Planting depth for oilseed calendula

Calendula (Calendula officinalis L.) is a popular ornamental and medicinal plant, but it also is a potential oilseed crop. Its seed oil has high levels of calendic acid, which makes it a highly valued drying oil with important industrial applications. Current agronomic information on calendula is not easily available, is limited in geographic scope, or pertains primarily to ornamental or medicinal varieties. Consequently, our objective was to investigate seedling establishment of oilseed calendula in response to planting depth and soil microclimate in field soils over two years in central Minnesota, USA. ‘Carola’ was used in all experiments; it is one of the few commercial oilseed varieties available. More seedlings emerged from planting depths of 1 and 2 cm than from 4 or 6 cm. Regardless of planting depth, time after planting to 50% emergence was less variable when estimated by hydrothermal time (HTT, 89 °C d, CV = 14) than calendar days (7 d, CV = 39). HTT was calculated best with a base temperature of 5.5 °C and a base water potential of −2900 kPa (−2.9 MPa). Thus, growers must plant calendula at 1–2 cm, but soil at this depth can dry rapidly, which slows accumulation of HTT and delays emergence of calendula seedlings.

► Calendula seeds produce a valuable drying oil that can be substituted for tung oil in paints and other products.
► Because little information exists regarding the agronomy of calendula, we examined critical planting depths and soil hydrothermal requirements of calendula seed emergence.
► ‘Carola’ is the primary commercial variety of oilseed calendula.
► It should be sown between 1 and 2 cm deep in soil, and about 90 soil degree-days should be expected for 50% emergence.
► These results should better alert growers and specialty seed companies to the sensitive nature of calendula to planting requirements.