Water and irrigation
Healthy roots from start to finish
By Tineke Goebertus
A means of supplying water and fertilizer to plants, roots need to function at their best.
By Tineke Goebertus
Ensuring the success of a year-round vegetable crop means making sure that the roots are in excellent shape from start to finish. This is not as easy as it may sound, because throughout the season there are many different growing situations to be dealt with.
One of the most important things you should realize is that the above ground parts of your plants need water and CO2 to function well; however, their below ground parts, the roots, require oxygen to function optimally.
Roots not only anchor your plants; they are the only means of supplying water and fertilizer to the leaves, stems and fruit. The better shape your roots are in, the better your plants will thrive, and the more, larger-sized fruit they’ll produce from start to finish. It is up to you to create the ideal conditions for the roots of your plants. By the time summer comes around, your crop should have a full-grown root system. One kilogram of roots requires at least 200 mg of oxygen per hour, and the root tips use even more. The water you are feeding is, at most, saturated with 9 mg of oxygen per liter. When feeding 1 liter per m2 per hour, the additional 191 mg of oxygen required (calculated from 200 mg minus 9 mg) should be present in the root area itself. Being short of oxygen will result in part of the roots dying.
This is where it gets tricky; you’ll have to feed enough water for your plants and at the same time make sure there is enough oxygen present around the roots. To complicate matters, as of the second half of July, the days will get noticeably shorter and the humidity inside and outside of your greenhouse will be higher.
That is why it is so important that you know your irrigation system well, keep it clean, and measure every day!
The amount – measure your water
You should measure the following every day:
- the amount of water fed to the plants
- the amount of drain water
These two measurements check that your plants are receiving an adequate amount of water in relation to their size, fruit load, and weather conditions.
Under the same conditions, plants require the same amount of water. Water uptake is very closely correlated to the radiation it receives, as the water is mostly used for transpiration to cool the plants.
Your objective should be to feed enough water for your plants’ needs, and NOT to have a lot of drain per se. The only reason a certain drain percentage is required, is to compensate for the unevenness of your watering system, your greenhouse climate, possibly the ground profile and your plants. For example, when growing on gutters with a clean irrigation system, you don’t require the same drain percentage as your neighbour who is growing on a bad ground profile and not such a great irrigation system.
That’s why some growers can operate with only 25 per cent drain and others require 50 per cent to ensure that their plants receive enough water.
Timing – check your graphs
Aside from the correct amount, it is even more important to water at a time when the plant requires water. Don’t start too early and don’t continue too late.
The humidity deficit (HD) in the greenhouse is a great tool to use. Only when the HD in the morning starts increasing should you start with watering. As soon as the HD in the late afternoon recovers (decreases), you should stop watering.
Obviously, there should be a correlation between the amount of irrigation water and the radiation sum; not only by the end of the day, but also throughout the day. After all, it does not help when you feed too much water in the morning and not enough in the afternoon. Your plants would still be overwatered in the morning (leaving too little oxygen around the roots) and be short of water in the afternoon (not enough for the plant). Both of these situations could lead to disease, such as blossom end rot.
Drain percentage per cycle also provides good insight on irrigation distribution over the day. The drain percentage should come down gradually in the last two to three cycles.
Note: the use of a roof cover, a shade screen or a misting system will all have their effect in the water usage of your plants. Make sure to properly correct for this.
The substrate – some tools
To promote healthy roots, it’s important that you consider the following factors:
Type: The substrate you use makes a difference in the watering strategy. Cocopeat will become more wet towards the end of the season than rockwool. This should be reflected in the start time, but especially in the stop time of the irrigation. With cocopeat, you should stop earlier in the day compared to rockwool.
Bag volume: Obviously, a smaller volume dries up faster, BUT it also saturates faster than a larger volume when watering. As previously discussed, under the same climate conditions, your plants will require the same amount of water regardless of the substrate volume your plants are growing in. Typically, with larger substrate volumes, you start watering later and quit earlier.
Drain holes: The only way for the drain water to leave the bags is via the drain holes. When you start feeding more and more water per hour, it is important that the capacity of the drain holes is big enough to allow the excess water to leave the bags as soon as possible. If not, the bags will be too wet. And where there is water, there is no oxygen.
Profile: It is undesirable for your bags to sit in puddles of water. Not only will this hamper the ability of excess water to leave the bags at a quick enough pace, they are also a breeding ground for fungi and insects.
Opening the top: One of the reasons why plastic bags on cocopeat are initially oversized is to leave room for the cocopeat and the roots to expand during the growing season. But with a good rooting system, the bags will “fill up” before summer and will therefore contain less and less oxygen. That is why you should open the tops of the bags in June at the latest. Bags with a pre-cut perforation between the plant holes are commercially available. This makes it easier to get the job done without slicing the roots with your knife.
Remember, roots require oxygen all of the time, but your plants only require water for a certain amount of time.
- you cater the watering to the needs of your plants;
- you make sure that the amount of drain water is not excessive; and
- you allow the drain water to leave the root zone asap
…your roots will thank you, and this will result in a resilient crop with good production to the very end.
Tineke Goebertus is a greenhouse consultant in BC with Vortus Greenhouse Consultants Inc. She can be reached at firstname.lastname@example.org.