I recently bought a Black Pine at a local Bonsai tree auction. I live in zone 5 and was wondering if they need a time in light frost for their dormancy requirements.
Many trees including black pines need a certain number of chilling hours (i.e. dormancy). Chillings hours are temperatures between 0 - 7 C (32F - 44F). You can look up the chilling hours needed for various species including black pines but most trees need about 40 days worth of chilling hours. So, yes the black pine will need to experience some cooler temperatures but given that you are located in zone 5 you will need to protect the tree during the winter (perhaps in an unheated garage that doesn’t go below 32F) as temperatures will get quite a bit colder and could easily kill the tree without protection. Hope this helps.
Thanks for that information. What if the tree is buried in soil to the pot line, covered with mulch to the first branch, and in a cold frame. Is it the foliage or roots that need protection or both.
Roots need the most protection. Please see the thread on Impact of Freezing Temps on Roots. Hagedorn lists the root zone kill temp as 10F (-12C) and the top hardiness for fully in ground tress as -20F (-29C). It sounds like your set up will work just fine. I keep mine in a greenhouse at just above freezing after losing a bunch in an unheated hoop house sitting in the ground.
Much appreciated. Finally I got something right??