Snow mold is a fungal disease that affects grass over winter and spring. It gets its name from the fact that it looks like snow on the grass, and it is most common in early spring when the winter snow melts. Cool-season grass species like bentgrass, ryegrass, and Kentucky bluegrass are most susceptible to snow mold.
Snow mold comes in gray and pink forms. There is no treatment for snow mold. Prevention is key. When snow covers the ground before it has frozen yet, it traps a layer of air between the lawn and the snow. As snow melts, it creates the ideal environment for mold spores to grow into fungus.
Snow mold creates ugly light brown patches on a lawn. Even worse, its fungal spores trigger many peoples’ allergies. Thankfully, there are ways to deal with snow mold and prevent it from wreaking havoc on your turf. This article is all about how you can identify snow mold and prevent it from returning.
What Is Snow Mold?
Snow mold is a disease that affects lawns in places that get a heavy blanket of winter snow. It is caused by fungi that naturally occur in the soil all year round.
During early spring, when winter snow begins to melt, the moisture in the soil causes the fungal spores to develop. The mold becomes visible to the naked eye when the grass is no longer blanketed by snow.
It mainly affects cold-season grasses, such as perennial ryegrass, Kentucky bluegrass, bentgrass, and fescue.
There are two types of fungi that cause snow mold:
- Typhula blight or gray snow mold
- Microdochium patch or pink snow mold
Gray Snow Mold
This is the slightly less harmful type. It only affects the grass blades, not the roots, so it does not often kill the grass. The snow mold looks like white-gray circular patches on a lawn. These become visible as snow melts.
Pink Snow Mold
The circular patches of pink snow mold are very light pink in color, especially around the edges. This snow mold damages entire grass plants and results in dead, straw-colored patches on a lawn.
How Does Snow Mold Develop?
When a thick, heavy layer of snow falls before the ground has frozen, a layer of air between the grass and the snow is trapped. The weight of the snow crushes the grass, especially if the blades are longer than 3 to 4 inches.
When daytime temperatures start rising in spring, and reach around 32 to 45 degrees F, snow will melt, and the soil beneath it will become saturated. These warm, moist conditions enable snow mold spores in the soil to sprout.
Snow mold will grow on the weakened grass under the snow, and as the snow melts, it reveals circular patches of gooey, cob-webby fungus.
Areas of lawn with piles of leaves or very heavy snowdrifts are more susceptible to snow mold because they stay moist for longer when the snow is melting.
Identify Snow Mold
Snow mold is very easy to identify as it is difficult to mistake it for anything else.
Symptoms of snow mold are:
- Small circular patches of crusty, straw-colored, matted grass
- White, light gray, or pink spots of cobweb-like fungus
- Small, round, hard structures on blades of grass (called sclerotia)
The patches range in size from a few inches to several feet in diameter. The white fungal mycelium is usually only visible under very wet conditions.
Treat Snow Mold
There are no chemical treatments you can use on snow mold once it has formed in spring. While this prognosis sounds gloomy, there is no need to worry too much about snow mold.
Snow mold will naturally die as soon as the grass and soil dry out, and the affected patches of grass will revive in a few weeks.
Heat and sunlight kill the fungus. To help the soil dry out faster and increase airflow between the grass blades, gently rake the areas with snow mold using a leaf rake. To prevent inhaling the mold spores when you do this, wear a mask!
When temperatures reach 45 degrees F, gray snow mold will start to die. When it gets as warm as 60 degrees F, the hardier pink snow mold will die.
If the snow mold has caused patches of grass to die, overseed those areas to fill them in. You do not have to wait for the grass to grow back naturally, but given time, it will.
Prevent Snow Mold
With snow mold, prevention is definitely better than cure. There are many ways to prepare your lawn for winter so that snow mold does not develop in spring.
- Mow your lawn late in fall, before the first snowfall of the season. Cut it around an inch shorter than you normally do because snow mold is more of a problem when the grass is left too long through winter.
- Dethatch your lawn in late fall. When a lawn is left with a thick layer of thatch through winter, snow mold is far more likely to develop. Thatch that is over ¾ of an inch thick needs to be addressed before the first snow.
- Avoid letting snow pile up on your lawn. When clearing your driveway, sidewalk, or garden paths of snow, do not make a giant pile of snow on your lawn. Instead, spread snowdrifts out into a shallower, more even layer so that it will melt faster.
- Avoid leaving piles of leaves on your lawn over winter. A deep layer of leaves will keep the ground moist when snow melts, creating a breeding ground for snow mold. Instead, mulch the leaves using a lawnmower and spread this evenly over the lawn as dressing.
- Do not apply quick-release nitrogen fertilizer to your lawn late in the season. Instead, feed your lawn during the active growing season. Fertilizing late in fall will increase the risk of snow mold forming in spring.
- Avoid walking over moist parts of the lawn when snow is melting. Foot traffic leads to soil compaction that further stresses the grass.
