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Dry matter production from temperate pasture species is typically very low during midsummer. Switchgrass (Panicum virgatum L.) is being investigated as an alternative pasture species to overcome this problem. We studied two populations over 3 yr to determine if they had sufficient cold tolerance to be used in these areas. Populations selected from cv. Pathfinder were grown in plots at Ottawa and sampled from September to late November in 1985–1987. Each fall, samples were dug at intervals of 2 or 4 wk, the tops and roots trimmed back, then placed in plastic bags in a programmed freezer to determine the temperature at which 50% of the plants were killed (LT50). Percent crown moisture was also measured in the falls of 1985 and 1986. Prior to the onset of cold hardening in September, the two switchgrass populations had LT50 values of −4 °C. Slow cold-hardening occurred to the end of September, followed by a very rapid increase in cold tolerance to −18 °C at the end of October. During November, the rate of hardening decreased and cold tolerance came to a plateau of −19 to −22 °C by the end of the month. The same range has been reported for winter hardy cultivars of timothy and alfalfa. Crown moisture decreased to a plateau several weeks before LT50 values indicating that moisture is of questionable use as an indicator of cold tolerance in switchgrass. Good cold tolerance in switchgrass combined with superior survival during the severe winter of 1982/1983 and vigorous regrowth from overwintering rhizomes lead us to believe that winter hardy cultivars for central and eastern Ontario could be selected from this material.Key words: Switchgrass, Panicum virgatum L., low temperature tolerance, winter survival, forage plant