Takeaway: Ebb and flow systems are currently, and will continue to be, one of the most effective and simple recirculating hydroponic systems available.
One of the most reliable and effective hydroponic systems available to gardeners is the ebb and flow hydroponic system. Like other hydroponic systems, ebb and flow systems aim to maximize nutrient uptake and available oxygen in the root zone. A properly operating ebb and flow system will rival any other high-performance hydroponic system in both the speed of growth and overall yield.
A closer look at the principles of an ebb and flow system will illustrate how this fairly basic hydroponic system has found a permanent place within the indoor growing community with both hobbyists and commercial growers.
The Ebb and Flow System
An ebb and flow system, also known as a flood and drain system, relies on intermittent water delivery (flood) to a plant or series of plants held in an inert medium. The medium is primarily for stabilizing the plant and is not required to have any nutritional value. In a true hydroponic ebb and flow system, all nutrition is provided by the nutrient solution.
The most common media used in ebb and flow systems include hydroton (clay pebbles), stonewool and rinsed gravel, but any inert medium that doesn't float will suffice. The ebb and flow system is one of the simplest hydroponic systems to set up and use, but it still takes some experience and effort to master. There are two common ebb and flow configurations used by indoor horticulturists: a tray and reservoir configuration (single pump system) or an individual module configuration (dual pump system).
Tray and Reservoir Ebb and Flow System (Single Pump System)
The most basic ebb and flow system uses one pump for nutrient delivery and relies on gravity to return the solution back to the reservoir. This ebb and flow set-up requires the tray or containers that hold the plants to be above the reservoir with the nutrient solution. In many instances, growers using this configuration will use a large grow tray for holding their plants.
The entire tray can be filled with the medium and plants can be placed directly in the medium. The disadvantage of this method is the plants’ roots will become intertwined throughout the growing cycle. This renders the plants unmovable and can become a hassle if pathogens arise in the system.
My preferred use of a tray and reservoir ebb and flow system uses individual containers, filled with the chosen inert medium, which are placed in the tray. When using individual containers, growers can easily transfer plants in and out of the system, which increases the versatility of the system and can be advantageous for growers with separate vegetative and blooming rooms.
For a tray and reservoir ebb and flow system to operate correctly, two holes must be drilled into the tray for two important purposes. The first hole is for the delivery of the nutrient solution and the second hole acts as the overflow drain and controls the height of the nutrient solution in the tray when the pump is operating. The hole for nutrient delivery is flush with the bottom of the tray and is directly plumbed to the submersible pump in the reservoir.
As soon as the pump is turned on, the nutrient solution starts to fill the tray from the bottom upward. The nutrient solution will continue to rise in the tray until it reaches the top of the overflow drain. Some growers build the overflow drain from a small section of PVC, but drain fittings specific for ebb and flow systems are inexpensive and sold at virtually all hydroponic retailer shops. The fittings found at your local hydroponic retailer will usually include a rubber washer, which is imperative for getting a water-tight seal around the holes in the tray. The top of the overflow drain is usually a few inches below the maximum height of the walls of the tray.
The purpose of the overflow drain is to stop the tray from overflowing all over the floor when the pump is operating. As the water is pumped through the delivery hole into the tray, the level continues to rise until it reaches the top of the overflow hole. At this point, the nutrient solution travels down the overflow drain and returns to the reservoir. This creates a continual and consistent depth of water in the tray (as deep as the overflow drain is tall) while the pump is operating.
One of the most common ways to set up a tray and reservoir ebb and flow system is to place the grow tray on a stand directly above the reservoir. This way the nutrient solution that flows into the overflow drain returns to the reservoir in the most direct manner. A timer controlling the submersible pump allows the grower to determine the desired watering intervals.
Once the power to the pump is shut off, the weight of the water in the tray causes the remaining nutrient solution to return to the reservoir through the delivery hole (back through the pump). One of the biggest advantages of an ebb and flow system with the reservoir located directly below the growing tray is the minimal distance the water has to be pumped.
The biggest advantage of this is a reduction in the head height requirement for the pump. The farther the required vertical distance for nutrient delivery, the larger the pump has to be. A large pump can be a disadvantage in any hydroponic system because mechanical pumps create additional heat. The reservoir’s height also plays a large role in a tray and reservoir ebb and flow system.
