Sunburn always happens the moment you let your guard down, so never count on luck.

Before I share my experience today, let me ask you a question first.

For example, yesterday's temperature was 35°C and the weather was sunny. Today's temperature is 33°C and the weather is partly cloudy. Your succulents have always been kept on the south-facing balcony. So, is it possible for them to get sunburned today?

Don't rush to answer yes or no. First think about why you would give that answer.

My answer to this question is: it is possible.

I don't know whether we see this question the same way, so below I will explain the reasons why I concluded that the succulents might get sunburned.

Succulents can get sunburned under two circumstances.

One is called ultraviolet burn, which is actually caused by succulents temporarily being unable to adapt to a large dose of ultraviolet radiation. Ultraviolet burn can happen in all four seasons, whether it is winter or summer, and whether it is sunny or cloudy. As long as ultraviolet levels surge and the succulents feel a little unable to adapt, it can happen. You might even shine a low-power ultraviolet sterilizing lamp on your succulents in hopes of reducing their etiolation, and that can also sunburn them.

Today let's set aside ultraviolet burn and talk about how the other kind of sunburn happens.

The other kind of sunburn, if we give it a name too, can be called high-temperature sunburn. The name is easy to understand: sunburn caused by excessively high temperature.

But we need to make it clear that the high temperature mentioned here does not mean high air temperature, but high temperature on the leaf surface. To put it plainly, so-called high-temperature sunburn does not happen simply because the air temperature is high or the sunlight is strong. Essentially, the succulent is scalded because its surface temperature is too high.

Most succulents that live in deserts or high mountains have a very strong tolerance for high temperatures. For example, one of the most common Lithops, Lithops 'Purple Haze' (Zixun), has a very strong heat tolerance. Test data shows that Lithops 'Purple Haze' can still survive tenaciously when its surface temperature reaches 60°C.

Human skin feels scalded at 50°C, and if it contacts 60°C for more than 1 minute, it may cause a third-degree burn. That is to say, if we touch a Lithops 'Purple Haze' that has been heated scorching hot by the sun, we may well get burned, but the Lithops 'Purple Haze' lives on completely unharmed.

This heat tolerance is not even the strongest. Many succulents in the Cactaceae family, such as the cactus that can produce pear-like fruit, can withstand 70°C and remain unharmed. Even the Snow Lotus (Saussurea involucrata), which is generally considered rather delicate, can remain completely unscathed when its surface temperature reaches 50°C. The legendary Little Red Cloak (Echeveria 'Little Red Riding Hood'), said to die every summer, also survives with no pressure when its surface temperature is 48°C.

From this point of view, the 35°C and 33°C we mentioned at the beginning of the article seem like merely cool temperatures to these succulents with super-strong heat tolerance, so why do we still need shade?

If that is what you think, then you are too naive. Look at the picture below.

This is a thermometer with a maximum range of 55°C. It was left in a closed car for only 15 minutes, and it had already completely maxed out. Moreover, what was tested here was the air temperature inside the car, not the surface temperature of the black interior under direct sunlight.

In a shaded environment, those succulents that are not heat-tolerant, such as Little Red Cloak and Echeveria 'Yu Die' and the like, have probably already left you without hesitation. And at that time, the real-time outdoor temperature was actually only 31°C.

So the weather forecast is not the only standard by which we judge whether to provide shade and cooling. Yesterday's 35°C air temperature may have left the succulents unharmed precisely because there was wind. And in still-air conditions, a 33°C air temperature can absolutely produce a leaf surface temperature high enough to kill succulents.

If the environment where you grow succulents is relatively enclosed, many other factors will also influence whether they get sunburned. For example, is there moisture in the potting soil? What is the air humidity? If there is moisture in the potting soil, the succulent has a chance to open its stomata temporarily to dissipate heat in a high-temperature environment and fight the heat. In a hot and humid environment, water evaporates very slowly, which also hinders heat dissipation in succulents and makes sunburn accidents more likely.

Many growers mistakenly think sunburn is a very long process. In fact, that is not how it happens at all.

We can make an imperfect analogy with the process of boiling mutton slices for hot pot. When we boil mutton slices, we hold the mutton with chopsticks and put it in hot water. For the first few seconds, the mutton seems to undergo no change at all, but in a certain instant, the color of the mutton begins to change from red to white, and then in just 1 second, the surface of the mutton is already fully cooked.

Sunburn in succulents is exactly the same process. Some important biological enzymes immediately decompose when they reach a specific temperature, causing the physiological functions of the epidermal cells to immediately collapse, and then the succulent is sunburned. This process happens extremely quickly; it is not a process that unfolds slowly.

Sometimes when we go downstairs to eat lunch, the strong outdoor sunlight suddenly reminds us that we forgot to pull up the shade net. At that moment we often have a thought: being in the sun for a while is no big deal; after lunch, when we get back, we will just hurry and pull up the shade net.

But perhaps in the time it takes for that one meal, the succulents that were just full of life may turn into dead flesh, and the only thing you can do is throw them away.

So what I want to say is: in summer, please do not indulge your wishful thinking; protect your succulents at all times and in all places.