Nature turns certainty on its head

WHAT makes deciduous trees and other perennial plants start to grow in the spring?

I used to think that I knew the answer but now I’m beginning to have doubts.

It’s fairly well known that the main factor that triggers grass into growth is soil temperature. In the spring rising soil temperature lags behind rising air temperature, often by a matter of weeks. But when the top few centimetres of soil reach 6 degrees the grass starts to grow. This can even happen in January if we get a prolonged spell of mild weather. But if the weather gets cold again the grass will stop growing.