Marijuana flood irrigation is one of the simplest methods, and cheapest, to get large quantity and quality crops. Growing cannabis plants could be a headache for many growers, that weary of braking their back, finally acquire dripping systems, difficult to set up and manage. In this post we will talk about advantages and disadvantages about flood irrigation systems, as well as giving tips and methodology to get best results.
Watering cannabis plants indoors is usually one of the main inconviniences that beginner, and more experienced, growers find. Marijuana plants not only need an optimal pH value of water, also they need an optimal EC value (ppm). Once the grower sets those values of the nutrient solution there's still the last step: watering plants.
Everyone that has taken care of a plant whenever has detected that watering plants properly is not an easy job. Less experienced growers usually overwater their plants, while those busiest growers occasionally have found their plants praying for some water. Consecuences of one and another are usually similar: less healthy plants, that show out how they suffer in multiple ways: Yellow leaves, sad droopy leaves, and lack of green in general.
If we want a plant to finish its cycle in the best way possible, the substrate must pass through dry periods and wet periods, creating a dry-wet- cycle that benefits the root system and substrate's oxygenation.
What is flood irrigation based on?
Flood irrigation system tries to copy land conditions when growing with pots. This conditions are equal to say that in a land, under wich there is a water flow, will keep substrate hydratated, allowing a life in it, assuring hydric needings of plants. Water will hydrate the substrate from what plants will drink through its root system by capilarity.
This phenomenom, very known by guerrilla growers that seek areas close to a water springs so they won't need to water plants daily, is copied by flood irrigation technique to water plants indoors.
How does flood irrigation works indoors?
Noticing that indoor growers use pots take maximum profit from a certain space, this system is based in offering plants a water supply to hydrate them with the necessary water for their evolution.
In this case what grower should do is supply enough quantity of water to plants so they can survive for 4-6 days, until they need next watering. This watering won't be done plant by plant, instead of that we will add higher amount of water directly to a tray containing the pots so plants will absorb it by capilarity.
Growers using this growing technique must know that it is not necessary to add water as soon as tray dries, the main goal of this growing system will be to increase the necessary time between waterings so every re-watering will be after 4-6 days (increasing a lot the distance between waterings and reducing the time necessary to water plants).
In general, 1 squared meter will need about 20-25 liters of nutrient solution, that will remain on the tray for about 3 days. After this period, we will need to wait for 1-2 more days so the water still contained in the pots dries, and the re-flood again with another 20-25 liters more.
What do I need to set up a flood irrigation system?
To set up a flood irrigation system we need few elements, non of them expensive, and in many cases grower already has:
Flood Tray: A flood tray can be constructued (with wood, and covered with a thick plastic film that assures watertightness), or purchased (one of the many plastic trays available in market). The target of this tray is double: it keeps the water amount in a certain surface space, and also work as a flat surface to put the pots in. It is important for the proper irrigation to assure it's flat so all plants recieve the same quantity of water. Some growers, constructing big pools that work as a big tray, have problems due to the impossibility of mixing strains in the same pool because of feeding differences, so from SantYerbasi we do recommend to divide those big pools in 1 sqm divisions, or a maximum of 2 sqm. Another important point if you like to build your own tray is the height. We recommend a minimum height of 6-12 cm (less would decrease the volume of water and more would be useless).
Pots: Plastic pots, or fabric pots, used can vary in size depending on grower's likes. the important point is not the size of the pots, what is important is to have a relationship between pot's size and number of pots used. Bigger the pots are, less plants we need, and smaller the pots are more plants we'll need to use. A very important point for a good working flood irrigation system is that if we use plastic pots they must have holes on its bottom, and also on its sides. Pots with holes only on its bottom won't be able to absorb the water properly. In case of using fabric pots, wich are specially recommended, the fabric itself will absorb water, and will deliver it all along the pot's perimeter.
Flood irrigation system complete description
We will give details as an example for a Sea of Green garden, with 16 plants in 7 liter pots, in a 1 sqm. We place the tray in a flat floor (a good tip is instead of place it directly on the floor use a solid surface with wheels, so we will be able to move it if necessary).
As we want to have maximum performance from our squared meter, we should place the pots in equal number of rows and columns (4x4). We fill those pots with good quality substrate ( we can use soil, coco coir, or even rockwhool, but we usually discard this last option for being more complicated to work with).
