Seed germination of calendula in response to temperature

• Calendula stand establishment failures are related to high seedbed temperatures.
• Base, optimum, and ceiling temperatures were 2 °C, 16 °C, and 30 °C, respectively.
• Calendula germination is more susceptible to temperatures above To than below.
• Thermal death of calendula seed resulted from high temperatures during incubation.

Calendula (Calendula officinalis L.) seeds contain high concentrations of calendic acid (C18:3) which can be used as tung and linseed oil substitutes. Calendula is adapted to temperate climate, but field studies in western Minnesota indicated that stand establishment was susceptible to high soil temperatures immediately after planting in spring. Consequently, understanding the temperature conditions that govern germination of calendula is necessary to incorporate the crop into crop rotations of the Upper Midwest, U.S. Temperature gradient bar and heat-shock experiments were used to characterize calendula (cv. ‘Carola’) sensitivity before and during germination. Seed germinated between 2 and 32 °C with the optimum germination temperature at 16–17 °C. Heat shock temperatures (35–40 °C) of less than 50 h duration reduced germination (at 16 °C) below 50%. At 45 °C, 100% seed lethality was induced within 24 h of heat treatment. Accordingly, calendula seed should be sown in the field only if forecasted soil conditions are expected to be below 30 °C during seed germination.