What does "baking in the sun" mean? A detailed explanation

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Images: Curator

This seemingly simple question about baking in the sun has caused quite a few misunderstandings, and has also led to the deaths of many succulents during the summer. We misunderstand the term "baking in the sun" because deep down we feel that succulents love sunlight. Sometimes we calculate how long a succulent is directly exposed to the sun, and hope that the longer the time, the better. Under strong sunlight, succulent leaves often look remarkably translucent, and their colors become very bright and vivid. This makes us even more likely to mistake sun exposure as something quite comfortable for succulents.

Many gardening enthusiasts assume that sunlight only needs to be blocked on clear days when the temperature rises above 35°C. Baking in the sun, after all, surely only counts as baking when it is under extremely intense sun. In fact, what the Curator wants to say is that the standard for baking in the sun is much lower than you think. Avoiding baking in the sun means: do not expose succulents to direct sunlight. "Bao" means "exposed," not "extremely."

For example: in summer, if we park outdoors, we all know to park the car in the shade, even if it is only dappled tree shade. If we casually park in the open, then in just one hour the temperature inside the car will become unbelievably high. There are many professional tutorials online teaching you how to cook all kinds of food in a car without a stove. These tutorials all begin by telling you to park a sedan with a black interior facing south on an open lot with no shade.

Even in weather that is not hot (below 30°C), shade and full sun exposure show an enormous temperature difference. A car parked in the shade will have an interior temperature at most 2-3°C higher than the outside air temperature. But a car baked in the sun will easily have an interior temperature 20-30°C higher than the outside air temperature.

If we use a light meter app, it is also easy to measure that in the same environment, the illuminance in the shade is only a few thousand, while once exposed to direct sunlight it instantly reaches tens of thousands. A difference of ten times or even more is very normal.

The main reason we overlook sun exposure is often the air temperature. We know that a rise in air temperature is itself the result of strong sunlight. If the air temperature is not high, then of course the sunlight is not that strong either.

For succulents, although they need plenty of sunlight, this need is limited, and more is not always better. We all know that succulents rely on photosynthesis to synthesize organic matter. Every chloroplast in a succulent cell is a solar-powered factory. Running this solar-powered factory requires the close cooperation of two raw materials and one energy source. The raw materials are water and carbon dioxide, and the energy source is sunlight.

In most cases, we assume that succulents have plenty of water. After all, aren't their thick leaves full of water? Actually, that is not the case. Succulents will not draw on the water in their leaves until they are in crisis. So no matter how much water is stored in their leaves, as long as the soil has dried out, they enter a state of water deficiency. Then photosynthesis weakens.

Once photosynthesis weakens, sunlight becomes superfluous for the succulent. In this situation, the succulent will consume nutrients to do things that protect against sunlight. One of these things is very welcome to us: the synthesis of anthocyanins, which colors the succulent and makes it beautiful.

Another thing is that transpiration becomes active. Unused sunlight causes the succulent's leaves to heat up rapidly. To lower the temperature of the leaf surface, the succulent has to open its stomata and cool down through water evaporation. Succulents with darker colors heat up faster than lighter-colored ones. If sufficient water is not replenished, the succulent's cells rapidly lose water, causing the leaves to shrivel.

Let me give a very rough example to illustrate the impact of baking in the sun on succulents. Suppose that when a succulent photosynthesizes, the ratio of water, carbon dioxide, and sunlight it consumes is 1:1:1. The succulent collected 10 units of carbon dioxide last night as a reserve. The succulent's roots can absorb 1 unit of water from the soil per hour. If we place the succulent in an environment that provides 1 unit of sunlight per hour, and it gets 10 hours of sun per day, the succulent can thrive.

But what if we provide stronger sunlight, 2 units of sunlight per hour? According to the 1:1:1 consumption ratio, the water the roots can provide is no longer enough. The succulent must consume some of the water stored in its leaves to use up the excess sunlight, and synthesize some anthocyanins to protect against the sunlight. At this point the succulent will become thinner, but also more colorful. At the same time, after 5 hours, the succulent's stored carbon dioxide is used up, and at this point it cannot synthesize any organic matter at all. Now the 2 units of sunlight per hour are all excessive, causing the succulent to heat up rapidly. To maintain normal life activities, the succulent is forced to start dissipating heat through transpiration. This consumes even more water, making the succulent even thinner.

Now do you know the secret of succulents not being afraid of sun exposure? A strong root system ensures the speed of water supply. Sufficient water ensures the roots have water to drink. Loose soil ensures the roots breathe smoothly and provide enough energy for cellular life activities. Lush foliage ensures the succulent can collect enough carbon dioxide at night. Good ventilation ensures that when the succulent cools down through transpiration, water can evaporate smoothly.

Conversely, succulents with poor roots or that are currently rooting should not be baked in the sun (such as succulents that are rooting or in the acclimation period), succulents with very dry soil should not be baked in the sun (such as water-deficient succulents), succulents should not be baked in the sun when the soil is too wet and not breathable (such as overwatered succulents), succulents with few leaves should not be baked in the sun (such as succulents that have just had leaves removed or old-stem succulents), and succulents should not be baked in the sun when the air humidity is very high (such as after rain).

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