Sea breezes: Anna Pavord celebrates the summer with memories of favourite seaside gardens

The sea was a long way from my childhood. It was another country, one we went to once a year for the whole of the summer holidays. For 12 years it was always the same place, always the same way of getting there: a series of blunt-nosed buses that smelt of oil and damp plush. You get the same sort of smell in small- town cinemas on a rainy Saturday night.

The journey took a whole day. The first leg took us from Abergavenny to Brecon, the second (the longest) on to Carmarthen, where there was a tricky manoeuvre with luggage across the town to catch the third bus to Haverfordwest.

Bus number four did the final haul to our destination at Little Haven in Pembrokeshire, through stone-banked lanes of scabious and yellow toadflax. There was a particular corner on that route where you got your first glimpse of the sea, glinting at the end of a dip between two cornfields. Each year, at this point, you felt your head was going to shoot off the top of your body with the excitement. You could not shout, or jump up and down. You just sat there, locked on to the sea, with your head about to explode.

Later, when we had our own children, we made the same journey every summer to the same place, but by car, not bus. We could slam the doors on the luggage and the dog and be in Little Haven in a quarter of the time. Undeniably, it was easier and I did not miss staggering through Carmarthen with an overpacked suitcase about to pop its locks, nor worrying about the bus connection. We always made it, when I was a child, but the worry never went away.

The car though is a banal means of transport. The ease of it reduces the importance of the journey. I missed the ritual of the series of lumbering buses, the sense of pilgrimage to that miracle of the sea beyond the fields. Speed, too, diminishes the strangeness of a journey. Travelling fast, especially on aeroplanes, there is not enough time to clear away all the mental baggage you have brought with you from the ordinary and make a space in your mind for the extraordinary.

Aeroplanes came into my life immediately after the Pembrokeshire period, when all I wanted from a holiday was a good suntan. The means was not as important as the end. But flying, detached from the physical world, wrapped in cotton-wool cloud, you have no sense of progressing towards a goal. There is only the start and the finish of the journey: in between is a void.

Sometimes you have no choice in how you travel. You would not have much left of a two-week holiday in the United States unless you flew there. But the problem remains. How do you re-invent a sense of having arrived, slow down the approach so that your destination, the object of your dreams, that sea glittering in the distance, advances imperceptibly until suddenly, wham, it fills your whole mind?

Boats do it beautifully. It was what made Tresco in the Scilly Islands so special before it got a helicopter landing pad. The helicopter used to go only as far as the biggest island, St Mary's. There you made your way down to the quay where one of the Bryher boatmen would be waiting in an open wooden boat to take you over the last stretch of water to Tresco's silvery landing stage.

Pure masochism, some might think. I think not. The point of an island is the water around it. Arriving by sea, you are more aware of the influences that have shaped the place, and accept, even welcome, its limitations. You can still ignore the Tresco helicopter if you want and arrange your own sea transfer, but the island has been subtly changed by its more direct link with the mainland. It has become slightly less different.

I went there first in September, a long time ago, when the dunes were thick with agapanthus. Ever since, agapanthus have remained seaside plants for me. I vaguely knew, way before I started to grow them myself, that they were from South Africa, but here, on Tresco, they had naturalised in a way I'd never seen before. Much later, I learned that originally they had been bought in to extend the growing season for the flower farmers of the island, whose income came mainly from spring flowers. But the huge heads of the cut agapanthus made them costly to transport. Not enough of them would fit into the boxes that the Scillonian transported over to the mainland. They took up too much room on the boat. So they were chucked out on to the dunes, which happened to be a habitat that suited them very well.

No garden is more dependent on the sea than the one that Augustus Smith started to make around the ruins of Tresco Abbey from 1838 onwards. Located 28 miles off Land's End, Tresco is surrounded by the Gulf Stream which gives it (usually) an unique microclimate. The sea moderates the temperature so that (usually) there is no more than 9C seasonal difference between winter and summer. Frosts are (usually) rare and rainfall remains relatively steady at an average 75cm a year. These "usuallys" are there because, over the past 20 years, Tresco has suffered some shocking frosts and even more terrible gales.

The gales, salt-laden of course, were a problem even when Augustus Smith started work on his new garden. He did not live long enough to see his great wind breaking belts of macrocarpa grow to maturity. But the site he chose for the garden sloped to the south and, carved into terraces, provided the kind of planting conditions found nowhere else in Britain. In the shallow soil at the top of the slope are South African proteas and silver-leaved leucadendron. At the bottom, where the conditions are shadier and damper, you'll find vast woodwardia ferns and Norfolk Island pine. Everywhere are mounds of Geranium maderense and sea-green Euphorbia mellifera with leaning spires of self-seeding echium in between; spilling over the rocks are sheets of fleshy mesembryanthemum. Palms flourish as the wind is filtered through a series of unusually high-clipped hedges.

Sea traders brought the first outlandish plants to Tresco: aeoniums, agaves, banksias and bizarre cacti. The sea is now their protector, ringing the island with a defence even more potent than Augustus Smith's windbreaks. Nowhere else in Britain is quite as fiercely strange, magnificent and exotic as this southern hemisphere garden by the northern hemisphere sea.

The gardens at Tresco Abbey are open every day from 10am to 4pm. For more information, see tresco.co.uk