# Cannabis root problems: the master decision tree

> Healthy cannabis roots are white to cream, firm and smell earthy or of nothing. Brown, slimy roots whose outer skin slides off and smell rotten are root rot (usually Pythium), driven by warm, oxygen-poor water or waterlogged media. Firm, odourless tan roots are usually stained, not sick. Check solution temperature (target 18–21 °C) and dissolved oxygen first, then texture and smell, then insects. Root problems show above ground as drooping with a wet root zone, yellowing and stunting.

- Canonical: https://genetixbay.com/knowledge/diagnosis/root-problems-decision-tree.html
- Category: Diagnosis · Level: Intermediate
- Evidence: Supported · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Seedling, Veg, Flower · Systems: Soil, Coco, Rockwool, DWC, RDWC, NFT, Ebb and flow, Drip, Aeroponics, Living soil
- 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

**What do healthy cannabis roots look like?** Healthy cannabis roots are white to cream, firm, branched and covered in fine root hairs, with bright white growing tips. They smell earthy in soil and [coco](https://genetixbay.com/knowledge/glossary/coco-coir.md), and of nothing or slightly sweet in hydro. Firm roots tinted tan or brown by nutrients, tannins or additives are usually fine. [Brown roots](https://genetixbay.com/knowledge/glossary/root-rot.md) that are soft, slimy, smell rotten and lose their outer skin when pulled are rotting.

**Why are my cannabis roots brown and slimy?** Brown, slimy cannabis roots whose outer layer slips off between your fingers, often with a rotten or sewer smell, are root rot, most commonly [Pythium](https://genetixbay.com/knowledge/glossary/pythium.md) in hydro and overwatered media. Warm solution above about 22 to 24 °C, low dissolved oxygen and stagnant or overwatered media are the usual causes. Cool the solution to 18 to 21 °C, raise aeration, remove dead roots and clean the system.

**How warm is too warm for a cannabis reservoir?** A cannabis hydro reservoir is too warm above about 22 to 24 °C (72 to 75 °F). Warm water holds less oxygen, about 9.1 mg/L at 20 °C versus 8.3 mg/L at 25 °C at saturation, while roots and microbes use more, so Pythium root rot becomes much more likely. Aim for 18 to 21 °C (65 to 70 °F) with strong aeration.

**How do I know if my cannabis is [root-bound](https://genetixbay.com/knowledge/glossary/root-bound.md)?** A cannabis plant is root-bound when roots circle the outside of the root ball in a dense mat, water runs straight through, the pot dries out in hours, and growth slows despite good feed and light. Slide the plant out to check. Up-pot into a container two to three times the volume, gently loosening circling roots, or increase watering frequency if you plan to finish in that pot.

**Can a cannabis plant recover from root rot?** A cannabis plant can recover from root rot if it is caught early and the conditions are corrected. Cool the solution to 18 to 21 °C, raise dissolved oxygen, trim away dead brown roots, change and clean the reservoir, and re-establish beneficial microbes or commit fully to a sterile system. Recovery shows as new bright white root tips within 5 to 10 days.

**What happens if my [aeroponic](https://genetixbay.com/knowledge/glossary/aeroponics.md) pump fails?** If a high-pressure aeroponic pump or timer fails, cannabis roots hanging in air can dry and be damaged within tens of minutes, and the plant wilts soon after. Treat it as an emergency: restore misting, or mist roots by hand with nutrient solution until it is fixed. A pressure alarm, accumulator and backup pump are the only reliable protection.

**Short answer.** Healthy cannabis roots are white to cream, firm and smell of earth or of nothing. Brown, slimy roots whose outer skin slides off and smell rotten are root rot, usually *Pythium* [1][2]. Firm, odourless tan roots are usually just stained. Check solution temperature and dissolved oxygen first (target 18–21 °C / 65–70 °F, strong aeration) [3][4], then root texture and smell, then insects. Above ground, root trouble looks like drooping with a wet [root zone](https://genetixbay.com/knowledge/glossary/root-zone.md), yellowing and stunting.

## What's happening?

Roots need oxygen to respire, and respiration powers nutrient and water uptake [11]. Anything that starves roots of oxygen (waterlogged media, warm or stagnant solution, a failed air pump) stops uptake within hours and weakens the root cells. Oomycetes such as *Pythium* and fungi such as *[Fusarium](https://genetixbay.com/knowledge/glossary/fusarium-wilt.md)* then invade the weakened tissue; both were isolated from rotting roots of hydroponic cannabis in British Columbia [1]. *Pythium* swims between plants in recirculating water, so one sick plant becomes a whole system [3][4].

