# Brown roots in hydroponics (DWC, RDWC, NFT): a differential

> Healthy hydro roots are white to cream and firm. Brown roots that are still firm and odourless are often just stained by nutrients or additives; brown, slimy, smelly roots whose outer layer slips off are root rot, usually Pythium, favoured by warm, oxygen-poor solution. Check solution temperature and dissolved oxygen first, then root texture and smell.

- Canonical: https://genetixbay.com/knowledge/diagnosis/root-browning-hydro.html
- Category: Diagnosis · Level: Intermediate
- Evidence: Supported · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Seedling, Veg, Flower · Systems: DWC, RDWC, NFT, Ebb and flow, Aeroponics
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**Why are my DWC roots turning brown?** DWC roots turn brown either from harmless staining or from [root rot](https://genetixbay.com/knowledge/glossary/root-rot.md). Firm, branching, odourless brown roots with new white tips are usually stained by nutrients, organic additives or tannins. Soft, slimy, sour-smelling roots are root rot, most often Pythium, which research has isolated from hydroponic cannabis. Warm, oxygen-poor solution above about 23 to 24 °C is the usual enabler.

**What causes brown slimy roots in cannabis hydroponics?** Brown, slimy roots in cannabis hydroponics whose outer layer slips off between the fingers, leaving a thin thread, are Pythium root rot, sometimes with Fusarium. Research on hydroponic cannabis links these pathogens to root browning, stunting, yellowing and daytime wilting. Pythium zoospores swim through the solution, so in [recirculating systems](https://genetixbay.com/knowledge/glossary/recirculating-system.md) the infection spreads quickly between plants.

**How can I tell root rot from stained roots in hydro?** To tell root rot from stained roots in hydro, pull a root gently between finger and thumb and smell it. Stained roots stay firm, branch normally, have no odour and still show new white tips, and the plant above is healthy. Rotting roots feel soft and slimy, smell sour, and their outer cortex slides off while the plant wilts by day.

**What water temperature prevents root rot in cannabis hydro?** A nutrient solution temperature of about 18 to 22 °C (65 to 72 °F) helps prevent root rot in cannabis hydroponics. Extension guidance shows cool solution holds more dissolved oxygen and reduces Pythium zoospore survival, while research in other hydroponic crops found risk rises above about 23 to 24 °C (73 to 75 °F). Keep dissolved oxygen near saturation as well.

**Why does my hydro reservoir smell and my cannabis plants wilt?** A smelly hydro reservoir with wilting cannabis plants usually means root rot in warm, oxygen-poor solution, often with bacterial slime and biofilm. Cool the solution toward 18 to 22 °C, clean or replace [air stones](https://genetixbay.com/knowledge/glossary/air-stone.md) and add pump capacity, change the reservoir and clean surfaces. A [peroxide](https://genetixbay.com/knowledge/glossary/hydrogen-peroxide.md) dose alone will not fix it if the solution stays warm.

**Short answer.** Healthy hydro roots are white to cream, firm and fuzzy with fine [root hairs](https://genetixbay.com/knowledge/glossary/root-hair.md). Brown roots that are still firm, branching and odourless are often just stained by nutrients, organic additives or tannins. Brown, slimy, smelly roots whose outer layer slides off between your fingers are root rot, most often Pythium, sometimes with Fusarium [1][2]. Warm, oxygen-poor solution is the usual enabler. Check solution temperature and dissolved oxygen first, then texture and smell, then act fast: in recirculating systems it spreads through the water.

## What's happening

Pythium species produce swimming zoospores that move through nutrient solution and infect root tips. In hydroponically grown cannabis in Canada, Punja and Rodriguez isolated *Pythium dissotocum*, *P. myriotylum* and *P. aphanidermatum*, and *Fusarium oxysporum* and *F. solani*, which caused root browning, crown and pith discolouration, stunting, yellowing and wilting [1][2]. Warm solution holds less oxygen and favours the more aggressive, warm-loving Pythium species (in hydroponic spinach, ~23 °C marked the shift between *P. dissotocum* and *P. aphanidermatum* dominance [4]). Extension guidance for hydroponic crops is to keep solution cool (~20–22 °C) and near oxygen saturation, which reduces zoospore survival and infection [3].

[HLVd](https://genetixbay.com/knowledge/glossary/hop-latent-viroid.md) also accumulates in roots and spreads in shared hydroponic water [5]; it does not brown roots by itself but is worth knowing when plants share a system.

## How to recognise it

| Root appearance | Smell | Plant above | Most likely |
|---|---|---|---|
| White/cream, firm, fuzzy tips | Earthy or none | Healthy | Fine |
| Tan/brown but firm, branching, new white tips present | None | Healthy | Staining (nutrients, organics, tannins) |
| Brown, soft, slimy; outer cortex slips off leaving a thin thread | Sour, rotten | Wilting by day, yellowing, stunting | Pythium root rot [1] |
| Brown roots plus discoloured crown and brown pith in stem | Variable | Wilt, yellowing | Fusarium (often with Pythium) [1] |
| Roots matted in a brown, gelatinous mass; biofilm on surfaces | Rotten | Decline | Rot plus bacterial slime; low DO |
| Few roots, tips dead, crust on roots | None | [Burnt tips](https://genetixbay.com/knowledge/glossary/nutrient-burn.md) | High EC / salt damage |
| Browning only where light hits roots, green algae | Earthy | Fine | [Light leak](https://genetixbay.com/knowledge/glossary/light-leak.md) into reservoir |

