Currently, invasive species are a major threat to biodiversity, human health, and economies [1-4It has been estimated that the fight against invasive species and the damage caused by them in European Union accounts for a minimum of 9.6–12.7 billion euros annually, and this amount is expected to rise to 20 billion euros annually [1, 5–7]. The most important elements of protection against invasive species are prevention of introduction and early detection. In the case of established invasive species, the most successful options are eradication or isolation [8–13]. Herbicide treatment is one of the most effective ways to control or eradicate invasive plants in large areas [10, 14–19]. Nevertheless, the prevention and control measures of invasive species with chemicals are rather arguable whereas their use endangers other species and the ecology and abiotic elements (above and below ground waters, soil and air) in protected areas [18, 20, 21]. Consequently, herbicides have to be carefully chosen (dosage, types and combination) based on the native species community [16, 19, 20]. Therefore, the herbicide application must be well planned and localized, the applied chemicals should be safe and effective. However, the use of these products in non-agricultural areas are very rarely accessible [cf. 10, 22, 23]. This knowledge gap also requires not only extensive research but also effective exchange of information and experience [cf. 10, 18, 19].
Clonality is common among invasive plants [24, 25, 26]. The common reed Phragmites australis [27], alligator weed Alternanthera philoxeroides [28, 29], Japanese knotweed Fallopia japonica [30–33], Solidago species [14, 34], and Canada thistle Cirsium arvense [35, 36] are examples of problematic invasive clonal species. Their success is partly due to translocation of water, nutrients, and photoassimilates among physically interconnected shoots [37–42]. However, pathogens can also be transported through the same clonal network [36, 43–45] as can heavy metals [46, 47] and herbicides [17, 48–51]. Bud banks on a clonal network play an important role in competition, vegetative multiplication, and resprouting [52, 53]. An extensive dormant bud bank can be activated, resprouted, and made able to colonize an empty niche or re-establish monospecific stands after disturbance. Subsequently, the succession of natural vegetation is impeded or completely obstructed. The mortality risk of clonal plants is low because death only occurs when both shoots and bud banks are simultaneously destroyed [38, 54–60]. This possibly explains why management of clonal spreading species is difficult even with herbicide treatment [61], and knowledge of invasive plant biology is essential for effective management [8]. While most studies involved a single year of monitoring, examination of herbicide treatment for several years before, during, and after treatment can provide useful information that will help guide management programs [18, 62–65].
The common milkweed Asclepias syriaca originated in North America but is reportedly established in Continental, Mediterranean, and Pannonian Europe [66]. It is a perennial clonal plant [66–69], and despite the fact that its shoots die back every autumn, it can resprout in the same place for extended periods [68]. The clonal structure of A. syriaca comprises solitary or few (2–5) groups of shoots that develop vegetatively by buds of plagiotropic rhizomatic roots [68]. Milkweed is one of the most dangerous invasive transformer species currently widespread in Hungary and is spreading in Czech Republic, Romania, Poland, Serbia, and several other countries [66, 68, 70–72]. It primarily endangers psammophilous habitats where its structure differs from that of natural vegetation [73]. The problems arising from the invasion of milkweed were primarily attributed to the assumption that it can inhibit the regeneration of natural vegetation [20, 68, 73, 74]. Despite the harmful effects of A. syriaca, it was only recently added to the list of Invasive Alien Species of Union Concern [75]. To adequately control common milkweed, the bud banks of its roots and lateral roots must be eliminated. Control or eradication is an increasingly important action from both agricultural and conservation perspectives [20, 68, 76–78]. Complicating matters is the fact that extermination itself can create suitable conditions for colonization (e.g., soil disturbance), and large areas can become permanently milkweed-free only with coordinated efforts and at enormous costs [68]. Nevertheless, herbicide treatment may be a cost-effective method to control extended stands of milkweed in strictly protected areas [10, 20, 66, 68]. The most frequently used herbicides for A. syriaca management are glyphosate and triclopyr, whereas fluroxypyr or dicamba are rarely used. These are often used individually or in combination with each other or with some level of mechanical control [10, 20, 76, 77, 79]. Relatively little information is available on the long-term or semilong-term effects of postemergent herbicides on A. syriaca. Here we report one of the first and longest monitoring period of one-time herbicide treatment on a common milkweed stand and analyzed the before, after and during treatment effects. We hypothesized that clonality is an important factor for resistance to herbicide treatment and proposed the following three questions: a) How does a single herbicide treatment influence the further spread and pod production of the invasive plant? How much the single treatment reduces the shoot, pod and pod-bearing shoot number and how long-lasting these effects are? We examined the shoot, pod and pod-bearing shoot number changes in long term (and not only in the year of the treatment). b) Which strategies are used by the invasive species to recover? The recover strategies of the invasive plant mean: vegetative or generative propagation (or both of them) was used by the stand in the re-establishment. Based on the answers of the first two questions could be determined the third one: c) Was treatment a successful environmental management approach? The duration of the investigation also plays an important role, because what effect it has on the stand and for how long. This is a kind of cost effectiveness of the single herbicide treatment. d) Which can be potential herbicides for controlling common milkweed or other clonal plants?