Water temperature matters twice. Warm water holds less oxygen: about 9.1 mg/L at 20 °C (68 °F) and 8.3 mg/L at 25 °C (77 °F) at full saturation in fresh water [5]. At the same time, roots and microbes use oxygen faster as temperature rises, and the warm-loving *Pythium* species grow faster [3]. That is why hot reservoirs and summer heat waves bring root rot.

Not every root problem is a pathogen. Roots also suffer from salt burn (high [EC](https://genetixbay.com/knowledge/glossary/ec.md) kills the tips), being root-bound, insects ([root aphids](https://genetixbay.com/knowledge/glossary/root-aphid.md), [fungus gnat](https://genetixbay.com/knowledge/glossary/fungus-gnat.md) larvae) and, in high-pressure aeroponics, simply drying out when the misting stops [7][8][10].

## How do I find the root problem? (the decision tree)

1. **What does the plant do above ground?**
   - Droops while the medium or solution is wet → root zone is short of oxygen, or roots are rotting. Go to step 2.
   - Droops while the pot is light and dry → underwatering, not a root disease ([drooping](https://genetixbay.com/knowledge/diagnosis/drooping-overwatering-vs-underwatering.md)).
   - Slow, stunted, no droop → go to step 5 (root-bound, insects, cold).
   - Sudden collapse of the whole plant in HPA or aeroponics → step 6, now.
2. **Look at and touch the roots.** In hydro, lift the net pot. In soil or coco, slide the plant out of the pot.
   - White to cream, firm, fine hairs, white tips → roots are healthy; the problem is elsewhere ([pH](https://genetixbay.com/knowledge/glossary/ph.md), EC, climate, light).
   - Tan to brown but firm, tips still white, no smell → **staining** by nutrients, tannins or additives. Usually harmless ([root browning in hydro](https://genetixbay.com/knowledge/diagnosis/root-browning-hydro.md)).
   - Brown, soft, slimy, outer skin slides off leaving a thin white core thread → **root rot**, most often *Pythium* [1][3].
   - Brown or reddish discolouration at the crown and stem base, plant wilting, often in soil → **Fusarium crown or root rot** [1][6].
   - Roots short with brown, burnt tips but firm elsewhere → **salt burn** from high EC.
3. **Do the smell test.** Put your nose to the roots or the reservoir.
   - Earthy or nothing → healthy.
   - Sour, rotten, sewer or swampy → **anaerobic rot**. Rot plus smell plus wet medium is the strongest root-rot signal there is.
4. **Take the readings.**
   - Solution temperature above ~22–24 °C (72–75 °F) → **hot reservoir** is the driver [3][4].
   - Dissolved oxygen below ~5–6 mg/L, or air pump off, stones clogged, little surface agitation → **low dissolved oxygen**.
   - Soil or coco: pot stays heavy for more than 3 days, or water pools on top → **overwatering anoxia**.
   - [Runoff EC](https://genetixbay.com/knowledge/glossary/runoff-ec.md) far above input (more than ~0.5 mS/cm) → **[salt build-up](https://genetixbay.com/knowledge/glossary/salt-build-up.md)** ([runoff testing](https://genetixbay.com/knowledge/ph-ec/runoff-testing.md)).
5. **Look for insects and root-bound roots.**
   - Small pear-shaped insects, often with white waxy residue, on roots or at drainage holes → **root aphids** [7] ([root aphids](https://genetixbay.com/knowledge/pests/root-aphids.md)).
   - Thin, clear larvae with shiny black heads in the top few cm, adults flying over wet media → **fungus gnat larvae**, which chew root hairs of seedlings and clones and open wounds for *Pythium* [8] ([fungus gnats](https://genetixbay.com/knowledge/pests/fungus-gnats.md)).
   - A dense mat of roots circling the outside of the root ball, water runs straight through → **root-bound** [9].
   - Few roots, cold medium below ~18 °C (65 °F) → **cold root zone**.
6. **HPA or aeroponics: is the mist running?** No pressure, blocked nozzles or a failed timer → **pump failure: critical**. Roots can be damaged within tens of minutes of drying [10]. Restore misting first, diagnose later ([HPA aeroponics](https://genetixbay.com/knowledge/systems/hpa-aeroponics.md)).

