If you’ve never encountered something called slime mold, then finding a pile of black ooze growing on your lawn is bound to be quite a shock. Don’t worry, though, because this goo didn’t come from a sci-fi novel or horror movie.
Rest assured, black slime mold is one of the most harmless organisms you can find growing in your lawn or garden. But that doesn’t really make up for its unsettling appearance.
In this article, I explain what black slime mold on grass is and how it spreads to new lawns. I also provide some options for removing and preventing slime molds found growing on turf grass.
What Is Black Slime Mold
Black slime mold is an unsightly organism that often grows in lawns and gardens during the spring or particularly damp summers. According to the Alabama Cooperative Extension Office, most slime molds affecting residential lawns belong to the genera Fuligo, Murcielago, and Physarum.
Despite common misconceptions, slime molds are not true fungi. They belong to the kingdom Protista which is separate from both plants and fungi. Other examples within Protista (known as protists) include amoebas, kelp, and red algae.
While biology distinguishes these organisms from fungi, in practice it’s okay to think of slime molds as simplified fungi. The way they spread and appear only when conditions are ideal is very similar.
Symptoms of Slime Mold On Your Lawn
In my experience, it’s hard to overlook a case of slime mold on manicured grass. However, you may mistake this turf disease for something else. When it comes to black slime mold, in particular, growths are sometimes mistaken for spilled motor oil or grease. Other color variations may be mistaken for vomit or rotted food.
Patches of slime mold are normally no larger than 8 inches across. In severe cases, however, you may encounter entire sections of lawn covered in the organism. Slime mold is usually dark in color but can also be white, yellow, red, brown, gray, and even purple.
While the primary organism tends to have an oily or greasy consistency, the mold may take on a crusty appearance as fruiting bodies emerge. This crust typically lasts for up to a week before the entire organism dries up and deteriorates.
The good news is that slime molds are almost always harmless to the grass they grow on. Slime mold will only damage grass if it covers large sections and even then the harm will be mild.
After the slime mold dries up and disappears, you may notice that the grass where it was looks yellow or lackluster. This is the result of the slime mold interfering with photosynthesis — the mold itself didn’t damage the grass. With continued maintenance, the lawn should recover in a relatively short time.
What Causes Lawns to Get Black Slime Mold
Slime mold spores are primarily carried by the wind. They may also be spread over shorter distances by rainfall or irrigation. However, it’s possible for mold spores to live in the thatch layer for a significant time before ever germinating and showing visible signs. Just because you suddenly notice slime mold growing in your lawn does not mean it was introduced recently.
Slime mold requires lots of moisture to bloom and become visible. You’re more likely to see signs of slime mold in your lawn when there are warm days and cool nights, which promote heavy dew formation.
Keep an eye out for new growth when the temperature is between 50 and 80°F. Heavily shaded lawns may be at higher risk for slime mold because they tend to stay calm and dry out less quickly than sunny properties.
For slime molds to grow, they also need a food source. Slime mold activity is more common in lawns that have high amounts of organic matter in the soil or a thick layer of thatch buildup beneath the turf.
Slime Mold Lifecycle
Like fungi, slime molds start as spores. These spores can remain viable for a long time, even several years, before germinating when the conditions are right.
Once the mold spores germinate, they grow into something called a plasmodium. Slime molds are extremely simple organisms in biological terms, and this plasmodium is its primary life form. To our eyes, however, this form appears as little more than a blob of slime growing on our grass or garden mulch.
As a plasmodium, the slime mold can absorb its food, such as bacteria growing in the soil and thatch layer, with ease. Once the organism fully matures, it will develop fruiting bodies.
Fruiting bodies are essentially the flowers of the fungal and slime mold worlds. Mushrooms are just one example of a fungal fruiting body. While slime molds do not have mushrooms, their fruiting bodies still produce and release new spores into the environment, allowing the original plasmodium to reproduce and spread.
How To Get Rid of Black Slime Mold
I agree that black slime mold on your lawn is both an unwelcome smell and sight, especially if you’ve spent a good deal of time making your lawn look its best. But, more often than not, trying to get rid of this growth is more effort than it’s worth.
Even when conditions are perfect, mature plasmodia are unlikely to survive more than a few days to a week. In most cases, they disappear just as quickly as they showed up. If you’re still determined to free your lawn from this ugly intruder, however, there are a few removal strategies that can help speed things up.
A standard lawn rake is one of the best tools for removing slime mold. You can use a rake to gently scrape the visible plasmodium from the turf. However, there’s a high likelihood that the slime will reappear in a few days since this method doesn’t remove the entire organism.
When using a rake to scrape away slime mold, be careful not to spread the growth into unaffected areas of the lawn.