Fungicide To Prevent Snow Mold
There are products you can apply to your lawn late in the fall to prevent snow mold from forming in spring. These are lawn fungicides that will kill the snow mold spores as well as any other fungal spores in the soil.
To control snow mold most effectively, it is best to use a combination of systemic and contact fungicides. Systemic fungicides are absorbed by the grass plants, so they must be applied while the grass is still growing in fall.
When applying fungicide, use an application rate of 2 to 3 gallons per 1000ft2 to ensure that the grass is thoroughly covered.
The best fungicides (according to research at the University of Massachusetts) are:
- Banner Maxx (propiconazole)
- Revere 4000 4F (pentachloronitrobenzene)
- Compass 50WDG (trifloxystrobin)
- Daconil WeatherStik (chlorothalonil)
- Terraneb SP (chloroneb)
Organic Snow Mold Treatment
If you are an organic gardener, the idea of applying a lawn fungicide may not sit well with you. These products are harmful to soil health because they disrupt the microbiome.
Organic gardeners have come up with creative ways to tackle snow mold. Because the fungi are sensitive to heat and sunlight, solarizing the affected patches in spring can kill the disease.
Solarizing means covering the grass in clear plastic so that the soil heats up and kills the fungal spores and mycelium. Clear painter’s plastic works well.
Lay the sheet of plastic over the affected areas, weighing the edges down with stones or bricks. As the sun shines onto the plastic, it will bake the grass and soil beneath it and kill the snow mold. Do this for 3 to 4 weeks.
Solarizing is most effective on parts of the lawn that receive direct sun. Areas with dappled shade or only morning sun will not heat up as much.
If you live somewhere that receives heavy snow every winter, it may be worthwhile to try using snow fences to prevent deep snowdrifts from forming on the lawn. With a snow fence, you can control where snowdrifts form.
A snow fence is a temporary structure that causes wind-blown snow to accumulate behind it. One can construct a snow fence from wood or from plastic barrier netting.
Snow fences are definitely not worth the effort if you have a small piece of turf, but for large areas of lawn, it can make a huge difference for spring snowmelt.
How To Keep A Lawn Healthy Through Winter?
It is a common misconception that you do not have to pay any more attention to lawn care after summer. Take excellent care of your lawn in the months leading up to winter, and you can prevent fungal problems like snow mold.
Here is some advice for keeping your lawn healthy through winter:
- Keep watering your turf during the fall so that the soil is moist when the winter frosts start. If the soil freezes without any moisture in it, the grassroots will be dry for a long time, stressing the grass.
- Aerate your lawn soil early in the fall. Loosening the soil allows oxygen to penetrate into the grass root zone, and it allows the roots to spread through the soil more easily. Aerating is especially important in high-traffic areas where the soil becomes compacted.
- On the same day that you aerate the soil of your lawn, feed it using granular fertilizer. Choose one that has a combination of slow-releasing and quick-releasing nitrogen. The extra nutrients will support the grassroots as they build their stores for winter. It is critical not to fertilize too late in the fall. This may exacerbate fungal issues.
- Rake up dead leaves and remove them from your lawn. Not only do they smother grass when it is trying to grow back in spring, but they harbor mold and increase the risk of snow mold.
- Many people stop mowing the lawn at the end of summer. Long grass going into winter is a recipe for springtime fungal problems. Mow your lawn right until the first winter snow. This will ensure that it is short enough to easily dry out in spring when the snow melts.
- Dethatch your lawn in late fall. A thick layer of thatch will prevent grass from drying out quickly in spring. Thatch can also harbor fungal spores.
Other Lawn Fungi
If the lawn is looking patchy, but you are not 100% sure that the issue is snow mold, consider the following common fungal diseases in turfgrass:
- Necrotic ring spot (Ophiosphaerella korrae) – the symptoms of this disease usually present in late summer, but the disease does most of its damage in spring and early summer. Irregular brown spots will show up on the lawn.
- Pythium blight (Pythium aphanidermatum) – this disease is prevalent in young lawns where soil drainage is inadequate. As a water mold, it thrives in soggy soil. Symptoms are 1- to 3-inch-wide circular spots on the lawn. You might see cobwebby mycelium in the early morning. The grass in the circles looks gray and dead.
- Brown patch fungus (Rhizoctonia solani) – symptoms of brown patch start to appear in early summer when temperatures are between 65- and 70-degrees F at night. As the name implies, large brown patches develop on the lawn.
Snow mold is an annoying fungal lawn disease that is, unfortunately, easier to prevent than it is to treat. There aren’t any fungicides that can be used on snow mold once it appears in spring as the snow starts to melt. All you can do is gently rake affected patches to help the soil dry out.
To prevent snow mold, act during the fall – water, mow, dethatch, aerate, and feed your turf. Taking good care of your lawn before winter will help it recover from a heavy dump of snow. Do not leave piles of dead leaves on the lawn over winter and avoid letting deep snowdrifts form on the lawn.