Its a willow, but what kind is hard to say

Down by the sally garden: the willow in spring. Illustration: Michael Viney

A blaze of golden yellow halfway down the garden proclaims an invader to our small estate, where a self-sown goat willow, Salix caprea , offers glorious catkins to the spring.

Others of its kind light up my morning march, leaning from the banks of streams beyond the snowy sparkle of whitethorn. These are all male willow catkins, despite the feminine flourish of the pollen: a twig or two, bracing narcissi in a vase, spread the table with gold dust.

The new trespasser more than earns its niche. But the acre we came to had a willow of its own, couched in the hollow where the hill stream cuts through beside the house.

The tree is now quite huge, leaning big, mossy elbows on the opposite bank of the stream. In summer its canopy blots out the mountain from our windows; in winter it cradles the summit and the rise of the moon.

Being so big, it is probably some class of white willow, Salix alba . I am allowed to be that vague, as the hybridisation of willows in Ireland is botanically remarkable. The old Webbs Irish Flora , everyday bible of Irish botanists, detailed 15 species, most of them native to the island but some half-dozen introduced.

Triple hybrids The new edition, updated in 2012, adds no more species but has plenty to say about how difficult it has become to tell one willow from another. John Parnell and Tom Curtis write that 18 hybrids have now been recorded in Ireland, with probably more to come. Even triple hybrids are known, but it is even more difficult to determine their parentage.

Our big white willow should have leaves made silvery green by a coating of silky white hairs. I remember a breathtaking tree of such pure heritage beside a river in Leitrim, a county made especially beautiful by its riparian willows in spring. Our trees leaves are bald and dull, but, hybrid or no, it has every ambition to spread its genes. Its winged seeds will float in a feathery blizzard for days and then spring up from every outdoor flower pot or root stubbornly into crevices in paths or walls.

The Salicaceae evolved for the cool moist soils of the northern hemisphere, into the High Arctic, where I walked on its catkins in a long-past July.

In Ireland it was one of the first plants to colonise the postglacial tundra; it survives in prostrate form now on coastal mountainsides and in the hollows of dunes.

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Its a willow, but what kind is hard to say