Succulent root pruning tips: Succulents should be watered based on light

  Today's topic is watering based on light. "Watering based on light" is a bit of jargon I invented myself. I once thought this phrase sounded as cool as Darwin's "natural selection, survival of the fittest," so I often brought it out to show off in front of novice succulent growers. Basically, very few people ever questioned me to my face about what "watering based on light" meant, so I got away with using this jargon for all these years without any trouble.

  When I again used the four words "watering based on light" to answer succulent growers' questions about watering, I often received very blunt feedback from them: What does "watering based on light" mean?

  I thought about it myself too. "Watering based on light" is still easy to understand literally: it means water more when there is plenty of sunlight, and water less when there is not enough sunlight. But that interpretation does seem to lack practical guidance. So today let's talk in detail about what "watering based on light" really means.

  How does the water in a flowerpot disappear? There are only three ways: First, soil evaporation caused by sunlight hitting the soil surface. Second, transpiration by the succulent. Third, the succulent using up water through water storage or photosynthesis.

  Among these, the third one - the succulent storing water and carrying out photosynthesis (growth) - is what we consider the most meaningful way water is used. Unfortunately, this part accounts for the smallest proportion, generally only 1% or even less. If watering really could make a succulent grow onto its body, then it would not be a succulent, but a vegetable.

  First, let's talk about how sunlight affects the evaporation of water from the soil. Sunlight has a very big effect on the water in the soil. After sunlight shines on the soil surface, it can raise the temperature of the soil surface and promote the evaporation of water in the soil.

  So does that mean the stronger the sunlight, the faster the water in the soil evaporates? Not necessarily. This is directly related to the soil's water-retention performance.

  What we usually call granular soil breathes through the gaps between particles, and transmits water through the internal pores of the particles and the contact points between particles. When sunlight shines on the soil surface, the soil surface heats up, providing the heat needed for water evaporation, and the water contained in the surface soil begins to evaporate. Water in the deeper soil is drawn upward by capillary action, supplied to the soil surface, and then evaporated away.

  When the water in the surface soil evaporates, the gaps in the surface soil become filled with the water vapor that has evaporated out. Let's imagine the process of steaming mantou and steamed buns: the fire provides heat at the very bottom, and the steam rises upward, then passes over the mantou and buns and steams them cooked.

  I wonder if any of you have ever thought about this: water vapor is actually heavier than air, so why does it keep rising instead of sinking? It is because water vapor keeps escaping from the water, causing the water vapor pressure near the water surface to become very high, and thus pushing the water vapor to move upward toward the area of lower pressure.