Abstract

Sunflowers (Helianthus annuus L.) grown in North Dakota and Minnesota are frequently exposed to freezing temperatures before the plants mature, and the seed they produce will often fail to germinate. This study was conducted to determine the influence of artificial freeze treatments at progressive maturity stages. Sunflower heads (capitula) were harvested at weekly intervals from 28 to 68 days after flowering (DAF) 1973 and 1974. Germination percentages, measured 7 days after harvest (DAH), increased from 6 to 84% in 1973 and from 0 to 48% in 1974 with increased time between flowering and harvest. Freezing prior to 42 DAF blocked or altered some physiological process necessary for germination and resulted in seeds with lower germination. Seed harvested later in the growing season had a shorter dormant period than that harvested earlier. Growing degree days from flowering to harvest were significantly correlated with percent germination at 21 DAH in 1973 (r = 0.95, significant at 0.01 level) and 35 DAHin 1974 (r = 0.99, significant at 0.01 level).

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