Ebb and Flow Hydroponics

“Ebb and Flow” or “Flood an Drain” hydroponic systems are, as the name suggests, systems that rely on flooding the plant roots for a while and then draining them until the roots are nearly dry; and the process repeats.

In its simplest form, the ebb and flow system can be built with inexpensive and non-specialized materials you may have at your shed (except perhaps from the water/pond pump).

Pros

Cons

Simple to set-up. Simple and relatively inexpensive materials (except for the surge tank set-up). The only moving part is the pump

More complex and relatively more expensive in its surge tank set-up

Can be used for all plants, fruit-bearing ones included

The plants need to be above (or generally higher than) the reservoir water line (except for the surge tank set-up)

Maintenance is easy and inexpensive. Just make sure the pump and the timer are in good order and work

Some experience is required

Allowing the roots to nearly dry out allows for oxygen uptake and the plants respond well

 

It is more suitable to people who hold some experience in hydroponics as the adjustment of the pumping time and sequence is tricky (more on that later). Although in its simple form it is easy and inexpensive to set up, there are forms that require more tubing, more pumping and more space – usually recommended if your plants are going to become tall and large.

The set-up

As its name reveals, the plant roots are submerged into the nutrient solution by way of flooding. That solution is then drained and the roots are left to start drying out (but not completely drying) when the cycle of flooding and draining is repeated.

There are three main set-ups:

Flooded tray

The simplest of the three set-ups is the flooded tray. It is a shallow tray (think of it like a table with a tall-ish rim) where you can place the pots/baskets with the growing medium and plants on. The height of the tray rim needs to be higher than your desired highest nutrient solution level on the tray so that there is no spills.

The tray is placed on top of the nutrient solution reservoir and it has two holes on the far ends, one for the water inlet and one for the drain. The holes are on the far ends to allow for even concentration of the nutrient solution on the whole tray surface when the pump is on.  

The water depth in the tray

The depth of the nutrient solution level on the tray needs to be as required to ensure that the roots get the required moisture. If the growing medium is not absorbent enough, then the water level needs to reach 1-2 inches (2.5-5 cm) below the root crown. If it is absorbent enough, the level can be lower. Test it in an iterative process. Leave the tray filled up with the solution until water is absorbed by the growing medium and observe when the absorption stops. Adjust the water level by pumping more or draining more until you have the desired outcome.

Pump and drain

The nutrient solution is pumped using a water/pond submersible pump via tubes to one of the holes of the tray. The pump is underneath the tray, in the nutrient solution reservoir. It operates on a simple timer and is on as long as required for the tray to fill up with the solution to the point of the drain height.

The drain height (i.e. the water depth before it starts to drain) is determined by the height of the drain pipe which raises from the drain hole up to the point you desire the water level to be.

So the pump keeps operating up until the growing medium has fully absorbed the solution. During that time the water level is maintained because the excess is drained through the drain pipe back in the reservoir underneath. Note, the drain pipe needs to be larger in diameter because it works with gravity (lower velocity) while the inflow is pumped (higher water velocity) in order to have equal volumes in and out of the tray.

Individual plant containers

This set-up is more complicated than the simple flooded tray as it requires more plumbing. The plants with their growing medium have their own individual container. They can be in a basket and then be placed in the container for ease of removing them later.

Like with the flooded tray, this set-up needs to be at a higher level compared with the solution reservoir in order to allow successful drainage.

The containers are connected together from underneath and all flooded together when the pump is on via a tube on one end of the set-up. The level that the solution reaches in the containers is the same in all of them and is determined by the drain pipe height which is placed at the opposite end of the set-up.

In order to avoid an air lock that will cause you problems in the equal distribution of the solution in all containers and/or in the drain outlet, you can install a T connector at the top of the drain pipe which will allow air to leave the system while the solution runs off on the other side.

Surge tank

This set-up is the most complicated and expensive of the three. It requires more plumbing, two water pumps and more accurate timings in setting up the pumps operation.

The main benefit of this set-up is that it can be placed lower (on the floor) than the solution reservoir water line because the drainage is done by a pump and not by gravity. That allows for larger and taller plants to be cultivated.

This set-up has a second tank (surge tank) which is smaller than the solution reservoir. It serves the purpose of maintaining the desired solution level in the plant containers. A submersible pump is in it to pump the solution back to the reservoir. there solution reservoir can be of any type as long as it is opaque as explained earlier.

The primary pump pushes the nutrient solution to the surge tank and from there the solution feeds the plant containers via tubes connecting the bottom of the surge tank with the bottom of the containers. Once the solution reaches the desired level a float switch turns the submersible pump on to send the solution back into the main reservoir. Both the primary and secondary pumps need to have the same flow rate in order to maintain a stable water level in the containers.

