Flood and drain systems are one of the more popular forms of hydroponic production. Also known as "ebb and flow" systems, the concept is fairly simple. Think of a single big round or rectangular container with an open top. It can be any size. The container is filled almost (but not quite) to the brim with growing media, and plants are rooted within the growing media. The container is irrigated with nutrient solution on a regular, frequent timetable, for instance once an hour. This irrigation cycle begins when a pump moves the nutrient solution out of a nearby reservoir, and into the container. That continues until the growing media is briefly submerged, but before the nutrient solution reaches the top of the container. As soon as the nutrient solution reaches a preset level, the pump stops. Then the nutrient solution is allowed to drain away by gravity back into the reservoir. As the nutrient solution drains away, fresh air is pulled down into the roots of the plants. After a certain interval, the cycle repeats. That, in a nutshell, is flood and drain hydroponics.
Flood and drain systems have a lot of advantages. The biggest advantage for flood and drain is the regular immersion, then draining of the growing media. This has the effect not only of providing the roots with regular nutrients, but the drain portion of the cycle also draws air into the root zone. That by itself provide a tremendous advantage over soil-based crops, and even some other forms of hydroponics. Most folks are very aware that a plant's leaves "breathe" for the plant, taking in carbon dioxide and exhaling oxygen. However, the roots are also very much involved with gas exchange, and a waterlogged soil quickly drowns the plant. Even carefully managed soil-based gardening and farming can't match the gas exchange rates for flood and drain systems. Additionally, while other methods of hydroponics offer improved gas exchange, flood and drain is the only system which refreshes that air supply with each new nutrient cycle. The only system which offers better gas exchange rates is aeroponics, where the roots actually dangle in mid-air and nutrient solution is delivered via a mist.
Another advantage of ebb and flow systems is that the cycle interval between each irrigation can be varied according to whatever the particular plants need. Some food crops, such as shallow-rooted onions or strawberries, benefit from a very short interval time since the root mass can't absorb very much at any given time. Frequent irrigation, say several times an hour, gives these plants as much nutrition as they can readily absorb. Other crops, such as deeper-rooted mature lettuces or tomatoes, have a much larger root system relative to the size of the plant, and don't need irrigation as often. In fact some crops, including tomatoes, benefit from regular "mini-drought" conditions. For tomatoes specifically, the plant will put more sugars into the fruits while stressed in a drought. However, that needs to be balanced against the tendency of tomato fruits to split if they have been in drought and then receive a large dose of irrigation. This balancing act can be dialed in very precisely with flood-and drain systems.
A third advantage is that the container can be designed with, or without, a shallow reservoir of nutrient solution at the bottom of the container. This reservoir is determined by the height of the drain, and it can serve as an emergency backup in case of a pump failure or irrigation line blockage. The depth of the reservoir can be varied, or eliminated entirely, based on the crops in the container.
A fourth advantage of ebb and flow systems is that they can be completely off-grid in areas where electricity for a pump is expensive or unavailable. In that instance, the reservoir could be as simple as a bucket off to the side of the ebb and flow container. To irrigate the container, simply raise the bucket above the height of the container. Gravity will drain the bucket into the container. To then allow the container to drain, lower the bucket back below the level of the container. While this takes a lot of manual effort, it's an option which can be used for extremely simple operations.
Finally, flood and drain systems can be very easy to transplant into or out of, with one caveat. The choice of growing media will determine how easy it is to transplant into and/or out of a flood and drain system. In general, the smaller the particle size, the easier it is to work with for transplantation. For instance, sand and small gravel, or a perlite/vermiculite combo, works really well for crops that need to be moved into or out of flood and drain containers. Conversely, relatively big hydroton or lava rock growing media would be difficult to work with because the roots tend to wrap around those larger particles and then try to take them along when pulling plants out of the container. When trying to transplant into a flood and drain system, the larger particles would be difficult to arrange around the roots without something being crushed in the process. One way to make this whole process easier regardless of the growing media, is to root the plants in a rockwool cube and then the cube itself can be transplanted into or pulled out of a flood and drain container relatively easily. The core root structure is protected within the rockwool cube, and that cube can be nestled down inside whatever the new growing media will be. Keep this idea in mind when we discuss germinating seeds in flood and drain systems below.
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