Freezing behavior of potato (Solanum tuberosum) tubers in soil

Volunteer potatoes are a major weed problem in potato rotations in regions with mild winter soil temperatures. Freezing dynamics of potato tubers in air have been previously reported, but freezing dynamics of tubers in soil may differ due to ice nucleation sites and soil water associated with soil. Laboratory experiments conducted in hydrated and dry soil columns and field experiments were conducted to determine cold temperatures required to kill potato tubers in soil. Potato tubers in air-dried soil columns exposed to decreasing temperatures typically supercooled to ?3 to ?7 C before exhibiting a distinct exotherm, which stabilized at ?1.4 to ?1.5 C, representing the freezing point of tubers. Tubers that were supercooled and removed from the cold environment before experiencing this exotherm were able to sprout and had no visual symptoms of freezing injury, whereas tubers that experienced the exotherm were nonviable and unable to sprout. Tubers in soil columns hydrated to 7% SWC supercooled much less than tubers in dry soil and exhibited an exotherm that stabilized near ?1.9 C. Tubers exposed to temperatures near the tuber-freezing point (?1.4 to ?1.9 C) for periods of 1 min to 24 h, but not undergoing an exotherm, exhibited varying degrees of injury, which increased with time of exposure. Tubers held at ?1.0 C for 4 to 24 h were unharmed and able to sprout similar to controls. In field trials conducted from 1993 to 1999 in the Columbia Basin of Washington, tubers buried at shallow depths (5 cm) were much more likely to experience lethal cold temperatures than tubers buried deeper. In general, when minimum soil temperature at tuber depth reached ?1.5 to ?1.9 C or lower, some tuber mortality occurred and when soil temperature at tuber depth reached ?2.8 C or lower, extensive tuber death occurred. Monitoring of winter soil temperatures by depth in potato- growing regions could be used to predict severity of volunteer potato for the subsequent growing season.