The largest recorded plasmodia cover more floor than a small bedroom, and every one of them is still a single cell. That gap is what makes slime mold size so odd. Most blobs you find on mulch measure 1 to 1.5 inches (3-4 cm) across. The largest slime mold plasmodia on record, logged by the National Park Service, spread past 320 square feet (30 sq m) of damp ground.
A plasmodium starts as separate flagellated cells that swarm together and fuse. The walls between them go away, so what you see is one bag of cytoplasm held inside a single membrane. Inside that bag sit millions of nuclei, and they all divide at the same moment. No colony of small bodies is stacked up here. The blob gets bigger the way a balloon does, by holding more, and that is why the ceiling on its size sits so high.
A cell you can spot from across the yard breaks the picture most of us got in school, where a cell is a dot under a microscope. A plasmodium keeps that same basic setup and scales it up. There are no internal walls to split the body into rooms, so growth adds volume instead of new units. One body can end up draped over a whole log.
Plasmodia of some species have been recorded at over 320 square feet. All of that is one cell with one membrane around it.
Plasmodium size follows food and water more than age, and slime mold size in the wild swings on the same two things. A Physarum blob on damp bark usually stays near 1.5 inches (4 cm) wide, because it runs short of both before it runs short of room. On a soaked log in deep shade, the same organism stretches to 12 inches (30 cm) or more and keeps a lobed, fanned out edge as it goes. Two blobs of one species can differ several times over in width.
Thickness is the part people skip. A well fed plasmodium runs 1 to 2 inches (3-5 cm) deep, so a foot wide sheet is not a smear of paint on the bark. It is a slab you could lift with a trowel. That depth is where most of the mass sits, since the blob puts on size in three directions at once.
Records at the top of the slime mold size range come from wet forest ground, not from a garden bed. A sheet over 320 square feet needs constant damp and a huge supply of food, and mulch dries out long before a blob gets that far. What shows up on wood chips after a rainy week sits at the small end of the scale, and that is the size worth planning around.
Speed is the other half of the answer. Top crawl for a plasmodium is about 0.04 inches (1 mm) per hour, so a patch shifts maybe half an inch (1 cm) over a long day. You will never catch it moving while you watch. Come back the next morning and the yellow has climbed the edge of the bed, which is why people swear the thing appeared overnight.
Measure yours two mornings in a row if you want a real number for your own yard. Push a stick in at the edge, check the same spot at the same hour, and you will see growth of about an inch. Almost every blob a gardener meets tops out between an inch and a foot across. The room sized ones live in soaked woods and stay there.
Read the full article: Slime Mold: Not a Fungus, Not a Problem
