Fraxinus excelsior - flowering biology of the Common Ash

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Flowering biology of Fraxinus excelsior L.
- the Common Ash, or European Ash

Fraxinus excelsior is an anemophilous (wind-pollinated) tree, distributed throughout Europe and Asia Minor, and it's the only species of ash in Sweden. It is deciduous and begins to flower before leafing, in Sweden around the end of April to beginning of May. The buds are black and the leaves emerge from the terminal buds after flowering around the end of May. Hermaphroditic flowers consist of one pistil and two purple stamens attached to the base of the ovary. There is no calyx or corolla. The flowers occur in many-flowered panicles (100-400 flowers) that emerge from lateral floral buds on shoots of previous year. The species is polygamous, which means that the sex expression of individual trees shows a continuum of gender, i.e. they have either staminate flowers, pistillate flowers, both unisexual flower types, only hermaphroditic flowers, or a combination of uni- and bisexual flowers on different branches (see photos below). There may even be different flower types within an inflorescence. Often the top flower in an otherwise male inflorescence is hermaphroditic. Staminate inflorescences contain more flowers than pistillate ones.

The trees seldom become older than 250 years. It starts flowering at an age of about 30 years and at around 100 years of age growth in height ceases. It can reach a height of up to 30 m and a girth of over 6 m. The tallest living ash tree in Sweden is 30 m (girth 4.72 m) and the biggest has a girth of 7.30 m (height 22 m).

The common ash has been studied as part of my PhD research and the results are included in my PhD thesis. Further studies have been conducted by the EU-funded research project FRAXIGEN.