Cannabis root problems: the master decision tree

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, 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 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 kills the tips), being root-bound, insects (root aphids, fungus gnat 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). - 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, 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). - 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.being reviewed - Soil or coco: pot stays heavy for more than 3 days, or water pools on top → overwatering anoxia. - Runoff EC far above input (more than ~0.5 mS/cm) → salt build-up (runoff testing).
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). - 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). - 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).

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

Which numbers matter?

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 (the GenetixBay preference, because bacteria arrive regardless) or commit fully to a sterile reservoir, keeping in mind that oxidisers such as hydrogen peroxide kill beneficial microbes too ^[3][4]. See root rot and Pythium.
• 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.
• 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; 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.being reviewed
• 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.