Dormancy and chill units

I came across a document from the Ottawa Bonsai Society prepared by Steve Urlich (who is a Mirai member) that points to the the concept of dormancy and chill units or CU for short. It basically quantifies the contribution to total dormancy of the number of hours at different temperatures that a tree experiences:

  • 1 hour below 1.4°C (34.5F)= 0 chill units
  • 1 hour 1.4-2.4°C (36.3F)= 0.5 chill units
  • 1 hour 2.4- 9.1°C (48.4F)= 1 chill unit
  • 1 hour 9.1- 12.4°C (54.3F)= 0.5 chill units
  • 1 hour 12.4-15.9°C (60.6F)= 0 chill units
  • 1 hour 15.9- 18.0°C (64.4F)= - 0.5 chill units
  • 1 hour above 18.0°C = -1 chill units

Interestingly, high alpine trees (or any living organism in Canada) endure much longer and colder winters but the coldest parts of winter don’t contribute to dormancy according to the scale above. In that sense, a tree with longer and colder winters have perhaps the same total contributing number of hours (in late fall/early winter + spring) than a tree in milder winter conditions, just over a longer period of the year.

The only data (in that link above) that I could find about the number of units required by any species is for apple trees (Malus sp.) requiring between 1000 and 1200 CU. I would really like to find this time of quantified information about the minimum requirements of different trees. In any case, I think that it is safe to assume that the requirements of an apple tree could be used as a minimum requirements for all trees. Even for high alpine trees based on the non-contribution of hours below 1.4C.

In my case my cold greenhouse has been between 2.4C and 7C for the most part of Dec-Feb, or 90 days. This gives 2160 hours or CU units in the temperature range above. For the past week or so my temperature has been between 10C and 15C and trees started to wake-up.

I hope this discussion is of help and if anyone can find a resource that quantifies the requirements of different species it would be great.


I have looked into this concept with no luck in order to go beyond hardiness zone data to determine a species ability to succeed in my area with short, mild winters. But when I asked on bnut I wasn’t able to ascertain any usable info. Internet searches led me to very similar info as to what you found. It appears research on this subject has only been performed in regards to fruiting trees as the economic impact can be significant for farmers.

I do suspect that the length of the days - hours of light- affect the plants dormancy cycles too. It cannot be just 1 parametre.

Dormancy and bud break are two separate things. Essentially once the minimal dormancy requirements are met the tree may or may not exit dormancy depending on external factors being favourable, predominantly temperature and daylight hours, it is called ecodormancy or apparent dormancy.

1 Like

“The Chill Unit” that’s what I call my living room.