I recommend cleaning the rake and any other tools with a mixture of water and bleach before putting them back into storage. Remember that spores may stick to the rake that you can’t see with the naked eye. These spores could survive for quite some time before being reintroduced to the lawn and creating a new patch of slime mold.
You can continue mowing your lawn as usual even when black slime mold is present. In fact, cutting the grass may remove some of the organisms and speed up recovery.
There’s no need to adjust your mower’s height when cutting grass afflicted by slime mold. However, removing excess length could improve airflow and reduce the chance of more patches appearing. Do not cut more than one-third of the lawn’s height at a time.
After mowing over an area of lawn with black slime mold, stop to collect all of the infected grass clippings. You don’t want these clippings to blow into other parts of the lawn or get mixed in with lawn waste intended for use as compost or mulch.
If you’re concerned about spreading or reintroducing slime mold spores to your lawn in the future, I recommend cleaning the bottom of your mower as well. Use diluted bleach to kill any spores that may be stuck to the blades, wheels, and exterior housing.
3. Jet Washing
It’s also possible to remove black slime mold using a garden hose. Be aware, though, that there’s also a chance of spreading spores to additional parts of the lawn.
Use a sprayer attachment with a strong jet setting. Aim it at the slime mold directly to break up the plasmodium until it’s no longer visible.
For the best results, you’ll want to spray off black slime mold growths when the weather is dry. Ideally, there should be several dry days in the forecast ahead. This increases the odds of any leftover spores drying out before conditions turn damp again and the organisms can easily germinate.
Preventing Slime Mold on Lawns
Preventing slime mold from growing in your lawn is not essential to grass health. However, it is a great way to ensure your lawn looks its best at all times.
Simple cultural practices are the most effective against black slime mold. Maintaining a healthy lawn that receives the right amount of moisture and sun exposure will stop most cases of slime mold in its tracks.
1. Avoid Overwatering
Since moisture is a key factor in slime mold growth, your watering habits could play a big role in its prevention. Good irrigation practices will prevent a variety of turf grass diseases. I recommend following these guidelines regardless of if slime mold is actively growing in your lawn or not.
Water early in the day so that the grass has time to dry out before evening. My preferred time to water is mid-morning. If the weather is scorching and dry, water is usually safe as late as the afternoon.
I also suggest coordinating your irrigation schedule with the weather. There’s no need to water if rain is in the forecast. Not only is it a waste of water but it also improves the conditions for slime mold growth.
2. Improve Drainage
Any experienced gardener knows that irrigation is just one part of the moisture puzzle. If your lawn has poor-draining soil, even infrequent watering could create an environment where black slime mold flourishes.
Some of my favorite methods to improve drainage include amending the soil with organic material and core aeration. Depending on the state of your lawn, these steps can be completed as often as annually.
Amending the soil for better drainage typically entails spreading a thin layer of compost or another rich, organic material over the lawn’s surface.
For aeration, you’ll need a piece of specialized equipment called a core aerator that pulls out small “plugs” of soil. The holes created by a core aerator allow water, oxygen, and nutrients to better penetrate the soil.
I mentioned above that aeration is a great option for improving general soil drainage. It’s also effective at increasing airflow which allows the top layer of soil to dry out faster after rain or irrigation.
4. Minimize Thatching
Thatch is the layer of dead plant tissue that forms between the top layer of soil and living grass blades. While thatch is a completely natural phenomenon that can actually benefit a lawn, it also creates the perfect home for fungal and slime mold spores.
Thatch becomes a problem when it’s allowed to grow too thick. A thick layer of thatch restricts the movement of air, water, and nutrients. Some grass varieties — most notably, those that spread via horizontal stems — create thatch faster than others.
The best way to manage thatch buildup is by removing a portion of it each year. You can reduce thatch by using a metal hand rake or a powered dethatcher. This is usually done in the spring or fall.
5. Minimize Shade
Excessive shade can also leave turf grass vulnerable to black slime mold, especially if your lawn isn’t planted with a species suited to low-light conditions. Eliminating unnecessary shade by trimming trees and shrubs, moving large pieces of furniture, and opting for light-permeable fencing can help your lawn stave off slime mold and other infections.
FAQ’s Lawn with Black Slime Mold
Verdict: Black Slime Mold on Grass
Black slime mold is certainly one of the strangest things you might find on your lawn. In my experience, however, a little patience is all you need to get rid of it.
While short-term solutions like mowing or raking can eliminate visible slime mold growths, preventative measures are by far the most effective at keeping this organism out of your lawn. A healthy lawn is far less likely to succumb to black slime mold — not to mention more serious fungal infections.