Anna Pavord: Late-summer rain prompts the arrival of some magical mushrooms

Four field mushrooms erupted suddenly on the lawn in front of the wooden hut where I work. The suddenness is characteristic of mushrooms. It's as though some creature that usually lies quiet underground suddenly takes to blowing soft white bubble gum. Fortunately I was able to watch them closely through the window by my desk. You have to. If you pick them too soon, the flavour is not sufficiently developed. If you leave it too late, some ignorant, flat-footed pheasant will have trampled them. Or tiny white grubs will have worked their way up through the stem, where they don't much matter, into the cap, where they do.

My mother was oblivious to the grubs. "Shut your eyes," she commanded. "If you can't see them, you won't mind them. They don't taste of anything." Which was true. But still… She adored field mushrooms and at this time of year trawled carefully through certain old pastures around our house, where, most seasons, mushrooms came up. Fields where cattle grazed were best, she said. Perhaps that was a bowdlerised version of an old belief that mushrooms were roused, stimulated into growth by the presence of male animals: stallions, bulls, rams. It was the semen that did it. Who knows? That may be why they are more difficult to find now. Too much AI.

Field mushrooms are fragile things. They don't have solidly anchored roots as plants do. You have to pick them carefully, sliding your hand, palm upwards, underneath the cap, with the soft stem between your fingers. Then gently you ease the creature from the ground; it comes away with no resistance. Afterwards, it's best to lay it cap upwards in your basket, so the delicate gills don't get crushed. The soft pink underside of a freshly picked mushroom quickly changes colour if it is bruised. It changes anyway, as it ages.

"Mushroom" to me means just this one mushroom, Agaricus campestris. But Roger Phillips lists 27 different Agaricus species in his superb field guide (Macmillan £18.99). Four of them are poisonous, but the bad ones all have giveaways. One has a yellow-streaked cap, another has a spindly stem, a third grows in woods, not pasture, a fourth has a horrid smell.

Smell is one of the ways to identify a field mushroom – earthy, damp, enticing. They peel more easily than other kinds (though you don't need to peel them before you cook them). And then there are the pink-beige gills, pleated more beautifully than a Fortuny silk. "Decked with fine gutters," wrote Gerard in his famous Herball, or Generall Historie of Plantes (1597).

The lawn, so-called, where the mushrooms came up is no more than a bit of flat land that we mowed until it looked flattish and greenish. But this is the first time that it has produced mushrooms. Conditions were ideal, a period of hot weather followed by quite heavy rain, which is exactly what these late summer, early autumn-season fungi need to prompt them into growth. But where have they come from? It's a mystery.

I gathered the mushrooms early in the morning and fried them for breakfast. What I really needed was bacon fat, but it's difficult now to find bacon sold as it should be, complete with its rind and a solid band of fat. So I used farm butter instead. I won't go on about the taste. Pseuds' corner looms. But it did remind me what a wasteland stretches between the real thing and the cultivated mushrooms we are generally offered – all texture and no taste. You get the same sharp jolt when you eat peas you've grown yourself (sadly, that season has just finished for us) and wonder how you could ever have been conned by Clarence Birdseye into believing his frozen versions were as good as your fresh ones.

As their name suggests, you are most likely to find field mushrooms growing in pasture. And yet the biggest crop I ever saw grew in a field of maize, where the land had been ploughed and the seed drilled in May. The farmer was a friend of ours and bought us a basket full of these mushrooms, a few weeks before the maize crop was harvested. He had never noticed them there before and they've never come up since.

Phillips marks the eating quality of field mushrooms as "excellent", a word he also uses in describing the parasol mushroom. These are much bigger creatures, with caps up to 25cm across. They grow in huge quantities in one of our fields, so prominent that from the terrace in front of the house, I can see them on the other side of the valley. But excellent? About as tasty as the sole of a washed up flip-flop, I'd say. There are evidently some taste buds that Roger Phillips and I do not share.