What's this yellow mushroom growing in my houseplant or garden

Nothing edible to share today, but I wanted to make a post about a mushroom that gets asked about all the time on Reddit and Instagram.  

The question "What is this yellow mushroom growing in my house plant / planter / garden?" is one I see every day, and today I'm here to answer it.

95% of the time you are looking at Leucocoprinus birnbaumii, sometimes called the yellow parasol mushroom, the painted dapperling, or the flowerpot parasol. Yes, it's so common in flower pots that it actually has that in the common name. The other 5% of the time you might be looking at the closely related Leucocoprinus straminellus, which is generally a paler shade of yellow, or Leucocoprinus flavescensm which generally has a brown area in the center of the cap.

Both L. birnbaumii and straminellus are tropical or subtropical mushrooms, so finding them in nature in the continental US, Canada or northern Mexico is extremely rare, but extremely common in potting and gardening soil. This is most likely due to the common usage of tropical materials, like orchid bark, in purchased soils. These organic materials may be contaminated with L. birnbaumii spores which then grow mushrooms when conditions are warm enough.

For most North Americans this means late spring, summer or early fall, but if you have a greenhouse, the yellow parasol might pop up anytime. 

Bell shaped when young

Yellow parasol mushroom description 

A bright, dainty mushroom that's yellow all over, cap, stalk and gills. L. birnbaumii is bright yellow and L. straminellus is pale or whitish yellow.

The cap shape starts out as a bell or cone or occasionally a marshmallow shape, then expands to an umbrella, and then the classic, nearly flat, parasol shape. When in the umbrella stage, the edges of the cap generally have fine striations (lines) and the cap will have small fibrous or warty raised areas all over, and the middle area will usually be raised -- this is called an umbo. At maturity, the cap will be anywhere from 1.5" to 3.5" in diameter. 

The stems are narrow near the cap and thicker at the base, even somewhat bulbous as they enter the soil. The stems will usually feature a ring, sometimes a double ring. The mushroom should be 2" to 3.5" high at maturity, but might be as tall as 4.5" in rarer cases. 

Is the yellow parasol mushroom poisonous or edible?

Reports are conflicting on whether L. birnbaumii is toxic, but it definitely should NOT be eaten. Many  Leucocoprinus species are severe sickeners, and symptoms can be dangerous. 

Is the yellow parasol mushroom dangerous for my plants?

The mushroom itself is not dangerous for the plant it's growing beside, in fact, it might be beneficial. Mushrooms like these are good at helping to convert raw organic matter in the soil into nutrients in a form that's easier for plants to absorb. The mushroom mycelium eat the organic matter, grow the mushroom and then when the mushroom decays, it breaks down into food for the plants. 

Mushroom "roots", called mycelium are actually the part of the fungus that is alive. The mushroom itself is a fruit, but the mycelium (which look like microscopic roots) are actually the organism. These mycelium are very good at improving the texture and density of the soil, they help break it up on a microscopic level, allowing better movement of moisture and air, in a way that's really beneficial for plants. 

However, if the plant you are growing is a food plant, like the basil in my pictures above, you want to make sure you rinse it thoroughly so that none of the potentially toxic spores are on the food when you ingest it. A good solid rain after the mushrooms are gone will take care of this as well!

Finally, if you see these mushrooms on indoor plants, especially succulents or cacti, they might be an indicator that you are watering too much. Mushrooms prefer a moist environment and so generally will not show up in a the pot of a desert-loving plant, unless that plant is getting too much water or not enough drainage.  

Should I remove the yellow parasol mushroom from my plant pots or garden?

If you have small children or pets you think might eat the yellow parasol mushroom, you should remove them from the pots, otherwise there is no harm in leaving them be. If you do choose to pick out the mushrooms, you won't be doing any harm to the mycelium underground, and you will still see many of the benefits of having them in your soil. 

If you live in Hawaii, you may also want to remove Leucocoprinus to help protect the island chain's delicate ecosystem. If you do this, make sure to destroy, rather than dispose, of the mushrooms. 

How long will these mushrooms stick around, and will they come back?

It generally takes 1-3 days for yellow parasol mushrooms to reach their full size, but after that they will decay in another 1-2 days. The fungus itself is not winter-hardy in most of the continental USA or Canada, and being left outside for a winter will most likely kill it, especially in smaller pots. If you bring your plants inside for the winter, or if you live in the Southern states, you can probably expect to see them again next season. 

The mycelium will continue to send up fruiting bodies (mushrooms) as long as they are alive and getting enough nutrients to do so. When the organic matter they prefer is gone, the mycelium will die or go dormant. This doesn't mean that your plant needs more nutrition, necessarily, since what feeds a mushroom and what feeds a plant aren't exactly the same things, however, it does mean that the soil is not quite as rich as before. If you add fertilizer, they might pop back up. 

Can the yellow parasol become invasive?

Throughout most of the US and Canada, no, this mushroom is far too cold-sensitive to risk becoming an invasive species. Even in the Southern states, where it might survive, it's unlikely to become invasive because it has a lot of competition that is already established here. Some small colonies might be found in the wild, but they will find a suitable ecological niche. Hawaii, with it's more delicate ecosystem, may be at greater risk.