Planting depth for oilseed calendula

Abstract

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 2cm than from 4 or 6cm. Regardless of planting depth, time after planting to 50% emergence was less variable when estimated by hydrothermal time (HTT, 89°Cd, CV=14) than calendar days (7d, CV=39). HTT was calculated best with a base temperature of 5.5°C and a base water potential of −2900kPa (−2.9MPa). Thus, growers must plant calendula at 1–2cm, but soil at this depth can dry rapidly, which slows accumulation of HTT and delays emergence of calendula seedlings.