Root Rot in Hydroponics: Signs, Causes, Treatment and Prevention
Few things worry a hydroponic grower more than lifting a lid and finding brown, slimy roots. Root rot can spread through a shared reservoir in a matter of days, but it is not random. It nearly always comes down to a handful of conditions in the root zone, and once you know what they are it is one of the easier problems to prevent.
This guide covers how to tell healthy roots from rotting ones, what causes root rot in hydroponics, how to treat an outbreak and the routine that stops it coming back.
What is root rot?
Root rot is the name for a set of symptoms rather than a single disease. In hydroponic systems the usual culprit is Pythium, a water mould that lives in water and growing media and attacks roots that are already stressed. Other fungi and bacteria can move in once roots start to break down. Because the nutrient solution reaches every plant, a problem in one pot can quickly reach every plant on the same reservoir.
Healthy roots vs root rot
| Healthy roots | Root rot |
|---|---|
| White to cream coloured | Brown, grey or tan |
| Firm, with plenty of fine root hairs | Soft and slimy; the outer layer slips off when gently pulled |
| Fresh, earthy smell or no smell | Sour, musty or rotten smell |
| New white tips growing | Few or no new tips |
Above the surface you may see plants wilting even though their roots are sitting in water, drooping in the middle of the light period, yellowing lower leaves, slow growth, and symptoms that look like a nutrient deficiency even when your feed is correct. Wilting in a wet root zone is the classic warning sign: roots that cannot breathe cannot take up water.
Tip: some nutrients and additives stain roots a tan or brown colour. If roots are discoloured but still firm, with no smell and plenty of white tips, they are probably fine. Feel and smell tell you more than colour.
What causes root rot in hydroponics?
1. Warm nutrient solution
Warm water holds less dissolved oxygen, and Pythium thrives in warm, low-oxygen water. Aim to keep your reservoir between 18 and 22°C. The risk rises sharply once the solution creeps above about 24°C, which is easy to reach in summer or when a reservoir sits in a hot grow room.
2. Not enough oxygen at the roots
Roots need oxygen as much as they need water. Stagnant solution, an undersized or failed air pump, clogged air stones, flood cycles that run too long, and pots that never fully drain all starve the roots and give pathogens an opening.
3. Light getting into the reservoir
Light in a reservoir feeds algae. Algae competes for oxygen, and when it dies it becomes food for the microbes that cause rot. Your reservoir, pipework and channels should all be lightproof.
4. Dirt and debris
Dead roots, plant debris, old nutrient residue and slimy biofilm in pipework all give pathogens somewhere to live. A system that was not cleaned properly between crops is one of the most common sources of infection.
5. Stressed plants
Big swings in pH or EC don't cause root rot on their own, but they weaken plants and make their roots easier to infect.
How to prevent root rot
- Check your water temperature. Keep a thermometer in the reservoir and check it daily. In summer, cool the room, keep the tank off warm floors and away from lights, and consider insulating it. In a cold garage or outbuilding in a UK winter, an aquarium-style heater on a thermostat stops the solution dropping too low. Our guide to temperature and humidity control covers the room itself.
- Keep the solution oxygenated. Run an air pump with air stones or an air curtain in reservoirs and deep water systems, and check they are still bubbling evenly. Clean or replace clogged air stones. You will find pumps, air stones and airline in our growing systems and pumps range.
- Make the reservoir lightproof. Use an opaque tank with a well-fitting lid and cover any gaps around pipes. Browse water tanks and reservoirs.
- Change the nutrient solution regularly. Many growers change their solution every 7 to 14 days and top up with plain, pH-adjusted water in between. Fresh solution keeps nutrients balanced and gives pathogens less time to build up.
- Get your flood timings right. In flood and drain systems, make sure pots drain completely between floods so the roots get air.
- Clean between every crop. Strip down and scrub the reservoir, pump and pipework, sterilise, then rinse thoroughly. Our guide to keeping a clean grow room covers the rest of the space.
- Look after the root zone. Enzyme products such as Bio Nova BN-Zym help break down dead root material before it can rot, and beneficial microbe products help crowd out harmful ones. Browse our beneficials and disease and pest control ranges.
How to treat root rot
Plants can recover from root rot if you catch it early. Act quickly:
- Find the cause first. Check the water temperature, air supply and any light leaks before anything else. If you treat the roots without fixing the cause, the rot will come back.
- Drain and clean the system. Empty the reservoir, scrub away any slime or biofilm, clean the pump and pipework, and rinse well.
- Trim the damaged roots. Gently rinse the roots and cut away brown, mushy sections with clean, sterilised scissors. Leave anything that is white and firm.
- Refill with fresh solution. Use correctly pH-adjusted solution at 18 to 22°C. Many growers run a slightly lower EC while plants recover.
- Treat the solution. A root treatment additive such as Silver Bullet Roots, which combines silver and hydrogen peroxide, is designed to clear pathogens from the solution. Use the shock dose on the label, then drop back to the maintenance dose.
- Boost aeration. Add an extra air stone or increase the air flow while plants recover.
- Watch for new white roots. Fresh white root tips within a week or so are the sign that plants are recovering. If a plant keeps declining, take it out so it does not reinfect the others.
Note: sterilising treatments based on hydrogen peroxide kill helpful microbes as well as harmful ones, so don't use them at the same time as beneficial bacteria. Once the system is clear, you can bring the beneficials back in to help keep it that way.
Root rot FAQs
Can plants recover from root rot?
Yes, if it is caught early and the cause is fixed. Plants with only some brown roots usually bounce back once the solution is cool, oxygenated and clean. If most of the root mass has turned to mush, recovery is unlikely.
What temperature should hydroponic water be?
Between 18 and 22°C suits most plants. Above about 24°C, oxygen levels fall and the risk of root rot climbs quickly. Much below 16°C, growth slows down.
Do brown roots always mean root rot?
No. Some nutrients and additives stain roots. Rotting roots are soft, slimy and smell bad, while stained roots stay firm and keep putting out white tips.
Can root rot spread between plants?
Yes. Wherever plants share a reservoir, pathogens travel in the solution. That is why it is worth treating the whole system rather than a single plant.
Need a hand?
If you are struggling with root problems, bring a photo to our shop and we will help you work out what is going on. Call 01744 382 066 or visit us at Unit 5A Rainford Industrial Estate, St Helens, WA11 8LS.