Recirculating Deep Water Culture (RDWC)

Introduction

Recirculating Deep Water Culture (RDWC) is a form of hydroponics in which plant roots are suspended in continuously flowing, oxygenated nutrient solution. Unlike basic Deep Water Culture-which usually involves a single static reservoir-an RDWC system links multiple buckets or reservoirs via piping, and a central pump circulates the solution through all of them. At the same time, air pumps and air stones maintain high dissolved-oxygen levels. This constant flow prevents stratification of nutrients, ensures uniform pH and EC (electrical conductivity) across all plant sites, and allows growers to support larger, more nutrient-demanding crops with minimal manual intervention. By combining aeration and circulation, RDWC maximizes root-zone oxygen availability and nutrient uptake, resulting in faster growth rates and higher yields compared to passive or static methods.

Maintenance

Maintenance primarily involves weekly pH and EC checks, topping off water to maintain reservoir volume, and replacing the full nutrient solution every 10-14 days to prevent nutrient imbalances or algae buildup.

Building One Yourself (DIY)

To build a DIY RDWC setup, you'll need:

Containers/Reservoirs: Typically, 3-5 gallon food-grade buckets with opaque, airtight lids. Each bucket holds one net pot.
Plumbing Components: PVC or vinyl tubing, bulkhead fittings, T-connectors or manifolds, and barbed adapters. These create the flow path between buckets.
Water Pump: A submersible aquarium or hydroponic pump sized to move at least 200-400 gallons per hour (GPH), depending on the number of buckets.
Air Pump and Air Stones: An aquarium air pump (2-4 outlets) plus air stones to provide continuous oxygenation in each bucket.
Net Pots & Growing Media: 3-4 inch net pots fitted into each lid, filled with an inert medium such as clay pebbles, rockwool cubes, or coco coir.
pH/EC Meter & Nutrient Solution: A reliable digital meter to check pH (ideally 5.8 - 6.2) and EC, plus a balanced hydroponic nutrient formulation.

Steps:

Drill and Assemble: Drill holes in each lid for a net pot and two bulkhead fittings-one for the "supply" (inlet) and one for the "return" (drain).
Plumb the Buckets: Connect the bottom of Bucket A's "return" fitting to the "supply" fitting on Bucket B, and so on in series. The last bucket's "return" goes back to the pump's inlet. From the pump's outlet, run tubing back into Bucket A's supply fitting (or use a manifold to feed multiple buckets evenly).
Install Air Stones: Place one air stone in each bucket, connected to the air pump via airline tubing and a splitter. Position air stones toward the center of the bucket to promote upward circulation.
Fill & Test: Fill the entire system with water, turn on the pump and air pump, and check that water flows evenly through all buckets. Adjust tubing or add union fittings if flow is uneven.
Add Nutrients: Once plumbing is leak-free, drain and refill with your calculated nutrient solution. Check pH/EC and adjust as needed. Place seedlings or clones (already root-developed) into net pots, ensuring their roots dip into the flowing solution.

What a Professional Kit or System Might Look Like

Commercial RDWC kits often arrive as modular, stackable buckets or a series of interlocking reservoirs designed for quick assembly:

Pre-Drilled, Color-Coded Reservoirs: Buckets come with factory-installed bulkhead fittings and quick-connect tubing, reducing assembly errors.
Integrated Manifolds: Instead of daisy-chain tubing, some systems use molded plastic manifolds that split the pump output evenly to each reservoir, ensuring consistent flow and simplifying plumbing.
Digital Controllers & Timers: Higher-end setups include a controller that monitors pH, EC, water temperature, and even dissolved oxygen (DO). Some kits offer automatic nutrient dosing with peristaltic pumps and auto-top-off modules to maintain reservoir levels within 1 cm.
Insulated Reservoirs & Reflective Covers: To prevent heat gain or loss, many professional kits use insulated walls or reflective foil linings. This keeps nutrient temperatures in the ideal range (18-22°C) for root health.
Add-On Accessories: Air chillers, inline filters, and UV sterilizers can be added to remove pathogens and stabilize water quality. Some manufacturers also include height-adjustable racks so plants can be moved up as they grow.

Overall, a commercial RDWC kit reduces setup time, ensures more precise environmental control, and often integrates data logging or smartphone app connectivity-features that streamline a medium-to-large operation.

What Cultivars Do and Don't Work with RDWC

Because RDWC continuously delivers nutrients and oxygen, it supports a broad range of plants-including heavy feeders and deep-rooted varieties-that outgrow simpler systems:

Leafy Greens & Herbs: Lettuce, spinach, kale, Swiss chard, basil, mint, and cilantro flourish in RDWC, often reaching harvest size in 4 - 6 weeks. Their moderate root mass benefits from constant oxygenation and nutrient flow.
Fruiting/Vining Crops: Tomatoes, peppers, cucumbers, eggplants, and even dwarf melons perform exceptionally well. RDWC supplies the high water and nutrient demands required for flowering and fruit development. Over the 10-12 week grow cycle, roots can expand deeper without risk of sitting in stagnant water.
Large-Leaf Varieties: Swiss chard, bok choy, and bigger basil cultivars can develop lush, expansive foliage because the system can handle the increased nutrient draw.

Less suitable:

Root Vegetables: Carrots, radishes, beets, and other tap-root crops need a loose substrate to expand-something not provided by a bucket or net pot environment.
Very Tall or Heavy Plants Without Support: While RDWC can sustain large tomatoes or heavy pepper plants, they require external trellising or support structures. If you neglect support, stems may buckle under fruit weight.
Long-Term Woody Perennials: Strawberries and perennial herbs like rosemary or thyme might survive, but over multiple months they can become root-bound or develop imbalances that static timing (like nutrient change intervals) may not address as effectively as a more customizable ebb-and-flow or aeroponic system.

In summary, RDWC's strength is its ability to feed and oxygenate fast-growing, nutrient-hungry crops. While almost any non-root vegetable can succeed, select cultivars based on their root architecture and support needs to avoid complications with space and plant stability.