First leaf fully open, and recognisably the shape, if not the full size of the adult leaf
- For compound leaves like ash the leaf must be erect but the pinnate leaflets don’t yet need to be
- For larch first leaf is the start of needle forming. At the base the needles are still joined in the bud cover but the tops of the needles have separated.
- Use mature trees (30 years old or more), as young trees show different responses
- Even trees of the same species, close to each other can behave very differently
- If in doubt, wait until first leaf happens on 3 trees of the same species
First bud or first leaf - why is it important?