I realize that water seems to be taking a leading spot in the subject matter for this month, but it has been hot and dry in most areas, so you probably understand. While some of this was covered a couple of weeks ago (Water Application Strategies), I want to address the topic from a slightly different angle (well, several, but you get the idea)
As the warm middle of summer really settles in, one of the main challenges of growing different horticultural crops is keeping the plants supplied with sufficient water. Plants all vary in how much water they need, how well they can seek out and find a water supply on their own, and when they absolutely must have water. The variation between soil textures within fields adds some more complexity to the mix. And, to top it all off, water (whether in the soil, on the leaves, or in the air) is likely in the top three of most important factors affecting disease development in crops. As with everything, there are always compromises that must be made with any decision, so let’s look at the pros and cons of the different watering methods and strategies.
Efficiency versus Speed
Delivering all the water that a crop might want/need at one time might be considered efficient from the perspective of time savings, but realistically, it can cause a lot of damage, and will be inefficient, overall. The rate of uptake of the water into the soil is limited by the capacity and texture of the soil. This can result in erosion and water running off the land before it can be used, which means waste. A heavy fall of water can also batter and weigh down plants, resulting in physical injury.
So, you need to find the sweet spot between getting the water onto the plants in such as a way as it isn’t lost to evaporation, but at a slow enough rate that the soil can accommodate the volume.
Foliar versus Surface Application
I talked about this a fair bit in the “Water Application Strategies”, but more from a water efficiency standpoint than from the perspective of disease management. If you can apply water to the soil surface, rather than wetting the foliage, you will generally help to reduce the rate and severity of disease development (at least aboveground diseases). This is because pathogens like moisture. They are spread by water splashing between plants. Precipitation triggers spore production in many pathogens, which irrigation mimics. A wet leaf is more easily infected than a dry one (for the most part), particularly by pathogens that want/need a film of water. Wet leaves need to dry off, which can increase the local humidity, or at least prolong the time that the humidity around the leaves is higher.
Sometimes we water for different reasons. In some situations, we water to cool the crop or to protect the plant from frost (essentially using water to absorb or release heat energy). In this case, watering the foliage is entirely acceptable.
As I’ve said before, if you can water the base of the plant, do that.
Light and Shallow versus Heavy and Deep
From an efficiency standpoint, for the most part, it makes more sense to apply more water less often. By filling up the soil profile completely and then letting things settle out, you aren’t keeping some areas dry, while others are saturated. This allows plants to have the water they need, but also encourages them to send out roots to find the water, rather than having all the roots at the surface.
The only time I would shift from this strategy is early on in growing, when you have small seeds in a potentially crusted soil. Then you should keep things moist up top, until they emerge. Then proceed to water more thoroughly.
Daytime versus Other Times of Day
Again, from a water efficiency standpoint, watering at a time when the chance for evaporative losses is lower should be more efficient and recommended. So, as much as possible, water when it is cooler, rather than when there is a lot of sunshine, wind, and heat.
Of course, if you have to get water on and your delivery system is the limiting factor, go ahead and water. For example, if you are using a pivot system, and it can only deliver a certain amount of water at a time, you had best water the crop.
Similarly, if the purpose of applying water is not to meet the water needs of the plant, but rather to cool off the air and the crop (something that is sometimes required), water to meet that need.
Early Morning versus Evening
If you are watering in the cooler parts of the day and you have the choice, water in the early morning, so that the plant can have a chance to dry off and remain dry for most of the day. This cuts down the period of time that the leaves are wet, which makes things less desirable for the various diseases. It also means that the plants have enough water to get them through the heat and stress of the day, rather than trying to help them to recover after the stress. I think of it as preventative hydration, rather than trying to fix a problem.
Conclusions
In the end, there are always trade-offs to any decision. Sometimes you choose efficiency. Sometimes you choose plant health. Sometimes you shoot for the ground in-between the two and hope for the best.