While all succulents and cacti have different light requirements, this guide will serve as a starting point from which to establish your own environment-specific light exposure practices.
Light Requirements for Succulents vs. Cacti
It's true, succulents and cacti can be grown in the same environment, in the same pot, under the same lighting conditions and do just fine. It's also true that succulents and cacti can both adapt to a particular environment's light exposure, whether that be under-lit or overexposed. However, if you really want your collection to thrive, your succulents and cacti should be cared for in slightly different ways.
One would think succulents and cacti have the same light requirements, right? Wrong! Cacti, on average, require about 20% more light than their spineless counterparts. Succulents do best when exposed to between 3,000 and 4,000 fc (foot-candles), where as cacti do best when exposed to between 4,000 and 5,000 fc. Sounds great, but what in the world is a "foot-candle"? Foot-candles are defined as "a unit of illumination equal to that given by a source of one candela at a distance of one foot (equivalent to one lumen per square foot or 10.764 lux)," which is basically a fancy, old-fashioned unit of measurement.
The easiest way to measure foot-candles is by using a light meter. There are a number of light meters available in the market, but the one we use here at Leaf & Clay is the LT300 by Extech. While using a tool like this allows you to establish the exact light exposure experienced by your plants, it is definitely not required to grow and maintain happy, healthy succulents and cacti.
Overexposure, Underexposure, and The Sweet Spot In-between.
A common misconception is that succulents can thrive in almost any light condition. While they can certainly survive in most, thriving is a much taller order. Succulents prefer to live in the "sweet spot" between too much sunlight and not quite enough.
When succulents get too much direct-sunlight (overexposure), the most common issue they face is sunburn. Yeah, that's right, succulents can get sunburned just like us humans. This occurs most prevalently when a plant that has been kept indoors, or in a shaded area, is moved abruptly into unobstructed sunlight. One of the easiest ways to permanently damage a beautiful, tender succulent is to leave it in direct sunlight for too long.
The best way to avoid burning your plants is to keep them in an area that gets partial-sun for the majority of the day, or full-sun for several hours a day at most. If your only option is to keep your succulents in full-sun, make sure to introduce them to it gradually. Start out by providing full-sun for an hour a day, and increasing the exposure time in small increments depending on how they react.
There are many varieties of succulent that do well in full-sun including agaves, sedums, aloes, and echeverias, but they should still be "hardened" or introduced to that level of exposure slowly. The easiest way to tell what varieties do well in full-sun in your area is to drive around and see what your neighbors have planted in their yards or containers under similar conditions.
At the other end of the spectrum we have low-light conditions (underexposure), where your plants are not getting enough sunlight. The most common ailment in this scenario is "etoliation," or the stretching of the stems and leaves.
Etoliation can affect to plants grown both indoors and out, but it's most prevalent among those grown indoors. The first sign that a succulent is becoming etoliated is that the growth center will begin to twist and turn towards the nearest natural light source. As you know, plants rely primarily on photosynthesis to synthesize their food and grow so sunlight is paramount. Second, as the plant continues to grow it will become taller and more stretched out. For example, with proper light exposure an Echeveria should remain tight and compact like a rosette, but too little light and it can end up looking like a Christmas Tree.
The second most common ailment facing succulents grown in low-light conditions is the prevalence of disease. They say sunshine is the best disinfectant and that's certainly the case when it comes to growing succulents. Generally, areas of low-light, whether indoors or outdoors, have much poorer air circulation than their brightly lit counterparts. Lack of light and air circulation means soil stays wet longer, and wet soil is the perfect breeding ground for disease.
Finally, one the most frustrating consequence of growing succulents in low-light conditions (primarily indoors) is the loss of vibrancy. Bright light causes succulents to become "stressed" and express those beautiful colors we've all come to know and love. If you take away bright light, you can all but guarantee those pretty colors will fade from your plants. This is an unfortunate truth but a reality nonetheless. That's why we suggest sticking to plants that are green in color for low-light or indoor condition. Not only do they photosynthesize better and remain healthier longer in low-light conditions, but their color stays constant. If you live in a climate that requires you to keep your succulents indoors, you can spice up your collection by getting plants of diverse shapes and textures as opposed to funky colors. The unique geometry and mimicry found in succulents is just as intriguing and fun to collect as color, believe us!
The TLDR (too long, didn't read) Summary
Succulents like everything in moderation, even sunlight. Providing them with too much or too little sunlight can both cause irreversible damage. The easiest way to make sure your plants are happy is to pay attention to what they are saying. If they're getting sunburned, provide less light. If they are stretching out and getting tall or developing disease, provide more light.
The Next Steps
Now that you've become acquainted with some light requirement basics, it's time to learn about the closely associated environmental factor of temperature.