On the East Coast of the US there is no need to fertilize from about late Nov. to late Feb, if your trees are outside under cover. If the soil is frozen, as is probably the case with most trees under mulch with minimal protection, the tree is incapable of using it. The warmer the conditions in the winter the shorter the period of no fertilizer. It's a sliding scale and depends where you live.
Also, you have to be mindful even when you begin fertilizing in the spring. I've had bonsai nursery people tell me that too much fertilizer in early spring can be counterproductive, since the plants can't really use Nitrogen efficiently without fully formed leaves...