## How do I tell root problems apart? (differential table)

| Pattern | Most likely | Rule out first | Key test |
|---|---|---|---|
| Brown, slimy, skin slides off, rotten smell | Pythium root rot [1][3] | Staining | Pull a root between fingers; smell test |
| Tan or brown, firm, odourless, white tips | Staining | Early rot | Texture and smell; new white tips present? |
| Wilting with wet medium, no rot yet | Low oxygen (overwatering, low DO) | Fusarium wilt | Lift the pot; DO meter or check air pump |
| Reservoir above 24 °C, rot spreading | Hot reservoir driving Pythium [3] | — | Thermometer at the warmest time of day |
| Brown crown and stem base, soil grow | Fusarium crown rot [6] | Pythium, physical damage | Cut the stem base: brown internal streaks; lab test |
| Burnt brown root tips, roots firm | Salt burn | Rot | Runoff or reservoir EC vs target |
| White woolly residue, small insects on roots | Root aphids [7] | Root mealybugs, perlite dust | 10–20x lens |
| Larvae with black heads in top 5 cm | Fungus gnats [8] | Shore flies | Potato slice trap for 48 h; yellow card |
| Dense circling root mat | Root-bound [9] | Root aphids | Slide plant out of the pot |
| Roots drying in an aero chamber | Pump or nozzle failure (critical) | — | Pressure gauge; listen for misting |
| Few roots, slow, cold medium | Cold root zone | Overwatering | Probe thermometer in the medium |

## Which numbers matter?

| Parameter | Target or threshold | Notes |
|---|---|---|
| Solution temperature, hydro | 18–21 °C (65–70 °F); trouble above ~22–24 °C (72–75 °F) | [3][4]; see [root rot](https://genetixbay.com/knowledge/diseases/root-rot-pythium.md) |
| DO at saturation, fresh water | ~9.1 mg/L at 20 °C; ~8.3 mg/L at 25 °C; ~7.6 mg/L at 30 °C | Physical constants [5] |
| DO target, hydro | Above ~5–6 mg/L, ideally near saturation | Extension practice [3]. |
| Root-zone temperature, soil and coco | ~18–22 °C (65–72 °F) | Grower practice |
| Runoff EC above input | More than ~0.5 mS/cm = salt build-up | Grower practice |
| Soil/coco pot weight | Water when the pot is noticeably light; heavy for 3+ days = overwatering | Grower practice |
| HPA pressure | ~80–100 psi (5.5–7 bar) | See [HPA](https://genetixbay.com/knowledge/systems/hpa-aeroponics.md) |
| HPA failure margin | Roots damaged within tens of minutes without mist | [10]; grower practice |
| Up-pot size | 2–3x the current volume | Grower practice; growth scales with [rooting volume](https://genetixbay.com/knowledge/glossary/pot-size.md) [9] |

## What should I check first?

1. **Plant behaviour vs root-zone moisture.** Wet and wilting is a root-zone emergency; dry and wilting is thirst.
2. **Solution temperature** at the warmest point of the day.
3. **Air supply.** Pump running, stones bubbling evenly, surface agitated, no clogged lines.
4. **Root colour, texture and smell.**
5. **Reservoir or runoff EC and pH** with calibrated meters.
6. **A lens on the roots** and the top 5 cm of the medium for insects.
7. **Pot weight and drainage** in soil and coco.

## How do I fix each one?

- **Pythium root rot.** Cool the solution to 18–21 °C (65–70 °F) with a chiller, frozen bottles in an emergency, or moving the reservoir off the warm floor. Raise dissolved oxygen: bigger pump, more stones, surface agitation. Trim dead, slimy roots with sanitised scissors. Drain, clean and sanitise the system, then refill with fresh solution. Then choose: re-establish a [beneficial-microbe reservoir](https://genetixbay.com/knowledge/glossary/beneficial-microbe-reservoir.md) (the GenetixBay preference, because bacteria arrive regardless) or commit fully to a sterile reservoir, keeping in mind that oxidisers such as [hydrogen peroxide](https://genetixbay.com/knowledge/glossary/hydrogen-peroxide.md) kill beneficial microbes too [3][4]. See [root rot and Pythium](https://genetixbay.com/knowledge/diseases/root-rot-pythium.md).
- **Staining.** Nothing to fix; keep the solution cool and the oxygen high.
- **Low dissolved oxygen.** Increase air pump capacity, clean or replace stones, raise surface agitation, and lower solution temperature.
- **Hot reservoir.** Insulate it, shade it, keep it off warm floors, and chill to 18–21 °C.
- **Overwatering anoxia (soil, coco).** Stop watering until the pot is noticeably light; improve drainage (more perlite, fabric pots, no saucers of standing water). Water to 10–20% runoff and let it dry back.
- **Fusarium crown rot.** There is no cure for an infected plant. Remove it with its medium, sanitise pots and tools, and avoid root-zone stress in the rest [1][6]. See [Fusarium](https://genetixbay.com/knowledge/diseases/fusarium.md).
- **Salt burn.** Do a root-zone flush (the one valid flush: it treats the medium, not the plant) with pH-adjusted water to 20–30% runoff until runoff EC is near input EC, then resume a slightly lower feed. In hydro, replace the reservoir at target EC.
- **Root aphids.** See [root aphids](https://genetixbay.com/knowledge/pests/root-aphids.md); soil-applied entomopathogenic fungi and predators are the main tools [7].
- **Fungus gnats.** Fix watering first, then Bti drenches, *Steinernema feltiae* nematodes or soil predatory mites, with yellow cards to monitor [8].
- **Root-bound.** Up-pot into a container 2–3x the volume and loosen circling roots gently. If you plan to finish in a small pot, water more often instead: big plants can come from small pots if you push water and feed frequently (house position on pot size) [9].
- **Cold root zone.** Warm the medium to ~20 °C (68 °F) with a heat mat under the pot or by raising pots off a cold floor.
- **HPA pump failure.** Restore misting immediately; hand-mist roots with nutrient solution until fixed. Fit an accumulator, a pressure alarm and a spare pump.

