Why do mushrooms suddenly pop up after rain?
โ Overnight growth isn't cell division
Walk through a garden or park on a rainy morning, and mushrooms are standing where nothing was yesterday. They can even grow several centimetres overnight. Plants never grow that fast. Where do mushrooms come from, and why can they get so big, so suddenly?
The morning after a night of rain. Along the lawn in your local park, on your usual route, white mushrooms stand in scattered clumps.
The evening before, when you walked the same path, there was nothing there. No sign of dug-up soil, either.
A few days later, the mushrooms melt away and the ground is bare again. It's as if they grew straight out of the earth and sank straight back into it.
Two main reasons
What we call a "mushroom" is only part of the organism. The real body is a mass of fine threads, spread quietly all year round through fallen leaves, soil, and rotting wood. Only the spore-making part pushes above ground.
When a mushroom gets bigger, its cell count barely increases. Water floods into parts that were already assembled in miniature, and each cell swells like a balloon. That's why mushrooms shoot up all at once after rain.
Put these two together, and the "it suddenly appeared" feeling starts to make sense. Let's look at each in turn.
A mushroom is less like a tree, more like a flower
The body of a mushroom is a fine thread only a few thousandths of a millimetre thick, called a hypha (plural: hyphae). Hyphae spread through soil and leaf litter like a net, breaking down organic matter around them for food.
In other words, a large body already exists underground before anything shows. In a wide-open space, this web of hyphae can stretch for many metres. The mushroom you see above ground is like a "flower" that the web sends up.
And even before rain falls, a tiny blueprint for the mushroom is already sitting just below the surface โ a grain a few millimetres across, called a primordium. That's the small grain on the left side of Figure 1.
Swelling alone can happen in one night
For most living things to grow, cells must divide to increase in number. That takes time. It's why a tree can only thicken by a few centimetres a year.
But inside a mushroom's primordium, the cap, stem, and gills are already built, just folded up small. All that's left is to add water. Cells soak it up, stretch lengthwise, and the whole thing swells up.
This "add water and swell" method needs only water and a way to carry it. Once rain soaks in and the soil turns damp, the hyphae draw up that water and send it to the primordium. That's how mushrooms manage several centimetres in a single night.
Mushrooms are also common in autumn, likely because rain and temperature happen to line up well at the same time. In dry summer soil, primordia just sit there, waiting for water.
Once a mushroom has swollen up and released its spores, it loses water and shrivels. Since roughly 90% of its weight is said to be water, once that water is gone, it can't hold its shape. It appears and vanishes fast because most of its body is water.
Sometimes mushrooms line up in a circle. The hyphae spread outward from the centre at roughly the same speed. The inner area, its nutrients used up, can no longer support growth, so only the outermost ring sends up mushrooms โ which is why it looks like a circle.
Summary
Mushrooms weren't born after the rain. The body was underground the whole time, with small parts already prepared and waiting. The rain just poured water onto them. Swelling needs no cell division, so they can rise in a single night.
Mushrooms didn't "grow" from the rain.
A shape prepared long before simply opened up with water.
For another story about carrying water from underground, see How can trees pull water up 100 metres? โ it shows how differently water moves there. For the work fungi do more broadly, see What's the difference between fermentation and rotting?. For another change happening in the same rain-soaked soil, see Why do earthworms come up to the surface on rainy days?. For another case of stretching by water rather than cell division, see Why can bamboo shoots grow dozens of centimetres in a day?. And for another food story about "seasonality," like autumn mushrooms, see Are in-season vegetables really more nutritious?.
- The morning after rain, walk around a park's lawn or the base of street trees. Remembering spots that had nothing the day before makes the change easier to spot.
- Photograph the same mushroom in the morning and evening, and compare. Lay a ruler beside it in the photo to see how many centimetres it grew in half a day.
- A few days later, check the same spot again. Try recording how fast it shrivels and collapses too. Always wash your hands after touching one.
Even experts say it's hard to identify wild mushroom species by appearance alone. Observe with your eyes only โ never put one in your mouth.
Want to know more? โ Terms, formulas, and textbook linksLabels show which level each part belongs to, from middle-school science to university specialist courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Biology Basics / Biology"
- High school+High-school advanced content, or textbook sidebar material
- UniversityNot covered in high school โ university-level specialist content (mycology, ecology)
- ResearchNot yet settled even at university level โ an area researchers are actively investigating
Middle schoolTerminology: this phenomenon has names
- Hypha: the fine thread that makes up a mushroom's true body. Only a few thousandths of a millimetre thick, spreading through soil and rotting wood like a net.
- Fruiting body: the above-ground part we call a "mushroom." An organ for making and releasing spores.
- Primordium: the small grain that forms underground and becomes the fruiting body. The shape of the cap and stem is already built in miniature.
- Turgor pressure: the pressure created when a cell absorbs water and pushes outward from within. The source of the force that makes a mushroom stand up.