## Numbers that matter

| Parameter | Target | Notes |
|---|---|---|
| Solution temperature | ~18–22 °C (65–72 °F) | [3]; above ~23–24 °C (73–75 °F) risk rises [4] |
| Dissolved oxygen | as close to saturation as possible; ~8 mg/L at ~25 °C | [3] |
| pH, hydro | 5.5–6.0 | Grower practice and cited literature |
| EC, veg DWC | ~1.2–1.8 [mS/cm](https://genetixbay.com/knowledge/glossary/ms-cm.md) | Llewellyn et al. used 1.8 dS/m [6] |
| Reservoir change interval | 7–14 days | Grower practice; weekly in [6] |
| Light into reservoir | none | Algae and root browning |

## What to check first

1. **Solution temperature** at the warmest time of day.
2. **Dissolved oxygen** (meter), or at least: are the air pump and stones working and bubbling evenly?
3. **Smell** of reservoir and roots.
4. **Texture.** Pull a root gently between finger and thumb. Firm = probably staining. Outer layer slips = rot.
5. **New tips.** White new root tips among brown roots = staining or a recovering system.
6. **Plant symptoms.** Daytime wilting, yellowing, stunting on plants with brown roots = rot.
7. **Additives.** Organic teas, molasses, humic/fulvic products stain roots and can feed microbes.
8. **Light leaks** into the reservoir or [net pot](https://genetixbay.com/knowledge/glossary/net-pot.md).

## How to fix it

- **Cool the solution** to ~18–22 °C (chiller, insulation, bigger reservoir volume, keep the reservoir off warm floors).
- **Maximise oxygen:** clean or replace air stones, add air pump capacity, keep return lines splashing in [RDWC](https://genetixbay.com/knowledge/glossary/rdwc.md).
- **Change the reservoir** and clean surfaces of biofilm.
- **Trim** dead, slimy root masses with sterilised scissors only if they are clearly detached; leave anything firm.
- **Water treatment classes** used in commercial hydroponics: chlorination, ozonation, copper/silver ionisation, UV and oxidisers such as hydrogen peroxide. Extension sources warn that some alter pH or nutrient availability and many harm beneficial microbes [3]. Check local regulations and the label for your crop.
- **Beneficial microbes** (e.g. root-colonising *Bacillus* and *Trichoderma* products) are used as a preventive in clean systems; evidence in cannabis is limited. See [root rot and Pythium](https://genetixbay.com/knowledge/diseases/root-rot-pythium.md).
- **Isolate** badly affected plants in RDWC by removing them from the shared circuit.
- **Sanitise between runs.** See [sanitation and prevention](https://genetixbay.com/knowledge/diseases/sanitation-and-prevention.md).

## How to confirm it worked

- New white root tips appear within 3–7 days.
- Plants stop wilting in the afternoon.
- The reservoir no longer smells and DO stays near saturation at the warmest point of the day.

## Common mistakes

- Panicking over firm, stained roots and dosing harsh oxidisers that kill healthy root tips.
- Fixing smell with a peroxide dose but leaving the solution at 26 °C.
- Adding sugar-rich organic additives to DWC.
- Sharing one recirculating system across [mother plants](https://genetixbay.com/knowledge/glossary/mother-plant.md) and production plants (pathogens and HLVd spread through water).

## Evidence

- Pythium and Fusarium as root pathogens of hydroponic cannabis: *Established* [1][2].
- Temperature and oxygen effects on Pythium: *Established* in other hydroponic crops [3][4]; extrapolated to cannabis.
- Water treatment efficacy in cannabis systems: *Grower practice*; little cannabis-specific data.

## Sources

1. Punja ZK, Rodriguez G., 2018. Fusarium and Pythium species infecting roots of hydroponically grown marijuana (Cannabis sativa L.) plants. Canadian Journal of Plant Pathology 40(4):498-513. https://doi.org/10.1080/07060661.2018.1535466
2. Punja ZK, et al., 2019. Pathogens and Molds Affecting Production and Quality of Cannabis sativa L. Frontiers in Plant Science 10:1120. https://doi.org/10.3389/fpls.2019.01120
3. University of Kentucky Cooperative Extension, E706 (hydroponic Pythium and oxygen management). https://greenhousehort.ca.uky.edu/sites/greenhousehort.ca.uky.edu/files/E706.pdf
4. Phytopathology 75:333 (1985) abstract on Pythium species and solution temperature in hydroponic spinach. https://www.apsnet.org/publications/phytopathology/backissues/Documents/1985Abstracts/Phyto75_333.htm
5. Adkar-Purushothama CR, Sano T, Perreault JP., 2023. Hop Latent Viroid: A Hidden Threat to the Cannabis Industry. Viruses 15(3):681. https://doi.org/10.3390/v15030681
6. Llewellyn D, Golem S, Jones AMP, Zheng Y., 2023. Plants 12(3):422 (DWC method reference). https://doi.org/10.3390/plants12030422