The size of a fungus depends on whether the temporary reproductive structure (the fruiting body) or the permanent, hidden network is measured. The common mushroom seen above ground is only the fruiting body, analogous to an apple on a tree, designed to disperse spores before quickly decaying. The main body of the organism, the fungus itself, exists almost entirely hidden beneath the soil or inside wood, forming a vast, intricate network of thread-like cells. To determine a fungus’s true dimensions, scientists must consider both the ephemeral structures we can observe and the sprawling, subterranean networks that represent the organism’s full scale.
Record Holders: The Largest Visible Mushrooms
The largest examples of mushroom size are measured by sheer mass or by the expansive diameter of their caps. The most massive single fruiting body ever documented belongs to the bracket fungus Phellinus ellipsoideus, a wood-rotting species. Discovered on Hainan Island, China, this specimen was a dense, shelf-like structure weighing up to 1,100 pounds (500 kg) and measuring about 35 feet (10.8 meters) long. This fungus achieves such weight because it is composed of hard, woody tissue that allows it to grow continuously over many years.
In contrast, the largest mushrooms in terms of volume and diameter are often the Giant Puffballs, Calvatia gigantea. These spherical fungi develop rapidly, sometimes reaching their full size in just a week. A notable record holder weighed over 51 pounds (23 kg) and another specimen measured 8 feet 8 inches in diameter. While these puffballs lack the height of a typical capped mushroom, they achieve impressive dimensions by ballooning with millions of spores and a spongy internal tissue.
For the familiar cap-and-stem structure, the limits are typically less dramatic, but still impressive. The tallest mushrooms rarely exceed a few feet because of structural limitations. Visible mushrooms, whether a cap, a puffball, or a woody bracket, are ephemeral and short-lived compared to the permanent fungal body beneath the surface. These records show the surprising capacity for rapid, concentrated growth under ideal conditions, yet they pale in comparison to the hidden networks of the fungus.
The True Scale: Mycelial Networks as the Largest Organisms
The vast majority of a fungus exists as a mycelium, a decentralized, interconnected mass of microscopic filaments called hyphae that permeates the substrate. This network is the fungus’s feeding body, constantly growing and colonizing new territory. When measured by area, a single fungal colony can become one of the largest individual organisms known on Earth.
The most famous example is a colony of the honey mushroom, Armillaria ostoyae, found in the Malheur National Forest in Oregon. This single organism is estimated to cover an area of 3.5 square miles (9.1 square kilometers). Genetic testing confirmed that samples collected across this vast expanse are all part of the same individual, making it a single continuous organism.
This “Humongous Fungus” is thought to have been growing for an immense period of time, with age estimates ranging from 2,400 years up to 8,650 years. Its total biomass is difficult to calculate precisely, but it is estimated to weigh up to 35,000 tons, making it a contender for the world’s heaviest organism. The fungus spreads its reach through thick, root-like structures called rhizomorphs, which look like black “shoestrings” and allow it to invade new trees and connect distant parts of its network.
The sheer size of these mycelial networks demonstrates that the growth potential of a fungus is determined not by a single, solid mass but by its ability to spread and colonize a massive area underground. Unlike the temporary fruiting body, this subterranean network represents the long-term, true scale of the organism.
Biological Constraints on Fungal Size
Despite the immense scale of mycelial networks, the size of the above-ground mushroom is limited by biological and physical mechanisms. Fungi lack the specialized vascular systems found in plants, which transport water and nutrients against gravity. Instead, they rely on the strength and hydraulic pressure within their microscopic hyphal cells to support their structure.
A primary constraint on the thickness and density of a mushroom is the need for oxygen. Fungi are aerobic organisms that require oxygen to survive, but they lack lungs or gills to actively move air. They rely on simple gas diffusion to move oxygen from the air into their dense tissue.
As a mushroom’s stalk or cap becomes thicker, the distance oxygen must travel to reach the innermost cells increases significantly. This reliance on diffusion creates a steep oxygen concentration gradient, meaning the cells deep within a large mushroom can be starved of oxygen. This physiological limitation prevents the formation of massive, solid fungal structures that could otherwise challenge the height and girth of large trees.
The physical strength of the fungal material, composed of chitin, also plays a role in limiting height against the force of gravity. While chitin is a strong polymer, the structure of the stipe, or stalk, is not reinforced like the woody trunk of a tree. This combination of structural and oxygen-transport limitations dictates that the visible mushrooms we encounter will always remain within a modest size range.