Freeze injury to southern pine seedlings

Freeze injury to roots and shoots of pines is affected by genotype and nursery practices. Local sources of shortleaf pine and Virginia pine that are grown in nurseries in USDA hardiness Zones 6 and 7a are relatively freeze tolerant. However, loblolly pine, slash pine, and longleaf pine seedlings have been injured by a number of freeze events (0 to 24 °F) in hardiness Zone 7b and 8. Some fast-growing half-sib families from the Coastal Plain are more susceptible to freeze than Piedmont sources. Temperatures that produce freeze injury symptoms are lower when pines are acclimated to cold weather than when seedlings have been deacclimated due to several nights of warm nighttime temperatures. Unusually warm temperatures in January of 2004 deacclimated pine seedlings, and this resulted in root injury from a hard freeze (18 °F). Since shoots typically do not show injury (unless they are actively flushing), root injury is often overlooked, and many freeze-injured seedlings died quickly after planting. Since freeze injury symptoms are sometimes difficult to identify, foresters typically offer various reasons (other than a freeze) for the poor seedling performance. This paper reviews some data on freeze events that have occurred over the past century.