Gardening during the summer months is often the most enjoyable and rewarding time of the year. Plants are in full bloom, and every morning is a chance to see what grew overnight.

Yet, summer months also introduce their own challenges. As chronic heat waves are becoming more and more common in the U.S., heat-related problems in the garden are unfortunately becoming an ordinary experience. Particularly in lower-latitude climates, from Southern California to Northern Florida, the heat can be unbearable.

If you’re having garden problems, though, it might not be just the heat. One problem that often gets overlooked is ultraviolet (UV) radiation.

Heat and UV rays often occur simultaneously, but each carries its own unique problems. For example, the UV index in your area may be high even though the temperatures feel mild for the day. Likewise, you might have adequate shade in your garden for UV protection, but the heat is still causing stress in your plants. If you’re having summer garden troubles, diagnosing what ails your garden is crucial – is it heat or UV?

Ultraviolet rays are invisible wavelengths that radiate from the sun, separated into -A, -B, and -C varieties (though -C varieties are largely blocked by the Earth’s atmosphere). It’s the UV-A and UV-B rays that can hurt plants and people alike.

We know that humans use UV-B to make vitamin D. So, you might imagine that plants also use UV light for photosynthesis. But, alas, this isn’t the case. Plants can’t use invisible light such as UV to photosynthesize. If they could, we’d be able to grow our gardens in darkness with only UV light. While that would be awesome, it’s not how nature works.

Plants require visible light to photosynthesize, which falls between Infrared (IR) light and UV light.

During photosynthesis, the chlorophyll in a plant’s leaves absorb mostly red and blue light, which are visible to the human eye and are the most efficient wavelengths for plants to produce energy.

When exposed to excessive UV light, plants can experience various negative biochemical reactions. Fortunately, plants have evolved to handle UV exposure from the sun - but prolonged exposure can still harm them.

So, do you need to worry about UV rays? Depending on your location and what time of day you enjoy gardening, consider these factors:

Geography:

UV rays are most extreme in lower-latitudes regions (e.g., Southern California, Arizona, Texas, Florida) where temperatures are coincidentally also higher. Consult your local UV index for your area’s current rating. Usually weather websites offer real-time UV indices, which can help you determine how much UV you and your plants are being exposed to.

Time of Day:

UV intensity is correlated with how direct the sunlight is. As the sun rises or sets, there is more atmosphere the UV rays must pass through, acting like a filter that weakens the rays.

By contrast, UV rays are strongest when the sun is directly overhead, also known as “solar noon”, because there is the least amount of atmosphere through which the light must pass. Thus, the period around solar noon, say, between 11am and 3pm, is when UV rays are the most extreme and dangerous. Coincidentally, this is usually the hottest time of the day as well.

For your own safety, it’s best practice to cover-up with long-sleeves and a large hat, wear sunblock, or avoid midday gardening activities. The morning and early evenings are best!

UV Plant Problems:

Ok, ok, so what about my plants? Are they going to die from UV? Probably not, but below are some common UV-related symptoms with the underlying causes:

• Symptom: yellowing or whitening of leaves, discoloration, necrosis, leaf drop

• Possible cause: Chlorophyll degradation

• Note: Excessive UV exposure can damage chlorophyll molecules, reducing a plant’s ability to photosynthesize, often causing discoloration in the leaves. If you see a combination of these symptoms, it would be wise to mitigate your garden’s UV exposure.

• Symptom: abnormal, stunted or twisted growth patterns; curled leaves; necrosis, scorched leaves

• Possible cause: Cell and/or DNA damage

• Note: Just as UV light can cause cell and DNA damage in humans, the same processes can happen to plants. UV can cause DNA strands to break, damage cells membranes, and “denature” plant proteins. In sum, UV can damage plants at the cellular level causing plants be weak, small, or “wonky”.

• Symptom: reduced viability of pollen, blossom drop, lack of fertilization, no fruit

• Possible cause: Damage to plant’s reproductive health

• Note: UV can damage pollen grains, reduce pollen tube growth, inhibit flower growth, mutate flower shape/sizes, and reduce the seed set and viability. The effects of UV on a plant’s reproductive health are far-reaching, and they can affect the seeds they produce. If you keep your seeds, then it’s a good idea to be aware of plant DNA damage in particular.

How have plants adapted to UV radiation? One theory is that plants create leathery leaves as a stress response to UV light. These leaves create a waxy cuticle at as a physical barrier to UV, kind of like sunscreen for humans. Such adaptation does provide the plant with more structural integrity, but it can also inhibit gas exchange. If you see your plants developing thicker-than-normal, waxy leaves in the middle of summer, it could be a sign UV radiation is affecting your plants.

Solutions to Excessive UV:

Sun shades: Install sun shades midday or during UV-intense days to shield your plants.

Wet your soil: Since water absorbs light, wet soil can actually reduce UV rays reflecting off your garden bed. Dry soil reflects more light than wet soil, so keep it moist!

Plant Sunscreen: I have no idea if this stuff works, but there are plant sunscreens you can spray on your plants. They’re very expensive and with mixed reviews. I’m doubtful…

Thanks for reading! I hope this article “sheds light” on UV radiation and its impact on your garden. Remember, UV rays are as harmful to humans as they are to plants, so protect both yourself and your garden!