Loving lichen

Here’s a little quiz for you . . . Which widespread organism is found on almost every garden surface (soil, trees, some evergreen leaves, walls, rocks, timber) and is used by birds, not only to line the inside of their nests but also to camouflage them? The same organism is used as a food source and as camouflage by moths and butterflies, and would once have been eaten by the now-extinct great Irish elk, the giant deer-like creature that featured on the old Irish pound coin. Not only that, the presence of particular species of this organism in a woodland is an important indicator to botanists that the same habitat once formed part of an ancient forest.

Remarkably, this organism also serves as what’s known as a “biomonitor”, allowing scientists to monitor levels of atmospheric pollution in any particular habitat. It’s also used in modern perfumery, in traditional medicines, and as a natural dye (the Romans, for example, used it to dye their togas purple, while the Scottish tweed industry still use it to colour yarns). Any clue as to its name?

It is in fact the humble lichen. That silver, moss-like growth that you often see clinging to the branches of mature garden trees, the one that flower arrangers love so much because of its delicate, filigreed beauty? That's lichen. The dappled, freckled patina that lends so much character to old garden walls, statuary and paving stones, prompting many of Chelsea Flower Show's top garden designers to trawl architectural salvage yards rather than to source freshly cut stone from a quarry? That's also lichen.

And those colourful, crusty, warty growths that you see growing on the trunks of trees, or on the surface of old garden fence posts and wooden garden gates? Lichen again. In fact, roughly 8 per cent of the earth’s terrestrial surface is covered in some species of this organism, which is so diverse that it can be found everywhere from the Arctic to the hottest deserts.

Earlier this month, a plaque was placed in the National Botanic Gardens in Glasnevin to commemorate Matilda Knowles, a remarkable Irish woman, lichenologist, botanist and former curator of the National Herbarium, who spent much of her life studying these strangely beautiful life forms. It's because of Knowles that I'm beginning to properly appreciate the wonderful variety and complex beauty of the many different lichens that grow in my own garden, as well as the fact that they form such an important part of its ecology and its biodiversity.

I can’t, of course, “grow” them in the same way I grow flowers, or shrubs, or trees (confusingly, lichens aren’t single organisms, but a symbiotic/ mutually dependent relationship between a fungus and algae/bacteria). But I can, within reason, encourage them to grow on ugly bare walls and on stone, concrete or terracotta pots, statuary, troughs and balustrades, by painting the surface with a watered-down solution of yoghurt, buttermilk or manure (don’t do this on paving, however, as it could make it dangerously slippy). Lichen-lovers should also bear in mind that lichens favour old trees over young specimens and grow best in gardens with high rainfall and clean air.

But even if we did nothing to encourage them, these decorative organisms would still gradually colonise our garden walls, pots, ornaments and plants, as evidenced by the diversity of lichens that can be found in old or established gardens. One example is Highgrove in Gloucestershire, the eccentric gardens of England's Prince Charles, where lichenologists recently discovered an extremely rare species (Caloplaca demissa) growing on the surface of the giant terracotta urns that decorate the garden's thyme walk, the first time that it had been recorded growing in England.

In another English garden – the National Trust-owned gardens of Cliveden in Buckinghamshire, the former home of the Astor family notoriously associated with the Profumo affair of the 1960s – a similarly rare lichen was recently discovered growing on its ornamental, 17th-century stone balustrade. A famous feature of these historic gardens, the travertine balustrade originally belonged to the Villa Borghese in Rome, and was brought to England in the late 19th century by Lord Astor. Intriguingly, when lichenologists were invited to examine it during recent conservation work, they discovered very old lichen communities including some species considered unique to the Mediterranean, suggesting that the balustrade was already populated in Italian lichens by the time it made the journey to England. A Mediterranean species of snail – rather touchingly nicknamed the "Cliveden snail" – that feeds on these particular species of lichens was also discovered living in the balustrade, presumably inadvertently imported at the same time. Not necessarily an especially welcome visitor, but proof of how intertwined and co-dependent are the lives of the plants and creatures that inhabit our gardens.

Gardens aside, you'll see various lichens growing in parks, on mountainsides, along coastal shorelines, even in graveyards. Some are even specific to regions of Ireland. Which is why, in honour of Matilda Knowles, the next time I visit the Aran Islands, I'll be keeping my eyes peeled for a tiny, limestone-loving one called Verrucaria knowlesiae. See lichens.ie for a detailed profile of Ireland's lichens.

Dates for your diary Kiltrea Bridge Pottery annual autumn sale continues until November 2nd (Mon-Sat, 11am-5pm, Sun 2-5pm), see kiltreapottery.com; November 1st-November 2nd, open days at Finlay Colley's Rare Plants Ireland nursery, Kilmatead, Green Isle Road, Dublin 22 (Exit 2 off the N7, signed Green Isle road,) Tel: 087 2473549 for details Attention all professional and amateur garden photographers; the deadline for entries for the International Garden Photographer of the Year competition is the 31st October, with the winners announced next year (the top prize is £5,000), followed by a touring exhibition of the winning images.

For details of the eight different categories as well as how to enter, see igpoty.com

This week in the garden Now's the time to divide rhubarb plants that may have outgrown their allotted space in the garden. Replant divisions (these should have two or three buds) into a fertile, moist but well-drained soil enriched with manure/ homemade compost, and in a sunny, weed-free spot far away from the competing root systems of trees/shrubs/hedges.

Healthy, virus-free, young plants are also available from most garden centres at this time of year. In particular, look out for “Timperley Early” (a vigorous, thick-stemmed, productive, early variety) and the more compact “Hawke’s Champagne” (classed as a second-early), while there’s also a new autumn cropping variety called “Livingstone”.

It may be late October but that doesn’t mean you have to do without fresh herbs such as sage, parsley, thyme, chives, lemon verbena and coriander.

Instead, prolong the herb-harvesting season by growing these under cover, in a polytunnel, glasshouse, or (at a pinch) in a cool but sunny porch.

Saved lots of seed, but wish you had some of those lovely, acid-free glassine envelopes in which to store them? Then you'll be relieved to know that Westport-based suppliers Seedaholic supply them, at €5.40 for a packet of 50. See seedaholic.com