It's far too early to see leaves yet although many woodland trees, such as hazel, are showing young catkins already. Wait until spring and they'll appear, if they grew now they would be killed by the cold.
Hya felix
Not really familiar with the plant, but i would imagine the buds appear quite early in the spring. As you may or may not know i am a complete novice with butterflies, but aren't Brimstones an early emerging species? Sorry i can't help more.