"20 or 30 years ago, it was slightly fanatical naturalists who came foraging with me," Dr Patrick Harding said, slipping out of his day socks, into his mushrooming socks. "It's only recently that people have wanted to eat them". That's lesson one. Mycologists shroom in the spirit of botanical inquiry, not because they are hungry; but somehow (Harding attributes it to restaurants and TV chefs, modestly not mentioning his own books on the matter) edible mushrooms have exploded (metaphorically; literally, they tend not to explode, being more likely to seep or curl). "For the first time in my life, I'm ahead of the trend."

Sheffield is famous for its magic mushrooms, and people of a certain age will tell you about a bus into the Peaks which people used to take on Friday night. They'd harvest their psychedelia, stay up all night seeing things, then come back in on a Saturday morning with eyes like saucers and dew in their hair. Nobody can remember what bus number it was, which I suppose is a lesson in itself.

"They're very easy to identify; called the liberty cap after the French shape, and they never have a straight stem. Considering the chemicals in them, I'm not surprised." The drugs economy has changed quite a bit, and even with the research possibilities of Google, young people probably no longer go to the Peaks when you can get an E for £3.50.

"Oh!" Harding yelps. "Look over there!" He looks like a baby who's seen a balloon. "This is only found growing with pine trees. Do we have a pine tree? Yes! Right above us." Harding has taught at universities all his adult life. "This has the same colour stem as the cap, which is quite unusual. You know the way cheap cheddar won't crumble, but a Wensleydale will? This crumbles like a good cheese." And that's why they call it the crumble cap. I have already forgotten whether or not you're allowed to eat it.

Previously, my rule with mushrooms was not to eat them if they were a weird colour, but I had reckoned without the Lactarius deliciosus, which has bright orange gills, goes a limey, unnerving green as it ages and weeps bright orange tears when you cut it. It's how you'd imagine the world, post-apocalypse. Apparently, it is also really tasty, with an al dente flavour. "Oh!" He's off. He's found a grey knight, which has a furry texture and a spindly stem. Harding's enthusiasm is so infectious that I almost don't care whether or not you can eat them anymore. But it is edible. If you're ok with eating something that looks like a mouse, hiding.

Before you go out on your own, here are some things you should know: Sheffield is brilliantly served this autumn, because it's been very wet. The fruits of fungi are 85% water, so are sunk by a drought. The whole country is groaning with mushrooms, in fact.

You don't have to go to a national park, you can be in the middle of the city, in a graveyard or some woods, and find specimen as fascinating as something you had hunted all day across wild terrain (maybe hunting is overstating it a bit, considering they don't move). We found a beautiful inkcap, the size of a marrow (a smallish one), up against a grave. "Look at those gills, they're pink, but black underneath. It's like looking up a lady's skirt. Sorry, that's sexist. I shouldn't have said that" (well… I'm prepared to overlook the use of the word "lady", though I would point out that our skirts haven't looked like this from the inside since the 1890s).

You have to use more than your eyes; also smell them, touch them, occasionally taste them, observe their habitat and, if you're really serious about eating them, do a spoor print, by leaving the mushroom on glass for three hours, and checking the colour of the residue.

You do no damage to the fungus when you pick the mushroom – it's just the fruit, it's like picking an apple from a tree – but you shouldn't pick them all because we're not the only ones who eat them. Badgers like them.

Almost everything that's edible has something that looks just like it that is inedible. This won't necessarily kill you – Harding has been out mushrooming hundreds of times, and never even had a stomach ache. But then, other people go out shrooming and then die, or have to have their kidneys replaced.

So the short answer, before you go, is don't go. Or you could go and have a look, marvel at nature's bounty, and eat when you get home.