Humidity, temperature critical in growing mushrooms.

A few years back, a colleague of mine bought a box filled with compost that had been inoculated with mushroom spawn. As a result of that purchase she says she enjoyed some of the most delectable flavor-filled mushrooms she's ever tasted anywhere ''and I eat mushrooms.'' she says with emphasis.

But, they also were the most expensive she had ever consumed.

For the $6 she paid out for the grow-your-own mushroom kit (at a local supermarket, incidentally), she harvested 10 mushrooms. That works out to 60 cents apiece. Even gourmet morrels flown all the way from Switzerland don't cost much more.

On the other hand, she doesn't blame the mushroom kit entirely. As she understands it the growing conditions in her apartment ''left something to be desired.'' Did they ever!

At the time she had a landlord who put the financial needs of his tenants first, or so he said. The landlord seems to have reasoned that the best way to keep rent increases to a minimum was to stop burning costly oil; or, rather, to burn it as infrequently as possible.

Some tenants even say he had a bumper sticker which read: ''Have you hugged your sweater today?''

Be that as it may, the temperature inside the apartments dropped to around 50 degrees F. on most winter nights and often struggled to do much better than that during the day.

Now, mushrooms tolerate such cold only marginally more than the folks who fly to Florida for the winter. They particularly hate the desert-dry air that often accompanies the more bitter spells and a chill breeze is fatal to their well being. To be abundantly fruitful, mushrooms prefer humid conditions and temperatures that start out at 70 degrees F. and thereafter stay above 55, but do not rise much above 60 degrees, although they can tolerate slightly higher temperatures.

It may seem strange that I should talk about my colleague's experience just before suggesting that I have an answer to ''what to give the vegetable gardener who has everything'' this holiday season. You guessed it: a grow-your-own mushrooms kit. The point is, if the right sort of conditions prevail beneath the kitchen sink, on top of the broom closet, or just to the left of the clothes dryer, to name a few of the preferred places, mushroom kits will perform wonderfully for you. If not, your gourmet treat will come in at, well, gourmet prices.

One of the advantages of home mushroom growing is that they take up very little space and they don't need sunlight the way other food-producing plants do. That's why mushrooms can be grown in secluded closet corners and other out-of-the-way places. At the same time, they are not bothered by light as long as they are away from direct sunlight.

Mushroom kits come as 7-by-12-inch trays and about 51/2 inches deep. They are filled with the composted growing medium that already has been inoculated with the spawn. When you get the tray home you cover the compost with a casing material about 11/2 inches thick. Many of the kits include a suitable casing in a separate bag or you can use topsoil from your own garden.

Once this is done, water the box and set it on one side in a moderately warm corner where the temperature is close to 70 degrees. Go around your house or apartment with a thermometer, taking the temperature in different locations. You will be surprised to see how it varies. There are many different microclimates even indoors. Often the warmer temperatures are nearer the ceiling.

For the next two or three weeks the mushroom spawn will grow underground, spreading out until all the soil is filled with micelium threads - mushroom ''roots,'' if you wish.

Now it is time to reduce the temperature somewhat to around 60 degrees and await the fruiting - the appearance of the mushrooms that literally pop up overnight. An office worker I know had one on the corner of her desk. When she left for home one Friday evening there ''wasn't a sign of mushrooms,'' she told me. But when she returned Monday morning the box was filled with them. She and her coworkers added fresh mushrooms to their lunch salads every day for almost three months and she regularly took home a batch to saute for dinner.

The mushrooms first appear as buttons and grow to their full size in about 24 hours.

Wait until the mushroom cap has split away from the stem and is fully open before harvesting. At that stage it has reached peak flavor. Never pull up a mushroom as you may well damage others that are just emerging alongside. Lightly press down on the soil on both sides of the stem and twist off the mushroom or, using a sharp knife, cut it off at soil level.

A tip for keeping the air appropriately moist for the mushroom is to cover the box with a layer of burlap which allows the air to circulate through to the mushrooms but traps just enough of the moisture to make all the difference to successful mushroom production. Water the mushrooms whenever the surface of the soil gets powdery, but never do it with near-freezing, straight-from-the-faucet water. Let the water stand until it reaches room temperature. At the same time much of the chlorine gas will escape from the water as well, something your mushrooms would prefer to do without.

Grow-your-own-mushroom kits are available at garden centers around the country. Nationwide, they are carried by True Value and Ace hardware stores, and by Stop and Shop supermarkets in the Northeast.

Three mail-order seed and nursery houses that also carry the kits are W. Atlee Burpee Company, Warminster, Pa. 18974; Lakeland Nurseries, Hanover, Pa. 17331, and George W. Park Seeds, Greenwood, S.C. 29647.

Of course, inflation being what it is, prices may have risen since my colleague began growing mushrooms.

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