Wildlife Federation stated that the population of the monarch butterfly is now down 90 percent in the last 20 years and cited the reduction in milkweed as a contributing factor. Many parts of the United States face a reduction in milkweed population due to factors such as increased habitat loss due to development, roadside median mowing, and herbicide use. Despite this, deforestation due to human settlement may have expanded the range and density of common milkweed in some regions. Milkweed has even become invasive as it is naturalized in several areas outside of its original native range, including Oregon and some parts of Europe. Over 40 distinct pathogens of Asclepias species have been identified, including two dozen pathogens for A. syriaca. For example, milkweed yellows is an infectious disease caused by the milkweed yellows phytoplasma, a strain of bacteria distinguished by the absence of a cell wall. The species features since 2017 on the list of Invasive Alien Species of Union Concern. This means that import of the species and trade in the species is forbidden in the whole of the European Union. Cultivation A. syriaca can become invasive. It spreads aggressively from rhizomes and may not be suited to small gardens and formalized plantings. The plant is winter hardy in USDA zones 3–9; it has a preference for moist but well drained soils, but is tolerant of dry conditions and clay soils. It is ideal in semi-dry places where it can spread without presenting problems for other ornamental species. Monarch Watch provides information on rearing monarchs and their host plants. Efforts to restore falling monarch butterfly populations by establishing butterfly gardens and monarch migratory "waystations" require particular attention to the target species' food preferences and population cycles, as well to the conditions needed to propagate and maintain their food plants. In the northeastern United States, monarch reproduction peaks in late summer when most of the plant's leaves are old and tough. Plants that are mowed or cut back in June – August regrow rapidly from their rhizomes in time for peak monarch egg-laying, when reproducing female monarchs have a preference for quickly-growing A. syriaca shoots whose foliage is tender and soft. A. syriaca is easily propagated by both seed and rhizome cuttings. The plant's seeds require a period of cold treatment (cold stratification) before they will germinate. To protect seeds from washing away during heavy rains and from seed–eating