear the number of associated species drops to 3-5, and these are usually ruderal, cosmopolitan plants, such as nettles and vertica – says Prof. Szymura, who deals with invasive plant species. Goldenrods are particularly dangerous for rare and valuable species that are under legal protection or threatened with extinction, such as the snake's head fritillary, the marsh gentian, the Turkish marsh gladiolus, Siberian iris, adder's tongue, or the globe flower, among others.

The most visible effect of goldenrod invasions is the decline in the diversity of native plant species, but they also harm animals, especially meadow pollinators. In occupied areas their diversity can drop by up to 90 percent. – For example, a bumblebee, which has become weakened after winter, needs food in early spring and is destined to starve if it crawls out of the ground in an area with goldenrods, which drowned out other plants – says Magdalena Szymura. They also reduce the abundance and diversity of bird species, which cannot move or nest in dense goldenrods.

– Analyses carried out by one of my doctoral students, Chathura Perera, together with Dr. Iwona Gruss from our university's Department of Plant Protection, also point to unfavourable changes in the diversity and abundance of soil invertebrates – adds Prof. Szymura.

Goldenrods are also not harmless to humans: allergy sufferers can be affected by their pollen. They also decrease the aesthetic value of landscapes and thus less attractive for tourism, which translates into economic losses. In fact, there are not many positive things that can be said about them, except that goldenrod extracts have been used as a urological remedy and that we can enjoy goldenrod honey. However, the latter is at the expense of bees: having gathered pollen so late, they are unable to prepare for winter. 

Experts believe that invasive goldenrods should be strongly limited, especially in naturally valuable areas, although they cannot be effectively eradicated. In the case of plant species that occupy more than 1,000 ha in a non-native range only control is possible. However, goldenrods occupy 130 000 hectares in south-western Poland, representing 4.5 percent of the total area of Lower Silesia.

From scalping to sowing

In their native North America goldenrods are not a problem. They live in harmony with other plants, and their numbers are regulated, for example, by a type of beetle that feeds on its leaves. Unfortunately, in Poland goldenrods do not have any specialised enemies. All that remains is to develop ways of reducing it – and this is what the Wrocław-Warsaw-Cracow team did, attempting to find the optimum method to control this troublesome plant. Various methods of eradicating invasive goldenrod species and adding seeds were tested to restore valuable grasslands, rich in various species.

The six-year experiment began in 2013 as part of the doctoral thesis of Dr. Sebastian Świerszcz, who observed changes in the composition of plant species during the four years of his doctoral studies, in which the supervisor was Prof. Karol Wolski. Professor Magdalena Szymura was the associate supervisor, and then continued with floristic analyses, as well as biomass production and groundcover value analyses.

Experiments were conducted in plots in Pawłowice, mainly with beds of giant goldenrods, with a small contribution of Canada goldenrods. Three methods of removing the enemy were studied: first, the top layer of the turf with goldenrod seeds was scalped, followed by soil tilling and the use of herbicides. Next came two methods of adding seeds: sowing a fast-growing mix of grass normally used in production meadows, and spreading fresh hay harvested from a semi-natural meadow together with control methods (no removal, no addition of seeds). Seeds of around 60 species, which were planned for recovery after the invasion, were introduced with fresh swath, including meadow cress, rockrose, ranunculus, chrysanthemums, bellflowers and lady's mantle. For six years the plots were mown twice a year: the first swath was before 15 June (to weaken the plant before intensive growth) and the second in mid-September. – We know from literature that a single swath is not effective – Magdalena Szymura points out. The vegetation composition, biomass production, chemical composition and quality of the forage were then assessed.

The combination of removing goldenrod by means of scalping and introducing seeds by spreading a fresh swath gave the best results in terms of the environmental impact (low level of damage), cost, and the botanical composition and nutritional value of the undergrowth. The methods developed enabled the restoration of a semi-natural meadow and reduced the coverage of goldenrod to 25 percent – a level that is no longer a threat and does not affect the abundance and number of species of pollinators. It also turned out that herbicides are no more effective than more environmentally friendly methods, i.e. mechanical methods followed by spreading fresh swath.

– This is a particularly important result, because invasive species are mainly controlled in protected areas, where the use of chemical methods to combat undesirable species is prohibited by law and only possible with prior ministerial approval – says Magdalena Szymura. – The results obtained after two years indicated a better efficiency of using herbicides, and it was only after the experiment had been running for a longer time that the effect of the procedure was no longer significant. When it comes to invasive species control, we are interested in finding methods that are effective in the long run and can be applied to all areas where the species is present, hence long-term monitoring of the effects of the treatments applied is very important – she adds.

The published research is unique precisely because of its long-term nature, allowing for the observation of a long-term effect. Experiments on the effectiveness of methods of invasive species control are conducted in different centres around the world, but usually for shorter periods. Meanwhile, the effectiveness of the control measures applied can only be determined in the long term.

