Short answer. Roots hang in a bucket of nutrient solution kept oxygenated by an air pump and air stones. It is fast and cheap to build, and unforgiving of two things: low dissolved oxygen (DO) and warm water. Keep solution at 18–22 °C (64–72 °F) and DO above ~6 mg/L (ideally 7–9 mg/L, near saturation) [1][2][4]. Run pH 5.6–6.0, EC 0.6–1.0 mS/cm (seedling) to 1.4–2.0 (flower). If the air pump stops, roots are in trouble within 2–12 hours [5].
How it works
Roots respire and need oxygen. Water holds little of it, and less as it warms (about 9 mg/L at 20 °C, 8 mg/L at 25 °C at sea level). Bubbles mix the water and keep it near saturation. Warm, low-oxygen solution suppresses roots and favours Pythium, which turns roots brown and slimy [1][2]. Leafy-green extension data show growth inhibition below ~4 mg/L DO and failure below ~3 mg/L [3][4]; cannabis-specific thresholds aren't published.
House position: living reservoir preferred. Bacteria will turn up in any reservoir whatever you do, so you may as well have the good ones: GenetixBay prefers a beneficial-microbe reservoir. A sterile reservoir is a valid choice, but only if you commit to it fully (continuous oxidiser or UV/ozone, clean top-ups, no organics) and never mix the two. It's a personal choice; the half-way house is what fails. See root rot and Pythium for both approaches.
Numbers that matter
| Parameter | Target | Alarm |
|---|---|---|
| Solution temperature | 18–22 °C (64–72 °F) | > 24 °C (75 °F) |
| Dissolved oxygen | ≥ 6 mg/L; 7–9 ideal | < 4 mg/L |
| pH | 5.6–6.0 (let it drift within band) | < 5.4 or > 6.4 |
| EC, seedling/clone | 0.6–1.0 mS/cm | |
| EC, veg | 1.0–1.6 mS/cm | |
| EC, flower | 1.4–2.0 mS/cm | |
| Air | ~1–2 L/min of air per 10 L solutionbeing reviewed | |
| Gap between net pot and water | 2–5 cm once roots are established | Roots above water get air |
| Reservoir refresh | Every 7–14 days or on mass-balance rules |
How to run it
- Opaque bucket (light causes algae), lid, net pot with clay pebbles, air pump sized for all buckets plus margin, air stones on the bottom.
- Start with the water touching the net pot base; lower it as roots grow.
- Top off with plain or quarter-strength water as plants drink; full refresh on schedule.
- Watch roots weekly: white and fuzzy is good, tan from nutrients is normal, brown slime with a swampy smell is root rot.
- In summer, use a chiller or keep the reservoir in a cooler room.
Failure modes
| Problem | Sign | Response |
|---|---|---|
| Air pump failure | Wilting within hours, roots dulling | Restart, keep a backup pump; Jardin alerts on zero pump power draw [5] |
| Root rot (Pythium) | Brown slimy roots, wilting despite water, smell | Cool the water, raise DO, clean system; biological root treatments; remove dead roots |
| pH swings | Wide daily drift | Check for root rot first; then buffer and refresh |
| Algae | Green film, roots stained | Light-proof everything |
Jardin notes
Jardin method "DWC": air pump on a monitored plug (power draw = proof of life), chiller plug, level floats, top-off, DO proxy from air-pump power and water temperature. On air failure Jardin escalates alerts, keeps the chiller on, and switches on a second pump if one is plugged into a spare port. Time-to-harm: 2–12 h for air, days for chemistry [5].
Safety
Water and mains electricity share the space. Use GFCI/RCD-protected outlets, drip loops on every cable and keep pumps above reservoir level where possible.
Common mistakes
- Warm reservoir in summer.
- One air pump, no backup.
- Adding hydrogen peroxide routinely on top of beneficial microbes (they cancel).
- Chasing pH 5.8 exactly instead of letting it drift within band.
Evidence
Temperature and DO effects on hydroponic root disease are Established in extension literature for other crops [1][2][3][4]. The hydro pH band (5.5–6.5, with drift inside it normal) is accepted practice from extension and grower consensus; cannabis-specific DO and temperature targets are Grower practice.
