Eliminate Mildew on Hydroponic Plants

Stop fungal diseases from damaging your indoor crop with these safe but effective tactics.


The warm, damp conditions in an indoor grow room are a breeding ground for fungus and slime. Mold, mildew, and other fungi are not only ugly and foul-smelling, they can ruin your harvest literally overnight. You might be tempted to reach for noxious chemicals whenever you see them, but there’s no need to risk your plants’ and your health (or to pollute the environment) to keep these problems under control. Take a few sensible precautions and rely on natural formulas, and you can live without fear of fungi.

Blow It Away

Fact: When air stagnates, fungal spores can begin colonizing on the droplets of moisture that settle on every surface. Nothing works better than steady airflow to dry up dampness and keep the spores in motion so they can’t germinate.

Do: A ventilation system in your grow room is ideal, but if you don’t have that, a small oscillating fan or two can dry off leaves and other surfaces where fungi settle.

Control Humidity

Fact: The ideal humidity level for healthy indoor plant growth is 45 percent to 50 percent, which is not enough for most fungi to survive. With a hygrometer, you can measure the humidity in your grow room.

Do: When humidity levels are consistently much higher than ideal, consider using a portable dehumidifier in the room. If running one throughout the growing cycle is too costly, a dehumidifier is especially valuable during the bloom stage, when all your effort can be ruined if gray mold (Botrytis blight) infects the buds, turning them mushy and funky-smelling before you can harvest them.

Clean Routinely

Fact: Dead leaves, spilled water and nutrients, dust, and dirt all play host to fungi. In just a few hours, spores on these seemingly minor messes begin to germinate.

Do: Discard dead leaves and the like in your compost pile. Wipe up spills and dirt right away. For extra protection, use an organic fungicide on any surface where a mess lingered long enough for spores to settle. If you recycle containers from one grow cycle to the next, be sure to clean them diligently and use the fungicide to ensure no spores remain on them.

Keep Outside Out

Fact: When you are outside, fungal spores—as well as insect pests—latch onto your clothes, shoes, hands, tools, and just about anyplace else they can. You won’t notice them, but they will come into your grow room with you.

Do: Avoid working in your grow room directly after being in an outdoor garden. If possible, wear separate clothes and shoes when working in your grow room. Thoroughly clean your hands and tools before they touch your indoor plants.

Feed Roots

Fact: Leaf surfaces do not absorb water as quickly as roots do, and wet leaves are inviting to several different types of disfiguring fungi.

Do: Skip foliar feeding, or applying nutrient solution to leaves. It can be helpful for outdoor plants, but in an indoor environment, the risks of inviting fungi far outweigh any benefits you gain from foliar feeding.

Act Fast

Fact: Fungi spread quickly, moving from one plant to the next, often in less than a day.

Do: Be swift and ruthless. If one of your plants has fungus on it, immediately isolate it from all the others. This can be hard when you’ve invested a lot of time and effort in nurturing the plant, but you must separate it completely from the rest of your crop. You can try to rescue it with applications of fungicide, but do it in a different room. Or just compost it and take the loss.

Target Gnats

Fact: Fungus gnats are tiny (1/8 of an inch), winged insects that live and breed around the roots of plants. The (wingless) larvae feed on fungi and plant roots. Even worse, the wounds produced by larvae are vectors of root rot fungi, meaning they carry it from plant to plant. And when the gnats die, they leave behind a sludge that harbors new fungus gnats.

Do: Capture fungus gnats by setting up sticky traps around your grow room. For severe infestations, target the pests with a natural insecticide with potassium salts of fatty acids (soap) to weaken their protective outer shells and pyrethrins (extracted from chrysanthemums) to kill them.

Remove Slime

Fact: Like fungus, slimy green algae grows in moisture, but it needs light rather than dark conditions. Algae does not harm plants directly, but when it decays, fungus gnats show up to feed on it.

Do: Use opaque containers for your nutrient solution to block the light that allows algae to grow. Also shield your growing media from light if possible—the moisture and direct light can foster algae growth on rock wool, clay pellets, and the like. Use a nontoxic soap spray to remove algae wherever it shows up in your grow room.