Definitely 15ºC degrees is Not enough, I doubt if you keep it in the fridge for 4 months(4-5 ºC) would be enough.

Yes, you need temperatures in the 2-4C range for at least 1000 hours to effect a proper dormancy. The tree will live without but it will effectively curtail its lifespan to only a few years (maybe 3) if not allowed this necessary period of chill.

These don't like desert conditions in general including dry air and hard water.

Have any of you all ever tricked one into dormancy? Wonder what it would take.

For about 20 years of my life I could have probably snuck a pot or two into a restaurant deep freeze or or walkin fridge. The deep freeze really would stretch the limits of cold hardiness of Acer palmatum, not to mention the need to acclimate the tree. The walkin fridge seemed humid enough and I have to admit our average temps so far this winter seem to be averaging from right about freezing for lows to highs in the forties (F) with only a week or so of really cold weather mixed in.

The primary driver moving trees toward dormancy is day length (actually it's an increase in the duration of the dark period, but that's a post for another time), with chill sealing the deal. Ideally, you would keep your tree in a temp range above -4C and below 5.5C (25-42* F). Once in deep dormancy, the tree needs a given number of 'chill units' to release it from dormancy. Once dormancy has ended, the tree moves into a quiescent state until temps above 5.5*C or 42* F stimulate growth.

Al