One of the most spectacular gardens I saw last summer was a completely unintentional one. Along the DART line at Killiney Bay, Co Dublin, the granite cliff-face was a rich, radiant brocade of self-sown wall valerian and silver ragwort. The valerian (Centranthus ruber) sang out in crisp sugar-cane tones: coolwhite, pink, red and deep, expensive crimson. The ragwort (Senecio cineraria) added two more layers of light with its shimmering downy leaves and glistening gold flower-heads.
Both types of plants are Mediterranean species that have escaped from gardens. Now naturalised here, they are as happy as Larry and in total harmony with their surroundings. This particular patch of Senecio - I was delighted to learn from a recently published Flora of County Dublin - originated from a single packet of seed sown around 1875 by Sir Francis Brady in his garden at Sorrento Cottage.
Meanwhile, in Howth, on the south-facing slopes over Dublin Bay, gardeners have been responsible for a thriving, settled population of plant immigrants: red hot poker (Kniphofia) and Hottentot fig (Carpobrotus edulis) from South Africa, Libertia formosa from Chile, Echium pininana from the Canary Islands and snow-in-summer (Cerastium tomentosum) from Italy. And on the site of the former dump at Ringsend, more introduced species flourish, including that architectural pair of tall, prickly biennials: teasel and cotton thistle.
All of the above plants are a pleasing addition to our landscape. Even the mightily-prolific valerian, which escaped to the wild around the beginning of the century and returned to invade many dry, Dublin gardens - including my own - is welcome. Unwanted progeny are easily weeded out at the seedling stage, with just a few being left behind to brighten awkward crevices.
But other naturalised aliens have become a major annoyance for tidy gardeners. Take Veronica filiformis, the little blue-flowered speedwell: originally from Turkey and the Caucasus, it arrived here as a rockery plant. Now it is the bane of lawn-proud people. And other imported plants are marching relentlessly across our soil, like the hideously invasive Japanese knotweed (Fallopia japonica) which was prized by Victorian gardeners for its heart-shaped leaves and frothy blossom. Not only do clumps of it creep massively out of control, but it regenerates from tiny bits of root - often in garden waste tipped over walls, onto railway cuttings or into dumps.
And then there are the genuinely dangerous foreign invaders, like the beautiful but noxious giant hogweed (Heracleum mantegazzianum) whose juices can cause painful skin lesions, especially among children who make blow-pipes and telescopes from its hollow stems. (It's worth mentioning here that two other plants, wild parsnip (Pastinaca sativa) and common hogweed (Heracleum sphondylium) contain the same irritant chemicals - but are less attractive to children)
But whatever the status of these non-native residents - welcome, irritating or threatening - they are all part of the late 20th-century picture of the wild plants of Co Dublin. And it's a surprisingly colourful picture composed of 1,300 species and hybrids - both native and introduced - that live in this metropolitan county, a county that has habitats as diverse as coastline, mountains, heathland, woodland, grasslands, wetlands, suburbia and the increasingly built-upon city.
Over the past decade a dedicated band of volunteer plantspotters from the Dublin Naturalists' Field Club (DNFC) has been tracking and plotting all of these plants. The results, published in the DNFC's new Flora of County Dublin, are essential reading for every Irish botanist and for gardeners curious about the plants beyond the garden gate. The last complete flora of the county was compiled in 1904 by Nathaniel Colgan.
The book, I must clarify, is not a plant guide, and does not carry identifying pictures or descriptions. (For that purpose you need something such as the Collins Pocket Guide, Wild Flowers of Britain and Northern Europe, or the excellent, but out-of-print The Concise British Flora in Colour by W. Keble Martin.) Instead, the Flora of County Dublin comprises 558 pages of lean, mean - and accurate - information.
The first part deals with topography, climate, geology, soil, the history of the study of the flora and includes accounts of the eight botanical districts of the county. The second part is composed of nearly 350 pages of systematic data on the 1,300 species and hybrids: historical records, frequency of occurrence, habitats and sites. Six Chinese ink drawings by botanical artist, Wendy Walsh are delicately delightful, while 16 photographs of different Dublin habitats are proof of the biodiversity on our doorstep.
It's true, the Flora of County Dublin may look like a dry volume of facts and figures to some people, but to me - a shameless botanical trainspotter - it paints a rich and many-layered portrait of my county. I love, for instance, to think of maidenhair fern slowly colonising the traffic-bound walls of Ranelagh - from Sallymount Avenue to Appian Way. And I feel a warm optimism on learning that the bog orchid (Hammarbya paludosa) is still to be found at Bohernabreena, where it was first discovered by John Templeton, an early plant enthusiast, in 1814.
Flora of County Dublin by the Dublin Naturalists' Field Club costs £25. It can be ordered from the Dublin Naturalists' Field Club, c/o 35 Nutley Park, Dublin 4. Tel: 01-2697469. Postage and packing is extra.
Diary date: South County Dublin Horticultural Society hosts an illustrated lecture, Container Gardening, by Linda Murphy at 8 p.m., next Wednesday, Royal Marine Hotel, Dun Laoghaire. Members free, visitors £2.