Different plants have different triggers as to when they break dormancy and start spring growth and bloom. For some it is the length of day. That is the most reliable to give a reproducible date of bloom. For others it is a cold spell which puts the plant into dormancy followed by a warm spell that will bring it out of dormancy and start it to bloom. Woody plants use a combination of the two.

Most rhododendrons are not tricked too easily into early bloom, but it is the norm with most crocuses. There is nothing you can do about it. One word of caution, pruning trees in winter can decrease the amount of shade and trick plants under the pruned trees into early bloom. Our October snow storm pruned our trees for us and this has caused some plants to come out of dormancy early this winter.