Silphium perfoliatum, the cup plant or cup-plant, is a species of flowering plant in the family Asteraceae, native to eastern and central North America. It is an erect herbaceous perennial with triangular toothed leaves, and daisy-like yellow composite flower heads in summer.Morphology of the plant. The typical height of S. perfoliatum plant ranges from 1–3,5 m (3–8 ft). The stem is stout, smooth, slightly hairy (glabrous) strongly 4-angled square, like mint plants. The leaves are opposite, toothed and ovate. The petioles are widely winged and fused around the stem, forming a cup. The first flower develops on the tip of the main stem, then more flowers develop on side branches.Generative reproduction. The ray florets have female characteristics, and eventually develop to become thin brown achenes with a marginal wing utilized for wind dispersal. Insect pollinators including bees, butterflies, and skippers help to cross-fertilize flowers to produce seeds. 20 to 30 seeds are created in each flower head. Each seed is about 9 to 15 mm long, 6–9 mm wide, flattened in shape, with a thickness of 1 mm.Use as energy crop. S. perfoliatum is considered a potential energy crop plant, especially because it has low demands on the climate, the soil and previuos crop and produces high amounts of biomass. Based on the results of the Thuringian State Institute of Agriculture the plant can be an alternative plant for biogas production. The annual biomass yields are very different according to the present literature: In the second cultivation year S. perfoliatumcan grow from 13 to 20 tons of biomass per hectare, an experiment done in Thüringen, Germany even showed that 18 to 28 tons of dry weight per hectare are possible.Cultivation. In Germany, the arable cultivation of S. perfoliatum is becoming increasingly widespread for biogas production. Its invasive potential for Mid European countries is considered as low, but a spread trough wind, birds and harvesting machines is possible. One has to be especially careful when growing S. perfoliatum near moving bodies of water, so that it cannot spread and germinate on river banks such as Buddleja davidii. This could be a potential problem, because these ecosystems are very sensitive. Sowing and planting. Due to low germination rate of 15-20% with untreated seeds S. perfoliatum is usually planted as seedling. This is the reason for high investment costs in establishing the crop. Furthermore, there is no yield in the first planting season as S. perfoliatum is only harvested from the second year on. In Germany, seeding time to obtain an acceptable seedling development is in the beginning of May. The highest biomass yield is achieved with 10 cm × 50 cm (within-row spacing: 10 cm; distance between the rows: 50cm). In European climate conditions the planting of seedlings is typically carried out at the end of May, or the beginning of June with planting machines from the vegetable and gardening industries. Low plant density (50 cm × 75 cm, 75 cm × 75 cm) can give a higher yield in the first harvestable year compared to the higher plant densities (50 cm × 50 cm). In the second harvestable year, they are similar, regardless of the plant density. In order to avoid reduced yields in the first year a possible solution could be sowing S. perfoliatum in combination with corn. Three quarters of usual corn yields can be achieved in the first year and S. perfoliatum can establish after corn harvest, achieving its full potential in the following year.Potential invasiveness if cultivated as an energy crop. Although it is a native plant, the cup plant has been declared invasive in several States in the US. Therefore, its invasiveness is in active discussion, especially when considering it as a potential biofuel crop.
To establish potential invasiveness, there is the Weed Risk Assessment (WRA), which can be modified for the local environment and be used to evaluate the potential invasiveness of species proposed as biofuels and to evaluate whether the traits selected for in biofuel species might predispose them to be invasive. There is the Australian- and the US-WRA. The cup plant, S. perfoliatum, has not been assessed yet. However, desirable traits for biofuels tend to overlap with those of invasive species, such as high productivity, low input requirements, and wide habitat breadth. S. perfoliatum fits these criteria, but because generatively reproduced seeds have low germination rates, one could hypothesize that the potential invasiveness is relatively small and that the spread of S. perfoliatum can be contained. Furthermore, one could harvest before the seeds reach their full maturity.