Americans, by then, were already crunching their way through the crisp, white inner leaves of the iceberg lettuce and its associated varieties. They liked them because they were sweet and juicy, could be kept for a long time in the fridge, and provided a suitable base for the choice of five salad dressings that was then obligatory in any restaurant worth its salt.
Slowly the fashion for crispness caught on here, and these days close to 70 per cent of the lettuces we grow and eat are of the iceberg type. But no sooner had we caught up than the Americans switched tack once again. Go to any New York greengrocer or restaurant today and you will find it extremely difficult to locate the iceberg amid the large choice of multi-coloured limp leaves, in their combinations of green, red, pink and purple. They go under mysterious names such as arugula (rocket), mache (lamb's lettuce or corn salad), radicchio, batavia (escarole), Lollo Rossa, frisee (endive) and many others. Although nothing yet threatens the supremacy of iceberg here, the first transatlantic breezes are beginning to stir. 'People are always looking for the unusual,' says Colin Randel, vegetable specialist for the seed merchants Dobies and Suttons. 'They see new things in the supermarkets and they want to grow them. They like the red ones for the edge of their potager, where they grow vegetables and flowers together.'
With lettuce more than any other vegetable, we have to define our terms. The lettuce pages of a seed catalogue typically present us with a bewildering choice of as many as 20 varieties, categorised under four headings: butterhead, crisphead, cos and loose leaf. These categories are sub-divided into green and red and large and small varieties. The crispheads cover two distinct sorts: the long- established Webb's Wonderful and the newer iceberg types from America, including Malika. Moreover, some varieties are bred to straddle the divides - for instance Avondefiance, a butterhead leaning towards the crisp.
For the faint-hearted (let alone the butter-headed), the initial temptation is to abandon the struggle and grow swedes instead. Yet for the salad enthusiast, it is worth persevering and sampling from across the range. Unlike, say, Brussels sprouts, there are genuine and marked differences in taste and texture between the varieties.
The iceberg types owe their popularity to supermarkets, yet there are advantages to growing your own. Although they keep better than butterheads, they lose sweetness with time and are at their very best picked fresh.
Gardeners growing them for the first time may be surprised to discover that instead of coming up as a tight-packed head of pale green and white leaves, as packaged in the supermarket, they are the shape and colour of regular lettuces, with outer leaves flopping about. This is because retailers take off the outer leaves, to give the lettuce a crisper look on their shelves.
Iceberg is the name of the original variety, but nowadays it seldom appears as such in the catalogues. The most common variant used to be Saladin, from California's Salinas Valley, which boasts the best lettuce-growing conditions in the world. Saladin has two disadvantages for British growers: it is susceptible to several viruses, especially downy mildew, of different strains from those it encounters in California; and because we have less sunlight it seldom develops its full sugar potential.
Scientists at Horticulture Research International (HRI) at Wellesbourne, near Warwick, are working to develop a variety that will resist downy mildew. Marshalls has already introduced Baltic, a variety that is better adapted to the British climate.
The fashion for red lettuces, until now mainly restricted to the loose leaf, such as Continuity, and cos varieties, has spread to iceberg types with the appearance of Sioux in Dobies' catalogue this year. Red heading lettuces (unlike red cabbage) are red only on the outer leaves: closer to the heart they are green and white. One of the most attractive is the French butterhead Marvel of Four Seasons, available from Johnsons as Chaperon.
Reddening is caused by sunlight and stress. If, while they are maturing, you give them only the amount of water they need to survive, they will take on a deeper colour. But be careful, because if you are too mean with water they will suffer from tip burn - a brown slime on the outer leaves. Butterheads are more prone to this than icebergs.
Popular red varieties include radicchio (not technically a lettuce, but a kind of chicory) and the loose-leaved Lollo Rossa and Red Salad Bowl. The last two have their green counterparts - Lolla Bionda and Salad Bowl. They are not generally pulled as single heads, but the leaves are picked individually and the plants should continue cropping through the summer.
Other small-leaved greens picked this way are corn salad and the more piquant rocket. These are not bona fide members of the lettuce family either, but hardy annuals with a long growing season. Corn salad is prone to mildew.
The cos varieties, sometimes known as London lettuces, are a little crisper than butterheads with tall, straight leaves. They have been around for decades, but were until recently a minority enthusiasm. Again the supermarkets have influenced growing habits with their recent discovery of the old variety Little Gem, a small cos with a reputation as one of the sweetest available.
Although easy to grow on any decently prepared soil treated with a general fertiliser, such as Growmore, lettuces are sensitive to temperature and sunlight. The range within which they do well is 13-25 degrees centigrade, which means that in the south of England they will crop from early June to late October, while the season is a good four weeks shorter in the north. Under ideal conditions it should take about nine weeks from sowing the seed to produce a mature head.
As for light, David Gray, Head of Annual Crops at HRI, explains: 'We don't always have enough light in England for them to develop a good heart, so we have to breed and select varieties that will do well in a dull climate.'
Success with lettuce gets more elusive as the summer goes on. Some gardeners, sowing seeds in July for a late crop, find they don't come up for weeks. This is because most lettuce seeds will only germinate at temperatures below 20 degrees centigrade. If it is too hot, they will stay dormant until the weather cools, then start to grow - but by that time it may be too late for the plants to mature before the end of autumn. Iceberg types will germinate at a higher temperature - up to 28 degrees - but need cooler weather to mature. Too hot and they get puffy and bolt.
Once you get the temperature right, lettuces are versatile and rugged enough to be grown in a variety of ways. You can start them indoors or on a seed bed, then plant them out, but you usually get better results by the more wasteful method of sowing a row and thinning out as they get bigger. (The thinnings make succulent early salads.)
They can be grown in containers or growing bags on a patio, but preferably not in individual pots, because they like room to spread their roots. Ideally they should be sown in small quantities every fortnight between March and June, for succession. But in practice few of us are well enough organised for that: so we get a glut and share with our friends, enjoying a short season of very large salads, knowing they are doing us a power of good.
Marshalls (0945 583407)
Dobies (0803 616281)
Johnsons (0205 351430)
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