As flood irrigation is based on root's capacity to absorb all flooded water in the tray, we will need to create a root system able to absorb that. Get those roots is an easy job, we just need to do some (4-5) regular watering until root system is big enough to absorb a flood irrigation. It is also important that pot's must be filled just with substrate, no need to use clay or pebbles on the bottom because that could block substrate's contact with water.
Once pots are filled, it's time to proceed with the first watering (before placing plants in their pots). Notice that a spongy substrate should absorb a 10% of its volume (7 liters > 0,7 liters of nutrient solution). This watering will be done in a regular way, through top zone, humidifying all substrate. After this step it's time to place the seedlings or cuttings in the center of each pot. Once plants are pkaced it only remains to wait until next watering (as plants are small, and containers are big enough, we should expect the next watering day will be in 4-6 days. If the substrate is pre-fertilized, we recommend to add just a root stimulator to avoid over-fertilizing problems in these young plants.
Once it's time to proceed with the second watering, the process it's the same, prepare the nutrient solution necessary to feed our 16 plants in 7 liter pots (16x7=112 liters of soil > we'll need about 12 liters of nutrient solution). In this second feeding it is recommended to add to RO water a root booster, and a low dosage of grow fertilizer (mix it together in the same water used to feed plants). If the grow-room temperature is always over 20ºC and lower than 28ºC, most probable thing is that we should wait for 4-5 days more until next watering. After this second watering plants will start to develope leaving back the first days adapting to the new pot.
After 10-12 days from the begining it will be needed to proceed with the third watering of plants. Following the same water quantity explained before, we will continue with a mix in the nutrient solution of root stimulator + grow nutrient.
If we started from seed, the most common thing is to keep doing these top side waterings during 3-4 weeks (until plants arrive to a height arround 30 cm, while growers working with clones will need less time (about 2-3 weeks) to reach the same height.
Once plants get that needed height we can switch lights to flowering (12/12) and start doing flood irrigation.
From this point every time we water the plants we'll need 20-25 liters of nutrient solution, that will be served directly into the tray, instead of watering each plant. Once those 25 liters are in the tray, we will see water plants absorb it along the 3 days following to the watering. Once water on the tray is absorbed by plants, we must wait for 1-2 days more so plants dry its pots (wait for this is specially important because if we add water again, we will drown plants). So if we follow this instructions we will have 5-6 days between waterings. After these period we only need to repeat the operation once and another until arriving to harvest day.
Advantages and disadvantages of flood irrigation
No that we have detailed how does the flood system work, we will detail its best advantages, and its possible disadvantages:
Advantages of flood irrigation: As it doesn't need pumps nor drippers, it is such an easy technique, compared with other expensive watering systems, that must be controlled weekly dripper by dripper. It saves a hige amount of hours dedicated to water every single plant (along all cycle). Helps to increase a bit the relative hmidity in the grow-room. Reduce the atention needed by grower, that only needs to check the plants once very 5-6 days.
Disadvantages of flood irrigation: This system is not compatible with usage of liquid organic nutrients, due to those trend to divide (like water and oil do) after some hours mixed. Impossible to mix strains in the same tray due to its nutrient demands differences (you can mix different strains but only if you know the have the same flowering time, similar stretch tendences, and nutrient demands). Increases relative humidity at late flowering (with the possible damages to the crop if the grow-room is not properly ventilated).
Recommended substrates for flood method
Flood irrigation can be used with almost all substrates, but our experience tell us that there are two specially recommended:
Soil: Soil is spongy enough to absorb perfectly. It is possible to use a high pre-fertilized soils, or less fertilized soils (grower will only know when plants have consumed that fertilizing before adding in a liquid way). The main advantage of using soil as substrate will be a better taste of the flowers (only possible in plants grown in soil).
Coco coir: Coco is another substrate fully valid for this system. It is spongy and has a great water retention. Coco must be fertilized from the first watering, because it's an inert substrate. It main advantage is plants will adapt faster to medium, because root system will conquere it quicker, what will derive in less days needed of growing phase to reach a certain height before flipping them to flowering stage.
Recommended fertilizers for automatic watering systems
As we explained before, flood irrigation is not compatible with organic liquid fertilizers, so we must use high quality mineral nutrients and boosters. Depending on the chosen substrate we should use a range of products or another (if plants are grown in soil we should use a soil fertilizer, chosing a coco fertilizer if are grown in coco coir).
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