Hazel grows across much of Europe, parts of north Africa and western Asia. In the UK it's often found in the understorey (lower, shrubby layer of the woodland) of lowland oak, ash or birch woodland, and in scrub and hedgerows.
Although Hazel (Corylus avellana) can grow up to seven metres tall, it is usually seen as a shrub with lots of stems. This is because most hazel trees are coppiced. Coppicing is a way of keeping a supply of wood without killing the tree! To begin with, the tree is allowed to grow and is then cut down. The next year, lots of new shoots grow from the stump. Over time the shoots grow into many new ‘trunks’. When they are the right size, they can be cut down and used as firewood, ‘wattle and daub’ building, thatching spars, hurdle fences and bean poles. The next year, new shoots will sprout once more from the stumps and the whole process starts all over again!
Hazel is wind pollinated and has separate male and female flowers on the same tree but it cannot pollinate itself. The male flowers are the yellow catkins, known as ‘lamb’s tails’, which appear this month. The female flowers appear on the same branches as the catkins but they are quite hard to spot! They look like a bud with a small red/pink tuft on the top, and it is these that develop into hazel nuts after fertilisation Although pollination happens straightaway, fertilisation is delayed until 4 or 5 months later! Birds and animals help disperse (spread) the nuts by burying them to store for winter and then forgetting where some were, allowing the nuts to grow into new trees.
For more info see https://www.woodlandtrust.org.uk/trees-woods-and-wildlife/british-trees/a-z-of-british-trees/hazel/