Kudzu (also known as kudzu bean or kudzu hemp) is becoming increasingly widespread in Ticino, especially around Lake Lugano and Lake Maggiore. The plant colonises gardens, roadside embankments and lake shores, where it smothers entire trees. Its presence results in substantial infrastructure maintenance costs and has significant adverse impacts on biodiversity and the protective function of the forest. Thankfully, the plant can be eradicated through targeted, effective mechanical control measures (cutting the vines below the root collar) and regular monitoring. Very early intervention increases the likelihood of such measures being successful.
Kudzu (Pueraria lobata) is a perennial climbing vine with three-lobed leaves from the legume family (Fabaceae). Native to Southeast Asia, it grows extremely quickly, up to 26 cm per day and 20 m per year! Kudzu forms an interconnected system of vertical and horizontal vines. The root nodes of the horizontal vines can develop into underground tubers that form a store of nutrients and water. These tubers can reach a considerable size and weigh up to 180 kg. The nodes can separate from the mother plant and become new, autonomous plants (generative reproduction similar to that of strawberries). When growing vertically and in direct sunlight, kudzu forms clusters of purple flowers from which dark brown pods develop. Each of these pods contains between three and ten seeds (mature seeds can be distinguished from immature ones by their darker colour). Due to the high germination rate of the seeds (around 75%) and their ability to survive for several years, it is highly likely that a seed bank will form in the soil.
History and distribution
Historically, Pueraria lobata is a traditional ingredient in Chinese cuisine and a treatment for various ailments. The plant's invasive potential has been largely underestimated. Kudzu was actually widely cultivated in the Southeastern United States in the first half of the 20th century, both as protection against erosion in agricultural fields depleted by intensive cotton production and as fodder for livestock.
Distribution in Switzerland
In Switzerland, kudzu is currently only found south of the Alps. It was first observed in Ticino in 1956. Extensive monitoring carried out since 2006 shows an increase in both the number of sites and the surface area affected, rising from 21 (15,290 m2) in 2006 to 55 (43,000 m2) in 2018, with over 60 sites affected in 2020 (data collected by the cantonal forestry service). All occurrences of the plant are recorded in a cantonal database, which is periodically updated and provides information on the location, the presence of seeds and the status of control measures. There are 30 registered outbreaks in forest areas, 12 of which involve seeds and 14 cover areas larger than 1,000 m2. Over the past 12 years, both the number of outbreaks and the total area affected have more than doubled, and the area of each individual outbreak has increased by an average of 68%. Five outbreaks have now been completely eliminated.
Most outbreaks are concentrated around Lake Lugano and Lake Maggiore, where the climate (hot summers and mild winters) encourages its growth. However, outbreaks in peripheral areas such as the Onsernone Valley and Lower Leventina are likely due to the illegal disposal of plant waste.
Impact on the environment
The warm climate and absence of antagonists (phytophagous insects and pathogens) encourage the spread of the plant. In just over 60 years, it has colonised more than 3 million hectares in the United States, infesting an extra 50,000 hectares each year and causing economic damage totalling at least $500,000 per year.
Pueraria lobata also has a major negative impact in our latitudes. As a climbing plant, it can exploit any vertical support (trees, walls, street lamps) to spread further. Forming a dense, opaque cover, it smothers all plant species below it. Because of these characteristics, kudzu is now listed as one of the 100 of the World's Worst Invasive Alien Species and has been added to the Black List of invasive neophytes (Info Flora 2013). As well as generating high maintenance costs for infrastructure and open spaces (e.g. agricultural fields, roadside embankments and gardens), kudzu also harms biodiversity. In forests, it kills the trees on which it climbs, and the dense carpet of vines hampers natural regeneration. In protection forests, kudzu outbreaks are cause for great concern, as the vine provides zero protection against landslides, rockfall or avalanches. It should also be noted that in winter, kudzu poses an increased fire risk due to the large amount of flammable material (dry vines) between the ground and the treetops.
Many years of trial and error have helped pinpoint the best mechanical and chemical control methods. Mechanical control is preferable to chemical solutions because it is not restricted by legal regulations and has no negative side effects for the environment.
Mechanical control involves 'collar cutting', i.e. separating the root system from the above-ground vines a few centimetres below the collar (the main root node that contains the vegetative buds and can form new shoots). Vertically growing vines are usually cut at around one metre above the ground so that they dry out because they are no longer in contact with the ground. Based on recent findings from the field, it is advisable to perform collar cutting by the May of the first year of measures being taken, followed by at least two inspections in July and September, when all shoots are to be removed. Where there is dense undergrowth, the ground should be cleared beforehand (during winter) so that it is easier to spot kudzu seedlings. Cut vines can be sorted and dried on the spot, but only if they cannot come into contact with the ground. If this is not possible, the material must be disposed of in a waste incineration plant. Vines cannot be taken to waste disposal sites, as there is a very high risk of kudzu spreading there. Affected areas must be checked regularly in the following years (twice a year, at the beginning and end of summer) to completely eliminate the plant.
Chemical control can be done in two ways: either by applying an approved triclopyr-based product (final concentration 2%) in August on the leaves of the plant's shoots cut three weeks before, or by directly brushing the product on the part of the vine that has just been cut. This means that the herbicide is applied selectively and in very small quantities.
Both methods (mechanical and chemical) are incredibly effective, immediately reducing the number of nodes drastically (or even eliminating them completely in the case of chemical methods). From the second year of intervention onwards, the time needed for control and regular checks falls rapidly. To monitor the effectiveness of the measures taken, each intervention is recorded in detail on a special form (stating the times, methods, time frames and quantities removed).
Kudzu is already well established in Ticino and is now in the expansion phase. It is advisable to take measures to completely eliminate the plant before the situation gets out of hand and the costs of control escalate. In 2018, the cantonal forestry service undertook a trial project at 14 sites of varying sizes and types to quantify the control costs. A canton-wide project covering the period 2019–23 is now being implemented.
Depending on the location of the outbreak, organising work and control measures may prove very complex. Interventions on overhanging rocks, along watercourses, roadside walls and lake shores require special safety measures and equipment (using ropes, regulating traffic or using barges). Plant material deposited illegally in forests or on roadside embankments hinders the effective search for root nodes and must therefore be removed at the start of the work (incurring additional costs). Large areas cleared of kudzu are also often easily colonised by other invasive neophytes such as tree of heaven (Ailanthus altissima), American pokeweed (Phytolacca americana), butterfly bush (Buddleja davidii) or Japanese knotweed (Reynoutria japonica), so it is important to actively manage these neophytes and immediately sow or plant native species.
Please notethat any vehicles and tools used in control measures must always be cleaned thoroughly, as parts of the plant could stick to them, be carried away and possibly resprout elsewhere.
Pueraria lobata is an invasive neophyte that has a major negative impact on our environment, and on protection forests in particular. Thanks to effective, viable control methods, kudzu could still be eradicated in Ticino if we immediately make every effort to prevent this plant from spreading further. The goal of elimination should not only apply to kudzu at the cantonal level, but locally to all those invasive neophytes that are not yet widespread.
An extensive list of publications on this topic is provided in the original article.