It happens after almost every good soaking rain. You glance out at the lawn or walk through the woods, and suddenly mushrooms are everywhere, as if someone planted them overnight. Yesterday there was nothing. Where did they come from so fast?
The mushrooms didn't really start growing overnight. Every mushroom takes several days to form as a tiny bud on a network of fungus threads hidden underground. When rain arrives, it can quickly pull in water and inflate cells that were already built, ballooning to full size in a matter of hours.
The Fungus You Don't See
A mushroom is only a small part of the fungus. It's the fleshy, spore-bearing fruiting body, a bit like an apple on a tree. The main organism lives in the soil or inside rotting wood as a mycelium, a mass of thin, branching threads called hyphae.
That hidden network can be enormous. One honey fungus in Oregon's Malheur National Forest covers about 3.5 square miles and is estimated to be some 8,000 years old. It may weigh as much as 35,000 tons, making it one of the largest living things on Earth.
Days of Prep, Hours of Growth
A mushroom begins as a pinhead-sized bud called a primordium, less than two millimeters across, formed within the mycelium. It grows into a small, egg-shaped button, packed with cells that are ready to go. In many species, the button is wrapped in a cottony layer called a universal veil. As the mushroom swells, the veil tears, sometimes leaving a cup at the base of the stalk or patches on the cap.
Once those early stages are in place, the mushroom can rapidly draw water from its mycelium and expand, mainly by inflating the cells it spent days preparing. Mycelium networks can carry water, so when the ground gets soaked, the mushroom fills up fast. That sudden swelling is the part we notice.
Here Today, Gone by Afternoon
Some mushrooms take this to the extreme. A tiny lawn species called Parasola plicatilis forms its buds in humid spots under the grass. After heavy rain or a dewy night, it balloons to full size in a few hours, releases its spores, and then collapses. On a hot day, it may be gone by late afternoon.
Not every mushroom is speedy. Some grow slowly, adding tissue bit by bit. One slow grower, Pleurotus nebrodensis, is now critically endangered, partly because people collect it.
The sudden appearing act gave English a few handy expressions. When something spreads quickly, we say it's mushrooming. When something shows up out of nowhere, it pops up like a mushroom.
Rings in the Grass
Sometimes mushrooms pop up in a near-perfect circle called a fairy ring. The mycelium spreads outward from a center point, and when the nutrients in the middle run out, the center dies. That leaves a living ring that can grow to more than 33 feet across.
Folklore has had plenty to say about these circles. In parts of Western Europe, they were seen either as dangerous places linked with witches or the Devil, or as a sign of good fortune.
So after the next good rain, take a moment to admire the overnight crop, but leave the tasting to experts, since some mushrooms are toxic. What looks like a magic trick is really days of quiet work underground, finished off with a big drink of water.