Because gravity is the force that returns the nutrient solution, the tray must be set above the reservoir. In most cases, a reservoir with a low height is a better fit for ebb and flow. The lower the reservoir’s height, the more vertical space the plants will have to grow.
The Pump in a Single Pump System
The pump is the heart of any ebb and flow system. Growers must be sure to purchase a pump that is adequate enough to deliver nutrient solution to their plants. The volume of water required to fill the tray, the height to which water must be delivered and water displacement (from media and planting containers in the tray) must all be taken into consideration when choosing the pump for an ebb and flow system.
A general rule of thumb is to choose a pump that delivers 10 times the needed volume of water per hour and has a maximum head height at least five times higher than required. For example, if it was determined that the required volume of water to flood a tray was 100 gallons after all displacement is considered and the pump needed to deliver the water to a height of 3 feet, the minimum pump would need a flow rate of 1,000 gallons per hour (gph) and a head height of at least 15 feet.
Individual Module Ebb and Flow System (Dual Pump System)
In recent years, indoor horticulturists have been taking advantage of a different style of ebb and flow system: the dual pump system. These systems require two mechanical pumps—one that delivers the nutrient solution to the plants and another that returns the nutrient solution back to the reservoir. There are a few advantages of a dual pump system. First, because gravity is not a requirement in dual pump systems, the planting containers can be placed directly on the floor.
This increases the vertical space in the grow room and is a huge advantage for gardeners who like to grow monster plants. Another advantage of a dual pump system is that the reservoir can be farther removed from the system, which makes reservoir adjustments and maintenance more convenient for the grower. Besides the addition of a second pump, a dual pump ebb and flow system operates under the same principles as the single pump system.
A nutrient solution is delivered to the plants, which are supported by an inert medium, for an allotted amount of time and is then drained. The ebb and flow of the nutrient solution is designed to maximize nutrient uptake and oxygen in the root zone which leads to faster growth and larger yields.
Timing the Nutrient Solution Delivery
Regardless of the type of ebb and flow system a grower uses, the most important factor affecting the way the plants will perform is the timing of the nutrient solution delivery. This will always require some experimentation by the grower as every growroom and every hydroponic system is different.
The goal—to deliver the solution and then let the medium dry out— is the same for all ebb and flow gardens. The amount of time the pump is actually delivering the nutrient solution can vary but most systems only require a few minutes of flooding to adequately moisten the medium. A common duration for beginner ebb and flow growers is a 15-min. pump-on duration. The pump-off time will vary and depends on multiple factors.
To maximize performance in an ebb and flow system, the medium should be allowed to dry out to the point just before stress to the plants occurs. This will vary greatly depending on the type of medium used and its moisture-holding capabilities. The most common visual indicator of stress caused by a lack of moisture is wilting. Wilting is not a good thing and growers should try to avoid it.
However, growers who observe the first signs of wilting while experimenting will have a better feel for gauging the perfect duration between waterings. In a perfectly operating ebb and flow system, the nutrient solution would be delivered right before wilting would occur. It is also important to take into consideration the plant’s size. As a plant grows it increases its requirements for nutrition and water. In fact, as plants reach maturity, it is not uncommon for the nutrient delivery intervals to double or triple per 24-hour period.
Because the optimal nutrient delivery requirements change throughout the life cycle of the plants, it is not uncommon for growers to have to experiment with various pump-on and pump-off durations for a few cycles until they find the ideal pump timing for their given system.
Aerating the Reservoir
Like any recirculating hydroponic system, ebb and flow systems benefit greatly from additional aeration in the reservoir. Some growers using the tray and reservoir configuration rely solely on the return of the nutrient solution through the overflow drain to aerate the nutrient solution, although the addition of an air pump and air diffuser is highly recommended. An air pump and air diffuser should always be used in a dual pump system.
Ebb and Flow is Here to Stay
Ebb and flow systems were one of the first hydroponic systems used by hobbyists and commercial growers. Commercial growers have used ebb and flow systems in large scale aquaponic applications because the medium used to support the plants can double as a bio-filter for the nitrates found in fish waste.
Although most commercial applications of ebb and flow systems are associated with aquaponics, hobbyist growers are still reaping the benefits of highly efficient ebb and flow systems in their indoor gardens. Ebb and flow systems are currently, and will continue to be, one of the most effective and simple recirculating hydroponic systems available.