Moldy Soil, Tiny Mushrooms, and 6 More Gross Plant Problems—and How to Fix Them

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The person you are on the first day you invest in a new houseplant and the person you are, say, three months later can be quite different. The first person is filled with excitement and confidence—a proud plant parent with nothing to fear. The second? They might be hovering over their struggling plant baby and wracked with confusion and guilt over why it’s failing to thrive.

If that sounds like you, don’t panic. And even more importantly, don’t throw in the towel just yet, because help is on the way. The answer to which houseplant problem is plaguing your greenery can be tough to puzzle out on your own, but a couple of pros have some answers: Jess Henderson, a plant-care expert and aspiring shop owner who lives alongside over 150 specimens (and counting!) in her 472-square foot studio, and Erin Marino of beloved plant purveyors The Sill.

From mushy leaves, to moldy soil, to all manner of pests, they talk through eight gross houseplant problems and exactly how to fix them.

Moldy Soil

If you start noticing that telltale mildew-y smell in the air, you might have yourself a case of moldy soil. But that doesn’t necessarily mean you need to toss your planter’s contents and start over. “Mold isn’t uncommon,” Henderson reassured, “especially if you’re using organic soil. It’s likely a harmless saprophytic fungus. On the other hand, the mold could be a sign that the plant isn’t getting what it needs, so look out for floppy, yellowing, or browning leaves.” 

If you notice other symptoms of distress, Marino says, “try pausing on watering and letting the soil try out completely in a sunny spot for a few days. If that doesn’t do the trick, it might be worth repotting your plant and providing fresh potting soil. You can use the same planter, just give it a quick rinse with some diluted soapy water.”

Fungus Gnats

If you’re a plant-owner, you’ve likely seen your share of fungus gnats—those tiny insects that you might have mistaken for fruit flies. “Fungus gnats look like little fruit flies, but their bodies tend to be slimmer and their flying poorer,” says Marino. “As their name implies, they eat the fungi that live in your plant’s soil.”

When you water your plants, you cause that fungi to bloom, which makes gnats very happy. They celebrate by laying more eggs in the soil, which is where your potential problem starts. “Adult fungus gnats are not harmful to your plant,” says Henderson. “But in large numbers, the larvae can stunt plant growth and damage roots.” 

If your fungus gnat population is on the rise, the culprit is likely to be overwatering, explains Marino, so “the first line of defense is to water less, and let the soil dry out completely between waterings. You can also work some diatomaceous earth into the first inch of the soil to target any fungus gnat larvae lurking below the surface.”

For Henderson’s part, she prefers to take a holistic approach, targeting the pests at every stage of life. “I do yellow sticky pads or a fancy bug zapper for the adults, and add hydrogen peroxide to my water on watering day for the larvae. Plus, I put a suffocating layer of sand on top of my soil.”

Credit: DAN559/Shutterstock

Tiny Mushrooms

They might not be aesthetically pleasing to you, but both experts stressed that tiny mushrooms in your potting soil are not typically something you need to worry about. “Plants have had beneficial relationships with soil bacteria and fungi for millions of years,” Marino reminded us. “It’s easy to forget that each planter is a tiny ecosystem where some microbes are beneficial and others are harmful. Those tiny white or yellow mushrooms are generally considered to be the former.”

For her part, Henderson is actually glad when mushrooms show up. “I love those little guys—they’re harmless and usually a good indicator that you have wonderful soil! If you’re blessed with a mushroom visit, I find it best to respect the process and let it do its thing.”

But if you absolutely must get rid of them, the process is a simple one, according to Marino. “Simply pluck them out of the soil and toss them into your compost bin or the trash. If you want to make sure they don’t return any time soon, let the soil dry out completely before watering again, and water less next time.”

Mushy Leaves

We’re mostly talking succulents here, where healthy leaves are plump and firm to the touch. If you spot a mushy leaf, Henderson says, “it’s usually an overwatering issue. I remove the leaves and repot with better drainage.” She also recommends employing bottom watering or terracotta spikes to help break your overwatering habit.

Marino concurs: “The best way to address mushy leaves is to prune them off your plant,” she says. “They aren’t going to bounce back, so trimming them lets your plant focus its energy on new, healthy growth instead. Once those leaves are removed, you’ll want to let the potting soil dry out completely. If it’s taking more than a few days, move your plant to a sunnier spot to help it along.”

Spider Mites

“Spider mites are very small and usually hang out on the undersides of leaves so they can be difficult to spot,” Marino warns.

