Field pennycress (Thlaspi arvense L.) was seeded in rows at approximately 10-day intervals from June 1 to August 30, 1963, and from July 12 to August 31, 1966, in subarctic, south-central Alaska (61.5° N.). Plants from June 20 and earlier seedings produced mature seed as summer annuals; those from seedings during the first 3 weeks of July produced elongated stems and green pods, while those from later seedings produced unelongated, basal rosettes. Seedlings that emerged during July or later were potential winter annuals. No study plants survived the 1963-64 winter. The 1966 plantings survived the 1966-67 winter as follows: July 12, 50%, July 20, 54%, August 1, 93%, and August 12, 23, and 31, 100%. Morphology, time of flowering, and seed pod production of plants surviving the winter also were influenced by planting date the previous growing season. To prevent field pennycress from producing seed as a summer annual, plants that emerge before the middle of July must be eradicated. Plants that emerge during July or later should be destroyed as seedlings or prior to seed production time the following year.