As we (on the Canadian Prairies and west) enter into an extreme heat wave, it shouldn’t surprise you that water is on my mind. Not only is it heating up earlier in the day and reaching higher daytime highs, but it is staying warm at night.
In my local area, we haven’t had much in the way of moisture to soak things down, at least not for a few weeks. That isn’t true of every area, but since the big clouds and rains have continued to slide past my place, it is dry here.
For the dryland farmers, they will have to hope and pray for rain and that there was enough moisture in the soil to sustain the crops through the heat, dryness, and rapid growth. For those that have some sort of irrigation capabilities, they will be putting on moisture to try and meet the crops’ ongoing needs.
Suggested strategies for efficient water application
Regardless of the year, the moisture situation, or the time of the growing season, there are strategies for applying water more efficiently, or in such a way as to reduce any negative impacts of applying water. “Negative impacts…”, you say? Well, yes, applying water to plants can have negative impacts, but not too many, most of the time.
The following are some concepts to consider when making decisions about applying water. I feel that it is important to point out that these are suggestions, rather than hard-and-fast requirements. Just like in almost every situation where a decision is required, sometimes you have to make tradeoffs. Sometimes you have to choose options that have less of a negative impact overall, rather than one that will tick all of the boxes. It’s just the way it is. In those cases, you do what you can with what you have and then you hope for the best. Hence, suggestions, with some flexibility.
Sprinkler versus drip irrigation
There are probably pretty accurate statistics for the overall efficiency of a sprinkler irrigation system versus other types of irrigation systems. Whatever the exact number, the basic fact is this. A sprinkler system has a greater amount evaporation, and therefore a lower efficiency than another system that puts the water down closer to the surface of the plants and the ground.
As you lower the distance between the emitter and the soil, theoretically your efficiency increases. As you put the delivery system on the surface of the soil and slow the delivery rate (e.g., to a drip or trickle), you have an increased efficiency, as you have less runoff and theoretically less evaporative losses. As you reduce the distance between the water and your crop roots, there is less waste and loss to competitive plants (a.k.a., weeds)
What is the tradeoff?
Drip systems, while very efficient at getting the water right next to the plant and therefore near to where the plant roots will take it up, take a long time to deliver water. They also can’t water a very high-density planting without A LOT of lines, at least not easily. So, a drip system works great with plants that aren’t tightly packed together, but not so much with other crops. Sometimes you trade efficiency for … efficiency of another type.
Foliage versus surface application
Plants take up most of their moisture through the root system, so any water applied to the foliage of the plants will have to make it way down to the soil before it can be taken up and used. This increases the chances of evaporation and loss. Applying the water to the surface means that the water can enter the soil and become more available to plant roots. If the water is applied nearby the plants, there is less waste and loss to other plants (like weeds).
At the same time, applying the water to the soil surface has the bonus of reducing the duration of leaf wetness, which favours the plant and slows or minimizes the development of diseases. Some diseases are triggered to release spores when precipitation falls on the plant (whether it is rain or irrigation). Prolonged leaf wetness is required for some diseases to infect plants. Keeping the foliage dry can make things harder for diseases.
What is the tradeoff?
Similar to the efficiency discussion, sometimes it just doesn’t make sense to surface apply water, particularly in a large scale, high-density situation. So, applying to the foliage is necessary. In cases such as this, do everything that you can to get the water on and let the foliage dry as quickly as possible, or implement other strategies for dealing with diseases.
Time of Day
Applying water in the heat of the day might make you feel better (you are doing SOMETHING) and will lower the canopy temperature of the crop temporarily (assuming a sprinkler application), but the amount of evaporation will be much higher. There is also the possibility of increasing sunscald in some sensitive plants, as the water droplets can act as magnifying glasses.
When watering, consider watering in the cooler parts of the day, to allow the water to enter the soil and plant before it is lost. Watering early in the day is best (in my opinion), as this loads up the soil profile for the plants and allows them to dry off more quickly (if watered from above).
What is the tradeoff?
Watering in the evening reduces the evaporative loss potential, but this also means that the leaves might stay wet for extended periods, opening the door for diseases that like/need water to attack and develop.
Watering in the morning might be best for the plants, most of the time, but by watering in the morning, you don’t cool anything off much. If you have heat sensitive plants (or plant parts, such as day neutral strawberry flowers), you might need to toss out efficiency to cool down the air and keep things growing, just so you get fruit later on.
Duration of application
I typically recommend giving plants a good, deep drink, rather than just wetting them down over and over. Watering frequently and shallowly just trains the plants to keep their roots closer to the surface, rather than sending them deeper looking for the extended moisture reserves. I’m not saying plants are foolish or lazy, but they aren’t stupid. Water is water, and close water is closer and available. It’s just that when things dry out, they’ll suffer.
What is the tradeoff?
If you have a sandy soil or heavy soil that is prone to crusting, you might need to change things up a little. For sandier soils, you might need to water more often to keep enough moisture in the available soil profile for plants, simply because it moves down and out quickly. For crusty soils, with germinating seeds and new seedlings, you might need to water light and frequently, to keep things soft enough for those tender seedlings to emerge.
Similarly, if you are fertigating, you might need to water more often to supply the needs of the plants. Balance it up with some deep flushes once in a while, just to keep the salts from building up.
Timing of application
While timing was already discussed, there is another time-related element to consider. All plants have a critical moisture period in which insufficient water will directly impact yield and productivity. If you have a limited amount of water, you should make sure that plants at least have water at their critical periods. For all plants, germination and emergence stages are a given. For fruiting plants, it is critical that they have moisture during fruit set onwards. For tender, leafy things, they can’t go without water at all. For root crops, it is when the roots are starting to develop and fill up.
What is the tradeoff?
If you are growing more than one crop, it is highly unlikely that you will only have crops that have identical critical moisture periods. So, you either short everything a little bit, or you pick the crop that will suffer from deficiencies the most, and has the highest value, and then favour it. It’s a tough decision, but it’s the reality.
Mulch
Using a plastic or organic mulch can keep the soil surface cooler, reduce evaporation, and will ultimately conserve moisture. It gives plants a chance to take up moisture over an extended period. Mulches also choke out weeds and other competitors, mostly. Mulch is nicely compatible with drip irrigation systems and less dense plantings.
What is the tradeoff?
Mulch doesn’t work all that great for plants that are planted closely together, either in the row or between rows. It is an added expense, from the perspective of both materials and labour (application and removal). It can backfire on you when you have excess moisture. Also, it might not be a great fit with larger scales.
What about for the non-commercial person?
Not all of us grow plants as a business, but we do have plants and they also need water. While some of the technology might be different, the principles are practically the same. If you look at each of the strategies, you should be able to see places where you can use the principle in your own space.
Water the surface, not the foliage. Water in the morning, not the heat of the day. Water deeply and infrequently (depending on what you are watering). Slow, dripping applications are more efficient than sprinklers/spraying. A layer of mulch is your friend.
On top of these principles, you can also add in the strategy of slowing any runoff from a hard surface to encourage infiltration into the soil. You can also capture moisture for later use (e.g., rain barrels).
Conclusions
When it comes down to it, plants need water to be able to grow, thrive, mature, and accomplish their purpose. When it is hot, they need more than when it is cool. Being efficient in your water application makes good economic sense, but you will have to weigh efficiencies of different competing priorities and then pick what works best for you. Being efficient also has other benefits, which might have nothing to do with the bottom line. Make your choices and do your best.