The common advice is to water in the morning and not at night because watering at night keeps leaves wet all night and allows fungal spores to infect leaves. This seems to make sense, but is this really true? Do leaves stay wet a long time if watered at night?
What do farmers do? If they have to irrigate 100 acres, do they stop watering in the afternoon to reduce fungal infection?
Another common piece of advice is to water the soil only. Don’t get the leaves wet and you will have less fungal problems. I have always felt that water evaporates fairly quickly so is this really a concern?
Best Time to Water
The Iowa State university says, “early morning (5:00 to 9:00 am) is the best time to water the garden when using a sprinkler, garden hose, or any other device that wets the plant foliage. When watering is completed, the plant foliage dries quickly. The rapid drying of plant foliage helps guard against the development of fungal diseases.”
The implication here is that leaves dry faster in the morning than at night.
Is this true?
It tends to be cooler in the morning which means the leaves are also cooler. Evaporation is higher in warmer conditions, so the warmer leaves you find in late afternoon should dry faster than the morning cool leaves. Mornings also tend to have less wind which means slower drying.
If the sun is up, like late morning or noon, it would speed up drying, but at 5:00 am the sun is not up yet, for many gardeners.
How Does Evaporation Compare Between Morning and Night?
I have never seen any data on this, but it seemed to be a simple experiment to run. I selected a lilac bush that has the front in direct sun from around 9:00 am until noon, after which it’s in shade. The back side of this shrub butts up against some large aspens and is shaded all day. At the specified times, I took a hose and watered the shrub until the leaves were dripping with water. I then timed how long it took for a pre-selected branch of leaves to dry completely. The temperature, wind speed and humidity are from the local weather channel.
At 10:00 am (23 C [73 F], 13 km/hr winds, 78% humidity), the drying times were:
- Front: 35 min (in full sunshine)
- Back: 70 min (full shade)
At 8:00 pm (25 C [77 F], 10 km/hr wind, 51% humidity), the drying times were:
- Front: 50 min (full shade)
- Back: 65 min (full shade)
The sun warmed the leaves and cut the evaporation time in half.
In shade, the drying time at night was about the same as in the morning. The listed wind speeds are general for the area, but this shrub is mostly protected from wind.
This is just one set of numbers, but the difference in evaporation rate for each measurement is not very large. With limited wind and no direct sun, evaporation takes about an hour. This is reduced by half in full sun.
Would an extra half hour make a difference for fungal infections?
Growth of Powdery Mildew
Powdery mildew is very common in the garden and next to black spot on roses, it is probably the most common fungal problem identified by gardeners. A common suggestion for preventing powdery mildew is to keep leaves dry and water in the morning.
It is important to note that powdery mildew is not one disease. Each plant family is infected by a different disease. So the powdery mildew on lilacs won’t infect the cucumbers or roses.
What few gardeners know is that “all powdery mildew species can germinate and infect in the absence of free water. In fact, spores of some powdery mildew fungi are killed and germination is inhibited by water on plant surfaces for extended periods.”
Clearly, if you are trying to prevent powdery mildew, it does not matter when you water . In fact, keeping leaves wet might just reduce the number of spores!
“Overhead sprinkling may help reduce powdery mildew because spores are washed off the plant. However, overhead sprinklers are not usually recommended as a control method in vegetables because their use may contribute to other pest problems.”
Growth of Downy Mildew
Downy Mildew is also common on some vegetables.
This fungus is even more species specific than powdery mildew, but it does require free water and humidity above 90% to infect a plant. “Infection occurs, generally in 8 to 12 hours.”
The tests above showed that there might be a drying difference of 30 minutes between morning watering and evening watering. That will have a limited effect on an infection process that takes 8 hours.
Growth of Black Spot
Black spot is common on roses. It is a fungal infection and “for spore production and infection, leaves must remain wet for more than about 7 hours.”
Watering at night, where leaves are wet for an hour, will not contribute to this disease.
What Do Farmers Do?
They irrigate when they need to irrigate and they usually keep it going day and night. That works for them, so why would it not work for gardeners?
A possible answer is that they use more fungicides than gardeners, but it could also be that it really doesn’t matter.
How Long Do You Water?
When you water it is important to water deeply. If you are using a sprinkler system this means that you water a long time. A few hours in the morning is not a great way to water any significant area. When I water, I set up the sprinkler in the evening and water all night – about 12 hours.
The other reason for watering in the morning is that the cooler temperatures reduce the amount of water lost to evaporation. More water ends up being absorbed into the soil.
However, wind and humidity also play an important role and on some days it is cooler at night than in the morning. A much better rule for water conservation would be to water at times that are cool, have low wind and high humidity. You might have noticed that these conditions are the opposite of those that suppress diseases.
There is one other factor that I have not taken into account. Watering in the morning may result in less loss due to evaporation, but then the hot noon day sun comes out and dries the soil. Watering at night allows more time for the water to seep deeper into the soil before it gets warmed by the sun.
What is not clear to me is which of these factors play the more important role?
Even if we knew how to evaluate these various parameters, it would be too complex for most gardeners. Based on this discussion I am not convinced that water is conserved by watering in the morning. It is however, a good idea to stay away from watering in the middle of the day.
Should You Water Plants From Above?
Lots of advice says that you should not get water on leaves. Water the ground, not the plant.
Many fungal spores, like powdery mildew, travel by air. They land on leaves and then wait for the right conditions to germinate and infect the plant. Watering from above washes these waiting spores off the plant, thereby reducing the degree of infection. It also removes dust from leaves.
Should you water plants from above? It probably depends on which problem you are trying to control.
Do Water Droplets Burn Plants?
No they don’t. See Myth #142, Garden Myths Book 2.
Do You Need to Water in the Morning?
Watering in the morning is common advice, but I can’t find any evidence that it makes any difference.
The three common fungal infections discussed above are not influenced by when you water. Other fungal infections may behave differently, but I find it hard to believe that an extra 30 minutes of drying time will make a difference.
The spread of disease and water conservation seem at odds with each other.
For disease control it is better to water when plants are in full sun because they dry quicker. Try to water when temperatures are high and humidity is low. However, this probably has very little effect on the development of fungal diseases.
For water conservation, water when it is cool, there is no direct sun, wind is low and humidity is high.