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Systems · Advanced · 2 min read

Recirculating deep water culture (RDWC)

RDWC links several DWC sites to a control bucket and circulates the solution with a pump, so one reservoir sets pH, EC and temperature for every plant and a single chiller cools the lot. It adds circulation and plumbing failure modes to DWC's oxygen and temperature risks. Targets match DWC: 18–22 °C, DO above 6 mg/L, pH 5.6–6.0.

Grower practice
Draft: awaiting expert review Updated
VegFlowerRDWC

Being reviewed. Our editors are double-checking a few figures in this article against primary sources. Marked passages may change.

Short answer. RDWC is DWC with the buckets plumbed together and a pump circulating solution through a control bucket. You get one place to measure and correct pH, EC and temperature and one chiller for every plant, plus larger total volume (more stable chemistry). The cost is plumbing: a split fitting, a blocked return or a stopped pump now affects every plant. Targets are the same as DWC: 18–22 °C (64–72 °F), DO > 6 mg/L, pH 5.6–6.0.

How it works

Each site has its own air stones. Solution is pumped from the control bucket into the sites (or the reverse), and returns by gravity through bulkhead fittings near the bottom. Because every site shares water, a root-disease outbreak in one site spreads to all [1].

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 Notes
Solution temperature 18–22 °C (64–72 °F) Chiller sized for pump heat + room
DO ≥ 6 mg/L Air in every site, not only the control bucket [2]
pH / EC As DWC
Circulation Turn total volume over ~1–4 times per hourbeing reviewed Too fast heats water; too slow leaves stagnant sites
Control bucket 20–30 % of system volumebeing reviewed Room for probes and top-off
Return plumbing ≥ 25 mm (1 in) for small systems Undersized returns overflow sites

How to run it

  1. Level all sites; test with plain water for 24 h and check every joint before plants go in.
  2. Probes, dosing inlet and top-off live in the control bucket.
  3. Inspect roots in every site weekly; any brown slime means system-wide action.
  4. Refresh on schedule; drain via a pump, not by tipping buckets.

Failure modes

Problem Sign Jardin response [4]
Split line / leak One site's level drops while control bucket holds Per-site level floats flag it; leak pad cuts the pump rail
Circulation pump dead Return flow stops; sites stagnate and warm "Stagnant bucket" alarm; proof-of-life by current
Air failure DO falls in all sites Alert, chiller stays on, backup pump if fitted
Root rot Spreads through system Clean and sterilise between runs; cool water

Jardin notes

Jardin method "RDWC": as DWC plus per-bucket level floats, circulation-pump proof-of-life and a stagnant-bucket alarm. Time-to-harm: 2–12 h for air, days for chemistry. Automation level 3 [4].

Safety

As DWC: GFCI/RCD on every outlet, drip loops, leak trays. RDWC systems hold hundreds of litres; plan for a full spill.

Common mistakes

  • Relying on air only in the control bucket.
  • Undersized return lines.
  • Mixing plants of different vigour on one system (one EC for all).

Evidence

Root-disease spread and DO/temperature behaviour in recirculating hydroponics are Established [1][2][3]. RDWC-specific design numbers are Grower practice.

Questions this article answers

Should my reservoir be sterile or full of beneficial microbes?

GenetixBay prefers a beneficial-microbe RDWC reservoir, since bacteria arrive in shared recirculating water whatever you do, so you may as well have the good ones. Running sterile is valid only with full commitment: a continuous oxidiser or UV or ozone, clean top-ups and no organics. In RDWC any outbreak reaches every site, so never mix the two approaches.

What is the difference between DWC and RDWC?

The difference is that RDWC plumbs several DWC sites together and pumps solution through a shared control bucket, while DWC keeps each plant in its own bucket. RDWC gives one place to correct pH, EC and temperature, one chiller and more stable chemistry. The cost is plumbing risk: a leak, blocked return or stopped pump, or a root disease, affects every plant.

How big should the RDWC control bucket be?

An RDWC control bucket is usually about 20–30% of the total system volume, a grower rule of thumb worth verifying for your build. It needs room for the pH and EC probes, the dosing inlet and the top-off. Pair it with return plumbing of at least 25 mm (1 inch) on small systems, because undersized returns overflow the sites.

How fast should the RDWC pump circulate?

An RDWC pump should turn the total system volume over about 1–4 times per hour, a grower rule of thumb. Circulating too fast adds pump heat to the water, which matters when you are holding 18–22 °C, while circulating too slowly leaves stagnant, warming sites. Circulation does not replace aeration: keep air stones in every site, not only the control bucket.

One plant in my RDWC has root rot; what now?

Root rot in one RDWC site means system-wide action, because every site shares the same water and the disease spreads to all of them. Cool the solution to 18–22 °C, keep dissolved oxygen above 6 mg/L, check roots in every site and remove dead root material. Clean and sterilise the whole system between runs to stop the pathogen carrying over.

Sources

  1. Colorado State University Extension PlantTalk 2043. Root disease problems and management in hydroponic systems. planttalk.colostate.edu/topics/miscellaneous/2043-root-disea
  2. Cornell CEA, 2016. Leafy greens (GPN): DO and pH guidance. cea.cals.cornell.edu/files/2019/06/GPNOctober2016_LeafyGreen
  3. Resh HM, 2022. Hydroponic Food Production, 8th ed. CRC Press. doi.org/10.1201/9781003133254
  4. GenetixBay Jardin engineering notes, 2026 (system profiles and failure margins)..

Related

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Deep water culture (DWC)

In DWC roots hang in an aerated nutrient solution. It grows fast but depends on two things: dissolved oxygen (keep above about 6 mg/L, ideally near saturation) and solution temperature (18–22 °C / 64–72 °F). Warm, poorly aerated water is how Pythium root rot starts. Run pH 5.6–6.0 and EC 0.8–2.0 mS/cm by stage, and treat the air pump as life support.

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Nutrient film technique (NFT)

In NFT a thin, continuous film of nutrient solution runs down sloped channels past bare roots. It is efficient and well proven for lettuce and herbs, but large cannabis root masses can dam the channel, and roots dry within one to three hours if the pump stops. Keep flow continuous (about 1–2 L/min per channel), slope about 1:30 to 1:100, and solution at 18–22 °C.

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Choosing your growing system: soil, coco, rockwool or hydro

Choose by how much attention and measurement you can give, not by yield claims. Soil and living soil forgive mistakes for days; coco and drip need daily feeding; DWC, NFT and aeroponics grow fast but can lose roots within hours if a pump or air supply fails. No controlled study shows one system out-yielding the others when light and climate are equal.

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Temperature by stage: day, night and root-zone targets

Run roughly 24–28 °C (75–82 °F) by day in veg, 22–26 °C (72–79 °F) in flower, cooling to 20–24 °C (68–75 °F) in late flower, with nights 3–6 °C (5–10 °F) cooler. Above about 30 °C (86 °F) photosynthesis in cannabis falls unless light and CO₂ are high. Keep the root zone and hydro reservoirs at 18–22 °C (64–72 °F).

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Root rot and Pythium in cannabis

Pythium root rot is a water mould that thrives in warm, low-oxygen root zones and swims between plants in recirculating water. Brown, slimy roots whose outer layer slides off, plus wilting with a wet root zone, are the signature. Fix the root-zone conditions first: solution at about 18–21 °C (65–70 °F), high dissolved oxygen, clean water and no overwatering; then remove dead roots and re-establish beneficial microbes.

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