– The results of works where further steps of community restoration are included, in the form of the introduction of seeds and appropriate maintenance, are rarely published. Yet, it is these very stages that are crucial for the successful control of invasive species, as there is no room for emptiness in nature. If we eradicate an invasive species, a free ecological niche is created, allowing for further invasions. By introducing seeds of plant species of the intended community we can fill this niche and increase the chance of community recovery compared to if the area was left to self-fertilise.

Observing the effects of the recommended method takes time, and is therefore costly. This is why, due to the length of time for which project research is funded – usually two to three years, such observations are rarely undertaken – says Magdalena Szymura. – Meanwhile, in our opinion, six years is the shortest period of time possible to record the actual, long-term impact of the measures implemented on the effectiveness of invasive species reduction.

The results of the research were influenced, among other things, by the weather. For example, in drier periods there was more Yorkshire fog, a grass that is not affected by drought.

– An important result of our experiment is also the observation that the community created by introducing seeds by spreading a fresh swath has a more diverse range of plant species and a more stable species composition, even under extreme weather conditions in the form of drought or flooding of the experimental area after prolonged rainfall – says the professor.  

She plans to continue with the experiment and publish the results after 10 years. – Over time, the experimental plots become more and more similar to each other as meadow species migrate. It would be good to see what they look like 20 and 30 years after the invasive species control treatments, seed introduction and extensive cultivation – she adds.

Deliberately or by accident

Goldenrods are not the only invaders in the Polish landscape. Plants, animals, fungi and micro-organisms are constantly migrating, accidentally or deliberately brought in by humans. Extraneous plants end up, for example, with seeds transferred on animals or clothes. They are also sometimes imported into gardens and nurseries from which they later escape, creating a threat to native ecosystems and human health. Particularly harmful for humans are Sosnowsky's hogweed (imported as cattle feed from the USSR) and Mantegazzi hogweed (used as a decoration in sanatoriums), which can cause severe burns. This is why we mow hogweed more than goldenrod and other very invasive plants such as knotweed (Japanese, giant and Bohemian knotweed). Equally unwelcome are steeplebush, black locust, Northern red oak, black cherry, Impatiens glandulifera and, in the case of invasive animal species, the Asian lady beetle, raccoon, red-footed tortoise, American mink, or boxwood moth, among others. The European Union regularly updates the list of species that pose a threat to European ecosystems. New uninvited guests continue to appear, such as the Grindelia squarrosa (curly-top gumweed), which was previously a problem in southern Europe and is now present in central Europe due to climate change.

Along with the fragmentation and degradation of natural communities, extraneous plant invasions are considered one of the most significant threats to biodiversity on a global scale. In 2020 the European Biodiversity Strategy 2030 was adopted, which aimed at, among other things, reducing the spread of, as well as combatting, invasive species. In Poland it is included in the Alien Species Act 2021. The most effective method, and also the first stage, is prevention: if a species has not yet found its way into an area, but is known to be invasive from other areas, then regulations should be put in place to prevent its introduction.

The second stage is the attempt to eradicate an invasive species already present in an area – this will only succeed in the early stages of expansion and depends on whether the plant is present on many sites, and on its volume.

At the third stage, when the invader is widespread and poses a threat to valuable areas, all that remains is to control the spread and try to contain it. Mechanical methods, such as mowing, digging up the plants, and covering the mown area with plastic sheeting to prevent regrowth, require a lot of labour and many years of repetition. However, they are harmless and can be used in protected areas. The use of herbicides is cheaper, but has a negative impact on the environment and co-occurring plant species, and cannot be used in areas of natural value and protection, or near watercourses and reservoirs. The most effective mechanical and chemical methods, on the other hand, are very expensive.

– There are also ecological methods, such as grazing, but many invasive species produce substances that are harmful to animals. There have also been attempts to import natural enemies of invasive species from their native range, for example insects that feed only on these species. However, there is a risk that the imported insects could 'change their diet' and become a threat to native plant species – says Prof. Szymura. Together with the University of Silesia in Katowice, during the last season she worked on a strategy for the eradication of knotweed, hogweed and steeplebush, which is to be used as a guide for local authorities.

– We cannot effectively defeat so many invaders, so we have to learn to live with them and try to limit their presence. The most effective way to combat invasive species is through rational, sustainable land use, not leaving uncultivated land, appropriate mowing of roadside verges and maintaining areas next to railway tracks. This approach is carried out in many countries, effectively stopping the invasion of goldenrods. In Poland areas covered with goldenrods are most often mown, but, in the absence of control treatment and the introduction of seeds of other plants, mowing alone is not very effective. In fact, if not done regularly, it can even have the opposite effect and contribute to an increase in the number of goldenrods – adds Magdalena Szymura.

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