Middle schoolHigh schoolCheck with a formula: how many mushrooms' worth of water is one night's rain?
How much water does rain actually deliver to the ground? Let's work it out for one square metre โ about half the size of a tatami mat. We'll keep units consistent: millimetres for length, litres for volume, grams for weight.
| In symbols | N = ( h ร A ร ฯ ) รท w |
| In words | Number of mushrooms = (rainfall ร area ร weight of 1 litre of water) รท weight of water in one mushroom |
| Where it comes from | The volume of water that falls is "depth ร area = volume," which we then divide by the water needed for one mushroom to share it out โ a water balance (water is neither created nor destroyed along the way) |
Symbol meanings: h is rainfall (millimetres), A is area (square metres), ฯ is the weight of 1 litre of water (grams), w is the weight of water in one mushroom (grams), N is the number of mushrooms. Since 1 millimetre of rainfall equals 1 litre over 1 square metre, h ร A directly gives litres.
| Rainfall over one night | 20 millimetres |
| Volume of water added per 1mm of rainfall over 1 square metre | said to be 1 litre |
| Weight of 1 litre of water (grams) | 1000 grams |
| Weight of water in one mushroom (grams) | roughly 18 grams |
| Volume of water added over 1 square metre (litres) | 20 ร 1 = 20 |
| Convert that to weight (grams) | 20 ร 1000 = 20000 |
| How many mushrooms' worth is that (mushrooms) | 20000 รท 18 โ 1111 |
One night's rain over an area the size of half a tatami mat works out to roughly a thousand mushrooms' worth of water. Of course, much of it soaks deep into the ground or runs off elsewhere. Even so, it's clear that a single night's rain provides more than enough water to raise a mushroom.
High schoolHigh school+Where does the swelling force come from?
High schoolThe fluid inside a cell has sugars and ions dissolved in it. When it's more concentrated than the water around it, water tends to move into the cell. This is osmosis. The water that enters pushes the cell outward from within, and the cell wall holds it in.
High school+When a mushroom's stem elongates, the cell wall is thought to loosen and change shape only in the direction it's being pushed. Because the loosening has a direction, the cell stretches lengthwise instead of widening. Plant seedlings that grow overnight work in a similar way.
UniversityFungi as an underground network
The force that pushes out a mushroom's stem is osmotic pressure, arising from the difference in concentration inside and outside the cell, and turgor pressure, produced when the cell wall holds that pressure in. For a dilute solution, osmotic pressure can be estimated with the van 't Hoff equation, which is proportional to concentration and temperature. Hyphae do more than just produce mushrooms. Many fungi connect with tree roots, passing nutrients drawn from the soil to the tree in exchange for sugars the tree makes. This relationship is called mycorrhiza. In forest soil, a web of hyphae is known to link many trees together. The number of mushrooms visible above ground is only a small part of that much larger underground body, shown only when conditions align.
๐ For the derivation of the formula and further reading: Osmotic pressure (Wikipedia, Japanese)
ResearchWhat's still not fully understood
- What triggers it? Besides water, falling temperature and day length are also thought to act as cues, but the exact conditions for each species haven't been fully worked out.
- How fast can it grow? Whether the rate of cell-wall loosening or the rate of water flow sets the speed limit is still debated.
- How far does the underground web extend as one connected organism? How far the hyphae of a single fungus form one continuous network depends on how you measure it, and the answer varies.
In other words, this article too describes only "what's understood so far." The ground beneath our feet is still a world full of unknowns.
Links to textbooks (by level)
| Level | Subject / Unit | Where in this article |
|---|---|---|
| Middle school | Science: structure and function of living things / decomposers | The section on hyphae breaking down fallen leaves for food |
| High school | Biology Basics: cells and osmosis | The mechanism of cells swelling with water |
| High school+ | Biology: plant growth and the cell wall | The section on cell-wall loosening and lengthwise growth |
| University | Mycology / forest ecology | The section on mycorrhiza and the underground network |
| Research | Research into fungal life cycles | The unresolved trigger for fruiting-body formation |
| โ | Everyday connections | Observing mushrooms in a park or garden after rain |
- Standard mycology textbook explanations of fruiting-body formation and mycelium
- Nicholas P. Money, Mushroom (Oxford University Press, 2011)
- National Museum of Nature and Science (ๅฝ็ซ็งๅญฆๅ็ฉ้คจ) exhibit explanations of fungi and the role of decomposers
- Forestry and Forest Products Research Institute (ๆฃฎๆ็ทๅ็ ็ฉถๆ) publications explaining the relationship between mycorrhizal fungi and trees
โปThis article is a general-audience science explainer. The figures given are approximations to help illustrate the mechanism. The species of a wild mushroom cannot be determined by appearance alone. Follow guidance from your local authority or a specialist organization regarding foraging or edibility.