Three white clover cultivars of differing geographical origin, namely AberHerald, Grasslands Huia and Sandra, were grown in binary mixtures with perennial ryegrass (cv. Preference) for a period of 3 years at Uppsala (59° 49′ N, 17° 39′ E) where the length of the main growing season is c. 200 days. Changes in growing point numbers, stolon length and stolon weight from autumn until late spring were followed by four to six destructive harvests each season. Changes in carbohydrate content (starch and water-soluble carbohydrates) were also determined on six or seven occasions during the same time. Cold hardening and dehardening were followed by artificial freezing tests on stolons removed from the plots on seven occasions during the last study season. The rate of node production was followed in the last spring. Temperatures in the stolon environment were recorded during the last two study seasons.

All cultivars suffered a loss of stolon length during the winters, with Huia being most affected. The extent of recovery during spring varied between years. Late spring frosts checked the growth of both AberHerald and Huia more than that of Sandra. The accumulation of starch appeared to depend on autumn temperatures and there was a decline in starch content during the winter. Sandra maintained a higher content of water-soluble carbohydrates during the winter and the spring than the other cultivars. Sandra also achieved a higher degree of freezing tolerance and dehardened later than both AberHerald and Huia. However, temperatures in the stolon environment always exceeded the degree of freezing tolerance attained irrespective of cultivar.

It is concluded that the accumulation and utilization of non-structural carbohydrates are important in determining the extent to which a white clover cultivar survives the winter and spring in a harsh climate. Freezing tolerance per se does not seem to be the most limiting trait, but in a climate where frequent spring frosts occur, a late dehardening seems to be valuable.