## How do I know the fix worked?

- New bright white root tips appear within 5–10 days after rot or anoxia is corrected.
- Wilting with a wet root zone stops within 1–3 days of correcting oxygen or watering.
- The reservoir smells clean at the next change.
- Runoff EC falls within ~0.5 mS/cm of input after a salt flush.
- Insect counts fall on cards and traps week on week.
- Up-potted plants push faster node growth within 7–10 days.

## What are the common mistakes?

- Adding nutrients or "root boosters" to rotting roots. They cannot absorb, and EC climbs.
- Dosing hydrogen peroxide into a reservoir that relies on beneficial microbes, then reseeding them the same day.
- Calling stained roots rot, then tearing healthy roots out.
- Watering a wilting, wet plant because it "looks thirsty".
- Ignoring the reservoir temperature at the warmest time of day; a morning reading misses afternoon peaks.
- Running HPA without a pressure alarm or backup.
- Up-potting a seedling straight into a huge pot that then stays wet (it causes the overwatering it was meant to prevent).

## How strong is the evidence?

- *Pythium* and *Fusarium* causing root rot of hydroponic cannabis is *Established* [1][2].
- Warm, oxygen-poor solution favouring *Pythium* is *Established* in hydroponic crops [3][4]; cannabis-specific temperature thresholds are *Supported* by extrapolation.
- Dissolved oxygen solubility values are *Established* physics [5]; cannabis DO targets are *Grower practice*.
- Root aphids and fungus gnats harming cannabis roots are *Established* [7][8].
- Growth scaling with rooting volume is *Established* across species [9].
- HPA failure margins are *Supported* by aeroponics reviews [10] and grower practice.
- Beneficial-microbe reservoirs over sterile ones is the house position; both are valid if done properly.

## 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, Collyer D, Scott C, 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. Pythium root rot in hydroponic leafy greens and its management (E-706). https://greenhousehort.ca.uky.edu/sites/greenhousehort.ca.uky.edu/files/E706.pdf
4. Colorado State University Extension PlantTalk 2043. Root disease problems and management in hydroponic systems. https://planttalk.colostate.edu/topics/miscellaneous/2043-root-disease-problems-management-hydroponic-systems/
5. US Geological Survey. Dissolved oxygen solubility tables (DOTABLES). https://water.usgs.gov/water-resources/software/DOTABLES/
6. Utah State University Extension. Hemp Fusarium crown rot (IPM Notes). https://extension.usu.edu/planthealth/ipm/notes_ag/hemp-fusarium-crown-rot
7. Utah State University Extension. Rice Root Aphids (hemp). https://extension.usu.edu/planthealth/ipm/notes_ag/hemp-rice-root-aphid.php
8. UConn IPM. Managing Fungus Gnats in the Greenhouse. https://ipm.cahnr.uconn.edu/managing-fungus-gnats-in-the-greenhouse/
9. Poorter H, Bühler J, van Dusschoten D, Climent J, Postma JA. 2012. Pot size matters: a meta-analysis of the effects of rooting volume on plant growth. Functional Plant Biology 39(11):839-850. https://doi.org/10.1071/FP12049
10. Lakhiar IA, Gao J, Syed TN, Chandio FA, Buttar NA, 2018. Modern plant cultivation technologies in agriculture under controlled environment: a review on aeroponics. Journal of Plant Interactions 13(1):338-352. https://doi.org/10.1080/17429145.2018.1472308
11. Marschner P (ed.). 2012. Marschner's Mineral Nutrition of Higher Plants, 3rd ed. Academic Press. https://doi.org/10.1016/C2009-0-63043-9