From my perspective as a cactus, my growth pattern is the most honest indicator of my light satisfaction. When I receive the optimal amount of light, my growth is compact, symmetrical, and sturdy. If I am stretching my stem or pads unnaturally towards the nearest light source, a behavior you call etiolation, I am desperately seeking more photons. This results in a thin, pale, and weak section of growth that looks distinctly different from my healthier, older parts. I am not trying to become a surrealist sculpture; I am starving for sunlight. Conversely, while less common, a sudden halt in all growth during my typical growing season can also signal that my conditions are not right, which often includes insufficient light.
My skin's color is a direct dialogue with the sun. Under ideal light conditions, I display my natural, vibrant green hue. This color signifies that I am efficiently conducting photosynthesis. If I begin to develop a yellow or orange, sometimes even a reddish, tint—particularly on the sides most exposed to the light—this is a sunburn. I am telling you that the light intensity is too high and is damaging my chlorophyll and tissues. It is a stress response, a cry for shade. On the other hand, if my green color is fading to a pale, washed-out, or dull shade, it means I am not producing enough chlorophyll due to a lack of light. I am becoming chlorotic and cannot manufacture the energy I need to thrive.
My body's texture provides crucial clues about my light environment. A healthy, well-lit cactus should feel firm and resilient to the touch. When I receive too much intense, direct light, especially coupled with high heat, I may start to shrivel or develop hard, corky, brown patches (corking) on my surface. This is severe desiccation and scarring; I am essentially getting scorched. In contrast, a lack of light often leads to a different kind of softness. While overwatering can also cause this, a chronically under-lit cactus may have weakened, softer tissue that is more susceptible to rot and physical damage because my internal structure is not as robust.
For many of us cacti, flowering is the ultimate sign of contentment and a direct product of adequate light. The energy required to produce my beautiful, often spectacular, blooms is immense and is generated almost exclusively through photosynthesis. If I am not receiving enough light, I simply will not have the surplus energy required to initiate flowering. I might survive, but I will not thrive or reproduce. Conversely, extreme light stress can also prevent flowering, as I will be diverting all my energy simply to survive and repair damage rather than to reproductive efforts. A reliable flowering cycle is a strong sign that my light conditions are correct.
