At this time of year, it is impossible to think of the place as a single country. A plane may whisk you in a few hours from San Francisco to Wisconsin, but it is more difficult to set your mind at the same speed. In the morning, I would be admiring the bizarre sight of Magnolia campbellii spreading its vast pink blooms over a collection of huge cacti. In the afternoon, I would be standing in a blizzard with the ground under my feet frozen 2ft deep.
British gardeners tend to introduce themselves by means of their soil. "How do you do? I'm on clay." "Oh poor you! I'm slightly acid sand." Americans talk zones. "Hi! Chuck's the name. Zone Five. Six, if I'm lucky." But you soon get the hang of it. Britain is so tiny that almost the whole of it sits in what American gardeners recognise as Zone 8, with winter temperatures rarely below 10-20F (-12 to -7C). Only the chilliest bits of Scotland qualify for Zone 7.
Being so small (and so obsessed with gardens), we are also used to having garden guides that tell us where to visit, whether in Cornwall or Caithness. You need to work harder to get similar information in the States. But if you're in the Pacific north west, look for Gardeners on the Go: Seattle by Stephanie Feeney. You'll find plenty of ideas for tours around Puget Sound, taking in nurseries, places to eat and good bookshops as well as gardens.
We had a fabulous drive up the coast north of Seattle with the snow-covered Cascade mountains glittering almost all the way to the Skagit Valley. The Skagit is the greatest tulip-growing area in the States, so I was drawn there as irresistibly as a surfer to California. I did not expect, though, to find descendants of two great Dutch tulip growers, the Roozens and the Lefebers, to be here, still growing.
I was too early for the Tulip Festival (2-18 April), but skulked instead in Scotts Bookstore in Mt Vernon, an undisturbed town with good turn-of- the-century buildings. I felt as though I had wandered on to the film set of a dislocated Western and half-expected Gary Cooper to lope through the doors of the store. No such luck, but the compensation was a wonderful sweetcorn chowder in the bookstore's cafe.
If you are in Mt Vernon in April, make for the Lefeber Bulb Company's 25-acre bulb field and its little Museum of Tulip History. Tulip- growing started in this area in 1906, when Mary Brown Stewart planted the first commercial bulb crop. Last year more than 40 million bulbs left the valley, many of them sold to Holland. Go to the Roozengaarde, too, to wander in the immaculately maintained display garden.
Less familiar to us - except under glass - are cactus gardens. I was staggered by the beauty and strangeness of the cactus garden at the Huntington in San Marino, California. I knew there was a great library there, but had not realised there was also a vast garden (and a house stuffed with treasures).
The cactus garden is only one of the many different gardens surrounding the library, and the people there were keenest for me to see the Japanese garden. Cactus gardens are nothing to them and it was difficult to explain to Californians why I found their mass plantings of cacti so spectacular; used in huge drifts and stands, as we might lay out a shrubbery or flower border, they became heroic. And I'd always supposed I didn't much care for cacti.
In Washington, it was still too chilly to garden anywhere but under glass. In the huge conservatory of Brookside Gardens on the outskirts, I eyed up orchids that seemed quite ludicrously lavish compared with the pinched, cold landscape outside. The begonias here were superb, too. Plants that are growing happily also seem to transmit those feelings to us. Conversely, stressed plants make us cross and anxious.
My final treat was the National Gallery of Art in Washington, with the great East Building by IM Pei (he of the Louvre pyramid).
The gallery is stuffed with Cezannes and Monets, but my goal was the From Botany to Bouquets exhibition. Its aim is to reveal the shifting ground of Flemish and Dutch flower paintings of the late 16th and early 17th centuries. At what stage did science turn into art? The drawing of a hellebore in an Iconographica Botanicae of 1500 was made when the naming of names had only just begun. Its purpose was to identify.
There were also masses of painted tulips, the icons of the age, by the best painters. The show is not to be missed. Washington is only six-and-a-half flying hours away. It takes me longer to drive to Yorkshire.
Useful book: Gardeners on the Go: Seattle by Stephanie Feeney (Cedarcroft Press, $15.95).
For information on the Skagit Valley Tulip Festival contact the Festival Office, PO Box 1784, Mt Vernon, WA 98273 (001 360 428 5959), for a free brochure (with a map). The Lefeber Bulb Co is at 14379 State Route 536, Memorial Hwy, Mt Vernon (001 360 424 6234). The Roozengaarde (now part of the massive Washington Bulb Co) is at 15867 Beaver Marsh Rd, Mt Vernon (001 360 424 8531).
The Huntington Library and Garden is at 1151 Oxford Rd, San Marino, CA 91108. Admission free. Open daily (001 626 405 2135).
To get to Brookside Gardens take the Washington Beltway to Exit 31A. Follow Georgia Avenue north for three miles and turn right at Randolph Rd. At the second set of lights take another right on to Glen Alan Avenue. The gardens are three quarters of a mile. Open daily. Admission free.
The National Gallery of Art is on Constitution Avenue between 3rd and 7th St, Washington NW. Closest subway station: Archives-Navy Memorial. Open Mon-Sat (10am-5pm) Sun (11am-6pm). Admission free. From Botany to Bouquets is until 31 May (001 202 737 4215).