Deep Water Culture (DWC) is the simplest active hydroponic method. The plant sits in a net pot, the roots grow through it and hang permanently in a container of nutrient solution. Oxygen comes from a small air pump feeding an air stone. That is the whole system — no tubing runs, no nozzles, no timer.
How a DWC is put together
- Reservoir — a bucket of 10 to 20 l per large plant, 4–8 l per head of lettuce. It must be lightproof; light in the water means algae.
- Net pot — usually 5–8 cm across, with a rockwool cube or clay pebbles.
- Air pump and air stone — running 24 hours a day, drawing 2–5 W.
- Lid — holds the net pot and blocks the light.
The detail beginners miss most often: after the first 2–3 weeks, lower the water level so that 2–3 cm of air remain between the bottom of the net pot and the surface. That band is where the so-called air roots form — white and fluffy — breathing straight from the air. They are your insurance during a power cut.
Oxygen and water temperature are the whole secret
Roots breathe. In soil, oxygen arrives through air pores; in DWC the water has to deliver it. The problem is physics: the warmer the water, the less oxygen dissolves in it. That is why the temperature of the solution — not the fertilizer — decides whether DWC works.
| Water temperature | Oxygen saturation (approx.) | What it means in practice |
|---|---|---|
| 16 °C | 9.9 mg/l | Very safe, growth only slightly slower |
| 18–20 °C | 9.1–9.5 mg/l | Optimum for lettuce and herbs |
| 22 °C | 8.7 mg/l | Upper limit without extra measures |
| 24 °C | 8.4 mg/l | Pythium risk rises noticeably |
| 26 °C | 8.1 mg/l | Hard to sustain without cooling |
| 28 °C and above | ≤ 7.8 mg/l | Root rot becomes the rule |
These figures are saturation values for fresh water at sea level. A real nutrient solution is rarely 100 % saturated, so budget 1–2 mg/l less. Practical target: 6 mg/l or higher.
How much air the pump has to deliver
- Rule of thumb: about 1 litre of air per minute for every 10 l of solution. A 20 l bucket is fine with a 2 l/min pump.
- One stone producing fine bubbles beats a jet of coarse ones — fine bubbles offer many times the contact area with the water.
- The pump has to push against the water column. At 30 cm depth, real output drops roughly 20–30 % below the rated figure.
- Air stones clog with salts: soak them in vinegar or replace them every 3–6 months. Pump diaphragms last 1–2 years.
- Noise: diaphragm pumps sit at 35–45 dB. Put them on a piece of foam, never straight onto tiles.
Spotting and preventing root rot (Pythium)
Pythium is a fungus-like organism (an oomycete) that attacks weakened, poorly aerated roots. It does not appear from nowhere — it appears because the water is too warm, too still, or full of organic sludge.
- Healthy roots: white to cream, firm, smelling of fresh soil or of nothing at all.
- Sick roots: brown, slimy, slippery; pull one and the outer layer slides off the core. The smell is sweet and rotten.
- Prevention number 1: keep it below 22 °C. A frozen water bottle in the reservoir drops summer temperature by 2–4 °C for a few hours.
- Prevention number 2: zero light in the water and no plant debris in the tank.
- Two schools of treatment: sterile (3 % hydrogen peroxide, roughly 2–3 ml per litre) or biological (Bacillus, Trichoderma). Combining them is pointless — peroxide kills the beneficial bacteria too and breaks down chelated iron.
DWC or RDWC — what actually differs
Classic DWC means every bucket is an island. You measure, top up and correct each one separately. With three plants that is relaxed; with twelve it becomes a chore. RDWC (recirculating DWC) links the buckets with pipes into one loop sharing a central reservoir and a circulation pump.
| Criterion | DWC | RDWC |
|---|---|---|
| Reservoirs | Separate, each on its own | Linked by pipes into one loop |
| pH and EC checks | In every bucket individually | Once, in the central reservoir |
| Temperature | Small volume, fast swings | Larger volume, far more stable |
| Disease risk | Stays in one bucket | Spreads through the whole system |
| Complexity | Practically none, just an air pump | Circulation pump, pipes, seals |
Limits of the method, stated honestly
- ⚠️ The air pump runs 24/7. There is no interval mode — the moment it stops, the clock starts.
- ⚠️ In summer, water in a dark bucket on a balcony easily hits 28 °C. DWC in full sun without insulation simply does not work.
- ⚠️ Small volume means fast change: in a 10 l bucket the pH can jump 0.5 overnight.
- ⚠️ Tall plants such as tomatoes need a heavy, stable bucket plus support, otherwise the whole thing topples.
- ✅ In return: the fastest growth on leafy greens, a cheap build and almost no maintenance beyond measuring.
If you like the DWC logic but worry about temperature, look at the Kratky method (same principle, no pump at all) or systems where roots are not permanently submerged, such as NFT. A vertical tower follows similar logic: the water sits in the tank at the bottom, the pump runs in intervals, and roots spend most of their time in humid air — which is why they shrug off a warm afternoon more easily than a classic DWC bucket.