Growth, Biomass, and Seed Yields of the Perennial Oilseed Crop Silphium (Silphium integrifolium) in Variable Nitrogen Rates.

Silphium integrifolium (Silphium), a perennial native warm-season forb, represents an alternative to annual crop production with the potential to sequester carbon, reduce soil erosion, and decrease nitrogen leaching into waterways. Silphium is more drought tolerant than sunflower (Helianthus annuus), produces seeds that have an oil profile that is similar to sunflower oil in flavor and composition, and provides forage for many native pollinators (Kowalski and Wiercinski, 2004). However, given that Silphium is a new plant in managed agroecosystems, nutrient requirements for maximizing seed yield are unknown. Five rates of nitrogen were applied to Silphium planted at two different densities. In 2016, Silphium biomass and seed production was greatest (10,000 kg ha^-1 and 1900 kg ha^-1, respectively) when receiving 180 kg N ha-¹, the highest rate tested, and when established in a high plant density (26909 plants ha^-1). However, when established in a lower plant density (11960 plants/ha) and receiving 180 kg N ha^-1, individual Silphium plants produced the most buds and flowers compared to all other treatments. Studies to determine optimum N rates for Silphium are ongoing and will be useful for supporting the cultivation of perennial oilseed crops.