Root Rot (Pythium): How to Spot, Fix and Prevent It

Root rot is what growers call roots that turn brown, slimy and smelly and stop taking up water. The usual culprit is pythium, a water mould that thrives in warm, stale water, so it is best known in hydroponics, but overwatered soil and coco get it too. This guide covers how to spot it, prevent it and fix it.

What root rot looks like

• Healthy roots are white or cream, firm and finely haired, with little smell.
• Rotting roots are brown, soft and slimy, and smell sour. Pull one gently and the outer layer often slides off, leaving a thin thread.
• Above the roots, the plant wilts although the medium is wet or the tank is full, growth slows and the lower leaves yellow.

What causes root rot

Pythium is widespread, so you cannot count on keeping it out. It takes hold where roots are weak and short of oxygen:

• Warm nutrient solution. Warm water holds less oxygen and suits pythium, so it is mostly a summer problem.
• Still water. Solution that is not moving or aerated soon runs short of oxygen.
• Light in the tank. Light grows algae, which clogs the system and rots when it dies.
• Dead roots. Old roots left in pots, pipes and tanks give disease somewhere to start.
• Overwatering. In soil and coco, a pot that never dries back leaves the roots short of air.

How to prevent root rot in hydroponics

• Keep the solution cool. The Hailea Water Chillers page says to keep it at 18-21°C, for the most oxygen and to help keep pythium at bay. A meter such as the Aqua Master Combo Pen P110 Pro reads temperature as well as pH and EC.
• Add air. An air pump and air stone keep the solution moving and oxygenated. Tetra APS Airpumps come in three sizes with adjustable airflow, and the Air Stones page says the stream of bubbles helps prevent anaerobic conditions and root rot.
• Keep light out. Lid on the tank, every gap covered. On NFT trays, Correx top plates shade the roots, which their page says helps reduce algae.
• Change the tank. Empty and remix it regularly - weekly is the usual rule - rather than only topping up (see flood and drain systems).
• Clean between crops. Take out every old root, then flush the tank, pump and pipework: the Silver Bullet Roots page gives 2 ml per litre for this. Wipe down trays, pots and tools too (see cleaning products).

Sterile tank or beneficial microbes: pick one

• Sterile. A cleaner in the tank kills what lives there, good and bad: Silver Bullet Roots or Liquid Oxygen.
• Microbes. You fill the root zone with helpful bacteria and fungi, so harmful ones find less room: Great White in soil, coco or hydroponics, Plant Magic Oldtimer Granules in soil or coco. Our root stimulants guide compares more microbial products; you will find them under root products.

Do not mix the two: the Oldtimer Granules page says peroxide-based products will kill or severely inhibit the micro-life. The Cannazym page says enzymes should not be used with liquid oxygen (hydrogen peroxide) either. Plant Magic Enzyme is the exception: its page says it is not bacteria-based, so it can be used alongside peroxide products. Check any other additive's label before it goes in a sterile tank.

Which one? As a rule of thumb, sterile suits a recirculating tank and microbes suit soil and coco.

Root health products we sell

How to fix root rot in hydroponics, step by step

1. Check the roots. Lift a plant, look and smell.
2. Cool the tank. Measure it. If it is above the Hailea page's 18-21°C, a chiller is the lasting fix.
3. Empty and clean. Drain and clean the tank, pump and pipework (the 2 ml per litre flush above is for between crops). Rinse the roots and take off the dead ones that come away easily.
4. Refill and treat. Mix fresh feed. The Silver Bullet Roots page gives 1 ml per litre as a shock treatment for root rot. The other route is CX Wilt Guard with CX Regen A Root, each at 1 ml per litre: the Wilt Guard page says the pair has saved crops from pythium. Use one route, not both: the pages do not say they can share a tank. Follow the label, and leave microbes and enzymes (Plant Magic Enzyme excepted) out while peroxide is in the tank.
5. Add air. Fit an air stone if there is none, and keep the lid on.
6. Watch the new roots. Judge the fix by fresh white roots and a plant that stops wilting.
7. After the crop, strip the system, flush it and start with fresh media (our clay pebbles and rockwool guide covers reuse).

Root rot in soil and coco

In pots the usual cause is too much water. If a plant is wilting in a wet pot, stop watering until the pot feels light, then:

• Water by weight. Lift the pot and water again when it feels light, and never leave it standing in run-off. Coco should not dry right out, though: see our coco guide. A small plant in a big pot stays wet longest.
• Choose the pot. The 5l Air-Pot page says its drainage and aeration reduce the risk of overwatering and root rot, and fabric pots drain well too: our pots guide compares them.
• Open up the mix. If pots take too long to dry, mix Perlite through before you pot up; it will not fix watering too often (see perlite and vermiculite).
• Clear dead roots. The Cannazym page says it breaks down dead root matter, and calls enzymes vital if you reuse coco. After root rot, start with fresh coco.

Quick answers

Can a plant recover from root rot?

Sometimes. Rotten roots do not come back, but a plant caught early can grow new white ones once the cause is fixed. No product can promise it.

Are brown roots always root rot?

No. Some feeds and additives stain roots brown. Stained roots stay firm and smell clean; rotten ones are slimy and smell sour.

Does hydrogen peroxide treat root rot?

It can clean up the system, but it cannot bring rotten roots back. The Liquid Oxygen page says its hydrogen peroxide solution helps prevent pythium by keeping the system clean and oxygen-rich. Peroxide kills beneficial microbes too, so pick one approach and use it at the label rate.

More Growers' Guides

• Flood and drain systems
• Root stimulants and rooting gels
• Fabric, Air-Pot or plastic pots?
• Hydroponics: the systems explained
• Growers' glossary

Not sure what you need? Call the shop on 0117 971 3000 or pop in to the Bristol store and we'll help you pick.