Does Watering Plants on a Hot Sunny Day Scorch Their Leaves?

One common gardening myth is that watering plants on a hot, sunny day will scorch their leaves. Is this a fact or a fallacy?

There appears to be no information published by university agricultural extension agencies, or in the horticulture literature, to support this idea. So, where does this idea come from?

In the scientific publications investigating the problem of leaf scorch, many possible causes have been identified and documented, but none of them are related to watering plants at midday during hot weather.

What is Leaf Scorch?

Leaf scorch is a form of leaf burn which appears as spots or burnt areas along the leaf margins (edges of the leaves), and between the veins of the leaves.

The primary cause of leaf scorch is excessive wind or direct/reflected sunlight during drought conditions, where there is a lack of moisture in the root zone.

Plants take up water through fine feeder roots, and transport it through their vascular system up into the leaves. When large amounts of water evaporate from leaf surfaces, and there is insufficient water in the soil around the roots, the roots are unable to supply enough water to the leaves to compensate for the amount lost through transpiration (evaporation through the leaf). Without water, the leaf tissue furthest from the major veins dries out first, because water has to travel a longer distance to reach those areas, and that’s why the leaf margins scorch first.

What are the causes of plant water shortages that lead to leaf scorch?

• Insufficient soil moisture or improper watering
• Water lost too quickly from leaves due to high temperatures and dry winds
• Roots killed by plant pathogens, excavation, over-watering or soil compaction
• Too much fertilizer creating high salt levels which ‘burn’ the roots
• Fungi or bacteria invading and blocking the water conducting vessels in the plant

To prevent leaf scorch, keep plants regularly watered, and don’t let them dry out. Protect plants from harsh sun and wind during drought conditions. Also, don’t plant in compacted soils or areas where the roots will be restricted.

Don’t take my word for it though, science is about experimenting, observing the results for ourselves, and explaining what we see. The best approach to garden mythbusting is to do real-world testing, to see what happens! We’ll need to use some basic physics and math to explain our results, but we’ll clearly explain it in such a way that everyone can follow.

In Hot Sun, Do Water Drops Burn Leaves?

To test whether watering in the midday sun can burn plant leaves, I devised a simple experiment, placing a large drop of water on a potato leaf that was exposed to harsh, direct midday sun. The temperature was 24°C (75°F) on the day, without a cloud in the sky, so the sun was persistent and constant.

A larger water drop was used, around 6mm (1/4″) in diameter, as it remains far longer on the leaf before evaporating, gradually shrinking in size as it does. This way, the water drop has more time to potentially cause any damage, through a range of droplet sizes.

I checked every few minutes to see whether the water had evaporated, and if there was any damage present.

In the picture below, we can see the water droplet is sitting on the leaf closest to the upper left-hand corner.

After a little over 20 minutes, the water had evaporated, and even after close inspection, no damage to the leaf was visible.

Why did the leaves not scorch? The results of this experiment raise a few pertinent questions!

One of the explanations of why leaves supposedly scorch when plants are watered on a hot, sunny day is what I would term the “lens effect“. The idea is that the droplets of water act as lenses, much like a magnifying glass, focusing the sun’s rays onto a small spot, burning the leaf. This idea has logical merit, as water will refract (bend or change the direction of) light much like a glass lens.

Can individual drops of water act as miniature lenses and actually focus light? To answer this we need to revisit some basic physics, which I’ll explain in simple terms that are easy to grasp!

What is a Lens?

A lens is a transmissive optical device, it allows rays of light to pass through it, unlike a mirror, which is reflective, and bounces light back.

When light passes through a lens, it is refracted, which means that the path of the rays of light are changed, they’re bent in a different direction, allowing the lens to either focuses or disperses a beam of light.

What kind of a lens is a droplet of water? Unlike a magnifying glass lens, which is convex (bulging out) on both sides, and is described as a double-convex or biconvex lens, a drop of water is convex on the top, and flat on the bottom, and is an example of a plano-convex lens.

Plano-convex lenses have one flat surface, and one convex surface. They are the best choice of lens for focusing (converging) incoming rays of light to a single point on the other side of the lens.

The point at which all the rays of light are focused is termed the focal point, which is always located a certain distance away from the lens, and this distance is known as the focal length.

Now that we’ve covered some basic optical theory, the next step is to go out and test it!

Testing Water Drops as Lenses

In this test, we’ll look at the ability of water droplets to focus light like a magnifying glass, to determine if they can focus sunlight to burn leaves, and if so, under which circumstances.

Holding a magnifying glass over a piece of paper, we can see how it focuses the sunlight into a small, bright point. As we move the lens closes or further away, we can focus all the light into the smallest spot possible, which gets intensely bright and extremely hot.

