Global: Seed Tuber Physiological Age — Why Chronological Age Misleads Planters
Two seed lots of the same calendar age can perform very differently; accumulated thermal time, not birthdays, determines vigor and stem number.
Physiological age — usually measured as day-degrees above 4°C accumulated from the start of sprouting — predicts field performance far better than chronological age, and seed specialists urge growers to ask suppliers for storage history, not just the production year.
Young seed, stored cold since harvest, produces few stems with high vigor and suits long-season processing production. Moderately aged seed, warmed during storage, produces more stems, earlier tuber set and faster bulking, while very old seed gives fewer, weaker stems — valuable for early table and seed crops but risky in short windows where early senescence cuts yield.
The practical lever is heat units: a lot held 4°C all winter differs fundamentally from one that spent weeks at 12°C during loading and transport, even if both were grown the same year.
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Growers can manipulate physiological age deliberately: pre-sprouting protocols raise effective age for early markets, while strict cold chains preserve youth for maincrop contracts.
For professional seed buyers, the storage temperature history of a lot is therefore as important as its production year.
Sources: O’Brien et al., Journal of Agricultural Science, 1983 (NIAB); NDSU Extension