Henderson describes the early indicators that tell her she might have an infestation: “First I can tell that my plant may look a little off; not as bright green or as buoyant as usual. Then I’ll notice what looks like dust or tiny webbing at the petiole, the stalk that connects the leaves to the stem.”

Further down the line, as the mites attack leaf cells, Marino says you’ll see stippling, mottling, or curling on the leaves, with only a skeletal leaf web left behind. If you notice any of these factors, she says, “Try holding a leaf still. If mites are present, you will see tiny brown dots slowly crawling up or down. That’s them.” 

Your first stop for an infested plant, according to Henderson? Quarantine. “Keep the plant separated from others while you treat it, and for at least two weeks after the mites are gone,” she says. “Even if your other plants aren’t susceptible to mites, they could be carrying something else that could infect your already-compromised plant.”

For treatment, Marino has advice: “You’ll first want to give your plant an incredibly thorough cleaning with soapy water, then spray it down with a pesticide or similar. The most effective pesticide against mites is elemental sulfur but we do not recommend it indoors,” she says. “Spider mites are also susceptible to horticultural oils and insecticidal soaps, which you’ll need to reapply once every one to two weeks as needed.” (Henderson specifically recommends neem oil, available online for less than $10, for treatment and to prevent reinfection.)

If that all sounds like a lot of work, that’s because it is, so Henderson’s preference is to stay ahead of the issue. “Spider mites love a dry warm environment, so during the winter, I keep my house at 50 to 65 percent humidity,” she says. “Thankfully, I haven’t had to deal with spider mites since I bought my humidifiers.

Scale Insects

While scale insects are typically easier to spot than spider mites, they’re particularly good at blending in with bark-like stems, Henderson says. “If you drag your nail across what looks like a little bark bump,” she says, “a scab-like bug will come off. That’s scale.” If left unchecked, scale pests will crowd new growth and suck the life out of it, so again, Henderson urges quarantine, this time with physical removal. “You have to remove scale the old fashioned way,” she says, “I really enjoy smashing them with Q-tips.”

That’s because “scale insects enclose themselves under a shell fortress, which makes them generally impervious to spraying insecticides of any kind,” Marino says. “Instead, you’ll want to scrape off the scale bugs first, and then spray generously with an insecticide. The spray will help kill the scale larvae that you missed when scraping off the more visible adults.”

Again, Henderson recommends treating the healing plant with neem oil, as it makes the plant less desirable to inhabit going forward.

Fertilizer Burn

When asked what other gross houseplant issues should be added to this list, Henderson was quick with an answer: fertilizer burn. “When fertilizer burn occurs, yellow and brown spots will appear and quickly spread across the leaf, giving you an abundance of new, underdeveloped leaves.” 

She acknowledged that she falls into this trap on occasion herself, caught up in wanting bigger, leafier plants. To avoid the issue, Henderson recommends using just 50 percent of the recommended dose of your favorite fertilizer

Credit: laksena/Shutterstock


For Marino, the issue she was eager to add was damage from mealybugs. “Ever see white powdery material on your plants that seems to appear out of nowhere?” she asked. “It looks like fungus but it might be mealybug colonies. These insects are white and produce a white protective powdery substance to nest in. You’ll usually find these nests in protected areas of the plant like the undersides of leaves.”

Thankfully, she says that mealybugs are generally easy to deal with. “They are susceptible to most pesticides, horticultural oils, insecticidal soaps, alcohol wipes, and systemic pesticides,” Marino says. “Give your plant a complete wipe-down, making sure to get underneath the leaves and into all the nooks and crevices where stems meet.” This wipe-down is intended to help your pesticide be as effective as possible. “Dilute your pesticide with water before using, then spray your plant—practically drench it. We’re talking the undersides of the leaves and every possible location where mealybugs can hide,” Marino says. And be thorough, she urges. “Remember that un-thorough cleaning, of any plant pest, can lead to re-infestation.”

At the end of the day, though, one important, oft-forgotten step for a plant owner struggling with an undiagnosed issue is to be kind to yourself. “Pests and other issues happen to even the most seasoned plant parents,” says Marino. “It’s not a sign you’re a cursed plant owner. Plus, these pests are almost always only interested in your plants. There’s no need to fret about yourself, your pets, or your furniture.”

Above all, Henderson’s best advice is to pay attention to your plant, because it’s doing its very best to tell you what it needs. “There’s no one-size-fits-all solution in nature, but if you see yellowing leaves, browning, or extreme leggy stretching, that’s your plant communicating, ‘Hey, something is wrong! Come check me out!’” If you’re struggling to figure out the issue, “lots of bright indirect sun, humidity, good drainage, and supplemental lighting are just a few ways you can make a good plant environment,” she says.