• The distance that we need to hold the magnifying glass above a surface to get the smallest, circle of the most intense light is the focal length of the lens, and varies from lens to lens.
• Additionally, a magnifying glass lens must be held at the correct angle towards the sun to properly focus light to attain optimum focus.

To use a drop of water in the same way as a lens, it needs to be placed on a thin sheet of glass or clear plastic. I used the latter, which was cut from the side of a soft drink bottle.

By lifting and lowering the water-drop lens above our test surface, I was able to get the sun’s rays to focus to a small point. We can see in the picture below that the sunlight is actually being focused quite well.

I found that to get the sunlight focused to the smallest point of intense light, I had to hold the water-drop lens approximately 1.5cm (5/8″) above the test surface. That means the water drop I used has a focal length of approximately 15mm, and will focus light effectively when suspended that high above the surface it is focusing light on.

But water droplets don’t levitate in mid-air, suspended above objects. Water always seeks the lowest point due to gravity, and tends to sit on flat surfaces it lands on, such as the surfaces of a leaf.

Let’s put a magnifying glass flat on our test surface, and observe what it does to the sun’s rays.

As we can see in the picture above, no discernible amount of light is being focused when the magnifying glass is laid flat on the test surface, because the lens needs some distance to focus the incoming sunlight to a single point.

If we do the same with our water-drop lens, which is a much smaller lens with a much shorter focal length, we still see the same result, there is no focusing of light.

From our experiment, we can see that with either lens, they need to be raised above the test surface, to a distance equal to their focal length, to focus light to a single point. Sitting on the test surface, they do very little to focus the light, and only converge the rays of sunlight to an insignificant degree that isn’t discernible.

Why Drops of Water Can’t Burn Leaves

All lenses have a particular focal length, they need sufficient distance between themselves and the object they’re focusing light on to refract (bend) the light to a single point. They can’t do that when they’re sitting flat against a surface.

Water droplets, which act as plano-convex lenses, and focus light quite effectively at their requisite focal length, obey the laws of physics, and behave just like any other lenses. They don’t focus light much when sitting flat on a leaf.

Is there another way a water droplet can focus sunlight and burn a leaf? The only way to get the required distance, equal to the focal length of the lens, is to shine the sunlight sideways into the drop of water, which will focus the sunlight on the other side of it.

A research paper published in 2010, titled “Optics of sunlit water drops on leaves: conditions under which sunburn is possible” looked into this very topic. To quote that study, the researchers concluded:

“We found that a spheroid drop at solar elevation angle θ ≈ 23°, corresponding to early morning or late afternoon, produces a maximum intensity of focused sunlight on the leaf outside the drop’s imprint. Our experiments demonstrated that sunlit glass spheres placed on horizontal smooth Acer platanoides (maple) leaves can cause serious leaf burn on sunny summer days.”

The sunlight needs to enter the water droplets at an angle of approximately 23 degrees, which is an extremely low angle in the sky, and only possible during early morning or late afternoon, and not during midday, when the sun is almost directly overhead in the peak of summer.

When the sun is at the critical angle of 23° and shines over a standard 1.8m (6′) fence, it casts a shadow that stretches for 4.24m (14′), shading many plants out (depending on their height), preventing sunburn from water drops.

To type this calculation on a scientific calculator, type 1.8 ÷ (TAN 23) and press the = button. There’s a simple rule for calculating this sun angle without a calculator – when the length of the shadow is 2.4 times the height of the fence, the sun angle is 23° above the horizon.

In our garden mythbusting, we can declare this garden myth as false. Watering plants on a hot sunny day will not scorch their leaves. In fact, watering them in the middle of the day, even though that may not be the optimum time for water uptake, may be the very activity that prevents leaf scorch!

References

1. Egri, Á., Horváth, Á., Kriska, G. and Horváth, G. (2010), Optics of sunlit water drops on leaves: conditions under which sunburn is possible. New Phytologist, 185: 979-987. https://doi.org/10.1111/j.1469-8137.2009.03150.x , https://nph.onlinelibrary.wiley.com/doi/full/10.1111/j.1469-8137.2009.03150.x
2. Linda Chalker-Scott, Ph.D., Extension Horticulturist and Associate Professor, Puyallup Research and Extension Center, Washington State University, The Myth of Hot-Weather Watering “Watering plants on a hot sunny day will scorch their leaves” https://s3.wp.wsu.edu/uploads/sites/403/2015/03/leaf-scorch.pdf
3. The Pennsylvania State University, College of Agricultural Sciences – Bacterial Leaf Scorch, Updated: June 16, 2014
4. Colorado State University Extension – Leaf Scorch-2.911 by S. Rose and C.E. Swift * (12/14) Revised by L. Langelo
5. Currituck Garden News – Leaf Scorch and Winter Drying
6. The Morton Arboretum – Horticulture Care, Leaf Scorch