In this post, we will talk about cactus light requirements and temperature requirements. How much light do cacti need, where to position them for the best light, and how to help cacti enter the dormant state. We will also talk about cacti temperature needs. While each cactus has its own light requirements, in this post you can learn about general cacti light and temperature needs.
In our previous post, we have discussed watering tips and techniques for cacti. You can read this article here.
How much light do cacti need – cactus light requirements
When keeping cacti indoors or outdoors, make sure they get maximum amount of light possible. If you keep your cacti outdoors, make sure they are in the spot to get maximum amount of light. And same for indoors – make sure to place them on a windowsill where they can get enough sunlight.
If you keep a cactus indoors and place it on a bookshelf, for example, your plant will not survive for a long time. While all cacti have different light needs, generally speaking, most cacti need bright and direct sunlight. To provide your cactus with enough light, place it on a windowsill, balcony, in an unshaded greenhouse or generally outdoors.
Cacti seeds don’t need much light for propagation. But seedlings and small plants start needing much more light right after propagation. Lack of light at this stage will most probably cause death of a young cactus.
Artificial lighting for cacti
If you can’t provide natural sunlight to your young cacti seedlings or adult plants, use artificial light instead. Providing artificial light can be expensive, but if choosing bulbs, don’t pick ones that get too hot and emit little light.
The best artificial light for cacti are fluorescent bulbs/tubes (and even compact bulbs) or LED grow lights like this. Don’t pick mercury vapor or incandescent bulbs. For example, with 75w grow lights can cover an approximately 5-10 square feet growing area. With fluorescent tubes, the number is often higher.
Placing your cactus on a windowsill for light
If you keep your cactus on a windowsill, there are some tips that can save your plant from a burn. In very hot weather, the sunlight going through the window will create a high temperature gradient behind the glass (around 50 degrees Fahrenheit higher), while the room temperature stays much lower.
These hot sun rays can seriously burn your plants. So, if the temperature outside reaches 100 degrees or over, make sure to shade your cactus from the direct sunlight on a windowsill. Cacti that have small or a little number or spines need more protection.
This is especially true if your cactus is on a south-facing windowsill, which gets most of the sunlight. In very hot weather, you can move your cactus to an east or west-facing windowsill. You can also keep your cactus on a balcony if you have one. As windows block most of the incoming light, try to take your cacti outside for summer months if possible.
Cacti kept indoors in winter
You are probably asking, “how to provide light to my indoor cacti in winter?” Well, in winter, you will need to place them on the brightest windowsill (often south-facing one) and give each cactus enough space to soak in as much light as possible.
Another thing to make sure is that windows are clean. Windows get quite dirty in winter, and this can block more than a quarter of incoming light.
Protecting your cacti from burns after the end of winter
In most cacti’s natural environment, they receive natural sunlight throughout the year, without being shielded from the sun. But if you live somewhere with cold winters with short sunshine hours, then your cactus might suffer from burns after winter. The problem is, your cacti haven’t received many hours of sunshine in winter, and can suffer from burns after being suddenly exposed to strong sun rays.
For example, in some USA states, sunshine hours start increasing in March or April. And if you don’t shield your cacti from so much sunshine, it will develop burns. These burns can cause necrosis (dying) of epidermal tissues on the stem, leading to disruption of metabolic and breathing processes. So, a burnt cactus can even die or at least lose most of its spines.
So, how to prevent any burns on your cacti after winter? Your aim is not to block the light fully, but to reduce its amount, to shade your cacti. To do this, you can use some sort of plastic sheets or paper, or even gauze and attach them on the window side.
You need to shade your cacti after the end of winter
Do all cacti need bright and direct light?
No, not all cacti need bright and direct light. Some cacti also prefer bright but indirect light, as full sun can kill these plants. But there are also cacti that hate bright light and it can seriously damage them.
For example, Rhipsalis is a genus of epiphytic cacti that hate bright and direct light – they prefer shade and mostly inhabit tropical rainforests. There are around 35 species of Rhipsalis – with one of the most popular ones being a mistletoe cactus, or Rhipsalis baccifera.
Some cacti prefer bright light but are fine with part sun – such as cacti in a genus of Mammillaria (including globe and nipple cactus), barrel-shaped cacti, small cacti of genus Gymnocalycium, or chin cacti.
Let’s also mention that even those cacti that prefer bright and direct light might sometimes need a period of time without much light. Make sure to shade these light-loving cacti from bright and direct light right after repotting or if you are treating them for parasites. Same goes for transitional period after winter.
Cacti and ventilation
Your cacti also need dry air and good airflow. If you are keeping your cactus on a windowsill, make sure to open a window to let the fresh air in. And if your cacti are in a greenhouse, make sure to open a vent (or have an automatic vent), for good airflow.
You must have a proper ventilation in a greenhouse, especially during spring and summer. Cacti hate stagnant air and high humidity. It is usually a good idea to place your cacti outside in summer, so that they can get more fresh air.
Cacti temperature needs – summer and winter
In cacti’s natural habitat, high temperatures in summer generally make them go dormant. This is when they slow down their growth to survive extreme heat.
And with the start of rainfalls and cooler periods, cacti start growing. If your state or country has naturally hot summers and warm winters with frequent rainfalls, you don’t need to change anything. And once again, everything will depend on cacti species.
But if you live somewhere with cold winters (for example most states in North America or UK), you need to let your cacti grow in summer and rest in winter. This is because in cold winter, there is no enough light for your cacti to grow.
So, if you live somewhere with cold winter, you must help your cacti enter dormancy by reducing the temperatures to around 45-55 degrees Fahrenheit (7-10 degrees Celsius).
You will also need to reduce watering frequencies (once in a week or so). Lowering temperatures is necessary. Because if you don’t, your cacti will have to evaporate water for cooling itself and as a result, will dry out. High indoor temperatures will also prevent your cacti from going dormant. Some cacti even bloom in winter.