When the main pump turns off, the secondary keeps on going until a second float switch located at the bottom of the surge tank turns the secondary pump off. And the cycle repeats when the roots are nearly dry.

What you will need

  • A tray or individual plant containers, depending on the set-up
  • A large nutrient solution container
  • Submersible water pump to circulate the solution (two of them if it is for the surge tank set-up)
  • Air pump with air tube and air stone (recommended, but not absolutely necessary)
  • A surge tank (smaller than the main reservoir)
  • Timer for the main pump
  • Float switches (for the surge tank set-up)
  • Tubes and pipes to transfer the solution from the main reservoir to the tray or individual containers
  • Growing medium
  • Containers and baskets to hold the plants
  • Electrical Conductivity (EC) sensor (measures the total salts in the solution)

Aeration

To maintain healthier roots and plants, it is advisable to aerate the nutrient solution, whichever set-up you go for, although not necessary. Aeration, apart from stimulating growth, it can act as a counter-measure against oxygen depletion caused by algae and other microorganisms (if you are not diligent enough with cleaning the tray etc).

So in this case, you will need an air pump, tube and air stone to the bottom of the solution reservoir.

Water level

We have touched on this already but here it is consolidated…

It is important to note that the roots should never been left to dry out. In any case, always leave around 1-2 inches (5cm) between the water level and the root crown to avoid root rotting. The moisture in the growing medium will be enough to moist that top part of the root.

Where the water level should be in relation to the baskets (which hold the growing medium and the plants) depends on a combination of parameters:

  • Growth stage. When the plant is still small, the roots are not long and if the holding basket is large they are unlikely to have come out of the bottom of it. So the basket may need to be submerged into the solution nearly to the top (leave 1-2 inches – 2.5 to 5cm – from the root crown) in order for the solution to be absorbed by the growing medium and reach the roots.

If the plant is larger and the roots have come out of the bottom of the basket, then make sure that all the parts of the roots are submerged in the solution (again, leave 1-2 inches – 2.5 to 5cm – from the root crown).

  • Growing medium. If the growing medium is not absorbent enough, then you need to:
    • increase the water level by increasing the height of the drain pipe or
    • add some volume in or
    • change to a more absorbent growing medium or
    • prolong the time the pump is on.

If on the other hand the growing medium is super absorbent:

  • lower the water level by removing some volume of solution out or
  • lower the drain pipe height or
  • shorten the time the pump is on.

Remember! Each time you add water to increase the water level, you need to make sure that nutrient concentration is the right one, pH and EC indications are within the set range suitable for the specific plant at its current growth stage.

 

Typical Problems

Algae, bacteria and other microorganism growth in the solution. Avoid using transparent containers/reservoirs where light can pass through. Light encourages microorganisms to grow which can change the chemistry of the solution, reduce the efficiency of water and oxygen absorption by the roots which can lead to root rotting, ultimately reducing yield.

Thirst. It is unlikely that thirst will be an issue in this type of hydroponics, because inherently you are flooding the growing medium. BUT that is not guaranteed; if you notice that your plants show signs of thirst or the growing medium is not wet enough, then try the following one by one:

  • Check if the water level in the tray or in the individual plant containers is proven to be too low and the growing medium does not absorb it efficiently.
  • Worth then considering either increasing the water level by increasing the height of the drain pipe or adding more solution volume in the reservoir (or both). By doing that the growing medium should be fully flooded to your required level.
  • If the problem persists, it probably means your growing medium cannot retain moisture and it is worth considering swapping for a more absorbent growing medium. But remember, you need to let the roots nearly dry out before repeating the flooding cycle. A more absorbent medium will only allow for the flooding cycle to be less frequent.

Oxygen starvation. When roots do not get enough oxygen, they can be damaged and die. Early signs can be leaves that become wilt, distorted or not growing to their expected size. Past that level, the leaves can yellow and drop. If the condition is not taken care of, the entire plant an wilt, stems will start dying and the entire plant will eventually die.

If you have observed these signs, try the following one by one:

  • Ensure that you clean the tray from algae that consume oxygen from the nutrient solution
  • Ensure that your water reservoir is opaque and does not allow for light to penetrate
  • Introduce aeration (air pump, tube and air stone) if you don’t have it already
  • If you do have it, check that the air pump works and that bubbles come out of the air stone
  • If the air pump works but little or no bubbles come out of the air stone, check for any air leaks along the tubes and joints
  • If bubbles form, check if the roots are dry which then is a problem of thirst

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