Plants need water to grow and develop. I suppose every grower gets this part. However, the questions how much, how often and when to water may be less obvious. Now, let’s get down to it by understanding and accepting a couple of key points and rules of thumb.
To begin, herbaceous root systems require near 100% humidity, ideally, at all times, otherwise the root tips die back. The root tip is the very small end of the root that is divided into three zones. The root tip is responsible for absorbing the vast majority of minerals and water. Root hairs facilitate this uptake and occur in the last or third zone. After the third zone the root tissue begins to lignify and becomes more impervious to water and nutrients. Kill the tips and the root has to regenerate one before going forward.
Roots grow in response to depletion zones. These are the areas where the root has absorbed all the minerals and water located there. When the material is not replaced, the root extends to find more. Roots have to grow. When nutrients and water are abundant, the root system does not develop in balance with the shoots and a carbohydrate limited condition presents itself, so weakening the plant. Allow the plants time to dry and thus use up the minerals present. Conversely, if you keep them too dry, a condition known as chronic underwater or underfeed can manifest itself. The root tips will also die back limiting further plant development (figure 1-3).
3. Keep all drains open
Over watering is keeping the roots submerged in water without allowing them access to oxygen. Never let roots stay submerged for more than 20 minutes as even then you will get some die-back. Remember, roots require oxygen to do their job, which comes through diffusion at the root surface. A well-drained medium can have water applied for a longer period (ON time) because the excess drains away quickly from the medium when the application ceases. Poorly drained media need a much shorter application time (but the application rate has to be slower for absorption) because it will take longer to drain the excess water away from the root surface. Very poorly drained media are impossible because the rate of application has to be slow to absorb and with the drainage time, can never be watered throughout (figure 1-4).
4. Determining root health
The general rule of thumb for determining the root health and irrigation needs of a system is that 1 square meter of bench top, covered with leaves, will use 4-6 liters of water a day. New plants, or where the square meter is not totally covered with leaves, will use an average of about 3 liters a day. This is true whether there are 2 or 20 plants per square meter. Use this figure to decide how well the plants are working.
5. Water cycle
When figuring out the water cycle for a crop of more than one plant, base your times on an average of all the plants. For instance, we want to water most media (except aeroponics) when about 50% of the total volume of the water is used or gone. Set your automatic systems to turn on when 50% of the crop is ready. To accomplish this, keep everything the same; medium, plant age and size, light exposure, air currents, and so on.
With an organic or inert medium, water when 50% of the water you applied last time is gone. In some instances, the grower can weigh the container bone dry, water until drainage starts and weigh again. The difference is how much water the container will hold. After planting, the same will hold true through the early stages. By then, the grower should also be able to see that the plant is gaining weight as well, so if you keep weighing, don’t forget to take the increased weight of your plants into account (figure 1-5, 1-6)!
An aeroponic system requires you to be good at judging when the root surface has just lost the free moisture on it while not falling much below 100% humidity (air). You will have to monitor this constantly, especially where the roots are exposed to free air.
8. Keep roots in the dark
Roots like to be in the dark and really try to grow away from light. Keep them as free of light as possible in systems such as thin walled PVC or an air chamber.
9. Never too much water
Remember, in a container with medium and drainage holes, you can’t put in too much water. For example: you have a 9 liter pot, it doesn’t matter whether you apply 5 liter or 40 liter in the space of 5 minutes (if the medium does not flush away) there will always be the same amount left in the container ten minutes later. This is the only important point.
10. Preferably don't water at night
Watering cycles have to be adjusted during the 'night time' period, because plants don’t use as much water as they do during the 'day time'. Don’t forget that dark cycle is critical to plant development. Media that hold a lot of water, such as peat or rock wool, hardly ever need watering during the night. But do be sure to adjust the irrigation cycle to water in the last or first half hour of light. Aeroponics or clay pebbles will need infrequent watering during the night.
Read more in the accompanying article; Thoughts on watering plants.