Chilling Hours: Myths and Facts

This paper is a critical review of over four decades of research on chilling with southern pine seedlings. For most pines, freeze tolerance, seed dormancy, and endodormancy of terminal buds are affected by natural chilling (0° to 8 °C [32 to 46 °F]). Unfortunately, in the field of reforestation, several myths have emerged regarding the importance of chilling. One myth is that chilling seedlings in a dark cooler will increase freeze tolerance of southern pine seedlings and another myth states that chilling must occur before pine seedlings can be successfully “hot-planted.” I once believed in the common myth that bud ecodormancy (a.k.a. quiescence) is directly responsible for acceptable storage potential of pines. However, a true “cause and effect” relationship does not exist. Several independent studies have shown that pine seedlings can survive four weeks of storage (2 °C [36 °F]) without a “well-formed” terminal bud and without any natural chilling. A critical analysis reveals that most chilling studies have confounded planting date, chilling, freeze tolerance, rainfall amounts, and photoperiod. Conclusions from these “confounded” studies were used to spread the ecodormancy=storage myth. To dispel this myth may prove difficult, since it requires establishment of studies that do not involve confounding factors. This paper also discusses a new theory about why root growth potential (RGP) of cooler-stored seedlings sometimes drops quickly when bareroot seedlings are lifted in the fall. The theory suggests low RGP occurs when certain fungi (e.g. Pythium) grow on succulent, wounded roots in a cool, dark, damp environment. Knowledge about early storage of pine seedlings would increase if research is directed at explaining why RGP can drop quickly after only 1 week of cooler storage. Data are not required to keep a myth alive; it only requires unquestioning faith in a theory.

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Author(s): David B. South

Event: Joint Meeting of the Southern Forest Nursery Association and Northeastern Forest and Conservation Nursery Association
2012 - Chattanooga, Tennessee