EFFECTS OF PLANTING DATE ON THE GROWTH AND DEVELOPMENT OF NARROW-LEAVED HAWK’S-BEARD

Narrow-leaved hawk’s-beard (Crepis tectorum L.) planted prior to 6 June bolted and produced mature seeds as a summer annual. Plantings on 16 June bolted but not all reached maturity during the same growing season. Narrow-leaved hawk’s-beard planted in early July developed in one of three ways: (1) bolted and produced mature seeds as a summer annual, (2) bolted but produced no mature seeds in the year of planting, or (3) developed as a winter annual. Plantings made after 15 July developed as winter annuals. Survival of winter annuals the following spring ranged from 12 to 46% for July plantings to 80 to 90% for August–September plantings. No plants survived from the mid-October plantings. In growth chamber studies at 20 °C, bolting occurred in all plants at an 18-h photoperiod but not in plants at a 14-h photoperiod. The percentage of plants bolting under photoperiods between 14 and 18 h was intermediate. When grown under the same photoperiod, reducing the temperature to 18 °C during the light period and 5 °C during the dark period did not affect the percentage of plants bolting. These results suggest that photoperiod plays a major role in determining the pattern of development of narrow-leaved hawk’s-beard.