Cacti need strong light. Many take direct sun for most of the day once they are used to it, and in hot desert climates most do better with some shade. A cactus burns when it gets more sun than it is used to, so how fast the light changes decides how the plant comes through.
The short version
- Give a cactus the strongest light you have, but introduce it slowly.
- A cactus burns when its light goes up suddenly. That covers a newly bought or shipped plant, a plant moved outdoors or out of shade, and a plant turned so a different side faces the sun.
- Start a new or moved plant in light shade: a bright spot outdoors with no direct sun from midday on. Hold it there for several weeks, then add direct sun a little at a time over several more weeks, morning sun first and afternoon sun last.
- Yellowing on the sun-facing side is the first sign of sunburn. Put the plant back in light shade, or cover it with shade cloth.
- A large plant that already grows in sun keeps the same side facing south when you repot or replant it.
- In hot desert climates, shade cloth over cacti through the summer protects them.
Terms used here
Full sun is direct sun for most of the day.
Light shade is a bright outdoor spot with no direct sun from midday on. 3 places give it: under an open tree or shrub that lets flecks of sun through, a spot that gets direct sun only in the early morning, or under shade cloth.
Shade cloth is a fabric sold by nurseries and rated by the percentage of sunlight it blocks. Cloth rated 30% to 50% is the range to buy for cacti. For a new or moved plant, 30% is enough.
Deep shade is a spot with no direct sun at all, which includes most rooms indoors. Most rooms are too dark for a cactus to grow well.
How much light a cactus needs
A cactus needs ample light to grow thick and sturdy. Outdoors, full sun or light shade both grow a healthy plant. Deep shade does not. Short of light, a columnar cactus (one of the tall, upright kinds such as San Pedro) puts on thin, stringy growth at the top, and the narrow section generally does not fill out later.
Indoors, use the brightest window you have. Direct sun through bare glass can still burn a cactus, because still indoor air does not carry the heat away. A plant new to a sunny window needs the same slow build-up as a plant moved outdoors, and a sheer curtain gives it light shade while it adjusts.
Hard-grown and soft-grown plants
A hard-grown plant has grown in full sun and open air. A soft-grown plant has grown under shade cloth, in a greenhouse or indoors. Many growers find that hard-grown plants carry heavier spines and that soft-grown plants are a lusher green or have blue tones.
You cannot always tell which you have, and you do not need to. Treat every newly bought plant as soft-grown and build up its sun. A greenhouse-grown cactus needs a gradual introduction to full sun whatever its size, and a plant grown in sun loses nothing by a few weeks in light shade.
A single plant can be both at once. On a cactus that has stood in one place outdoors, the south and southwest sides take the most sun and become toughened against sunburn. The north and east sides stay tender. (South and north swap in the southern hemisphere.)
When a cactus burns
Sunburn most often follows a sudden increase in light:
- A newly bought or shipped plant goes straight into sun. After your cactus arrives covers the first days out of the box.
- A plant moves outdoors from a room, or from shade to sun.
- A plant loses its shade when a tree or shrub beside it is cut back.
- A pot is rotated, or a plant is replanted with a different side to the south, which puts the tender side in the sun.
The risk is highest when the change happens in summer. Late winter and early spring, when the sun is less intense, are the easiest times to move a plant into a brighter place.
How to build up sun
- Start in light shade. Use one of the 3 spots described above. Under shade cloth, 30% is enough.
- Hold it there for several weeks. If you see new growth at the tip before then, the plant has settled and you can move on. In winter, when a cactus may not grow, go by the weeks.
- Add direct sun in steps over a period of weeks. Give morning sun first, then extend it toward midday. Midday and afternoon sun are the most likely to burn, so afternoon sun comes last. Move the plant, or pull the cloth back, a little further at each step.
- Check the sun-facing side every few days. If the skin there turns pale or yellow, return to the last spot where the color was normal. Stay there until the pale patch has greened up or stopped changing, then try the step again.
In summer, make each step smaller and leave more time between steps.
A cutting that has not rooted starts in light shade like any new plant. How to root a cactus cutting explains how to tell when it has rooted. Build up its sun after that.
Keeping the same side to the sun
When you repot or replant a cactus that has been growing in sun, set it back facing the same compass direction, so the toughened side still takes the strongest sun. Before you move it, mark the south side with chalk on the pot or a loop of string knotted on that side, and put the mark to the south again. Do not rotate a potted cactus that lives in full sun outdoors.
Orientation counts most for a large plant that has stood in one place for years. A plant or cutting that arrives by mail has no side to track. Build up its sun from light shade and every side toughens together.
Shade in hot climates
In hot desert climates, shade cloth rated 30% to 50% protects cacti and other succulents, and it stays on through the summer. A spot with morning sun and afternoon shade does the same job.
Many growers give San Pedro afternoon shade once temperatures climb past the mid 90s F.
Potted cacti need more shade than plants in the ground. In desert climates most potted cacti do best in light shade through the hottest part of summer, and plants 2 to 6 inches tall should stay in light shade.
Avoid planting against a south- or west-facing wall in a hot climate, because the reflected sun can burn the plant. Do not move a plant into stronger sun during a hot spell.
What sunburn looks like
Sunburn appears on the side that faces the strongest sun, usually the south or west side.
- Early: the skin turns pale, then yellow, on that side only. Shaded at this stage, the color can recover.
- Later: the yellow patch turns straw-colored or brown. The skin there has died. It may crack and form a rough, bark-like scar.
A brown scar is permanent, and the plant usually lives and grows on.
If a plant burns, cover it with shade cloth or move it to light shade the same day. Once you see new growth at the tip, build up its sun again from step 1 in "How to build up sun".
Burned skin can let rot in. A patch that is soft, dark and wet, or one that keeps getting larger after the plant is shaded, is rot and needs treatment promptly.
To match these steps to your climate and your spot, read Growing cactus for your conditions.
Sources
Problems and Pests of Agave, Aloe, Cactus and Yucca. University of Arizona Cooperative Extension, 2011. https://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1399.pdf
How to Transplant a Cactus. University of Arizona Cooperative Extension, 2020. https://extension.arizona.edu/sites/default/files/2024-08/az1376-2020.pdf
Cactus, Agave, Yucca and Ocotillo. University of Arizona Cooperative Extension, 2009. https://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1225.pdf
Saguaro Problems, Pests And Disease. University of Arizona Cooperative Extension, 2021. https://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1962-2021.pdf
Care for Potted Cacti and Other Succulents in Desert Climates. Arizona-Sonora Desert Museum. https://www.desertmuseum.org/visit/sheets/1Cact_pot.pdf
Planting Cacti & Other Succulents in the Ground. Arizona-Sonora Desert Museum. https://www.desertmuseum.org/visit/sheets/1Sucgrnd.pdf
Six Tips To Care For Your Plants This Summer. Desert Botanical Garden. https://dbg.org/six-tips-to-care-for-your-plants-this-summer/
Cactus Care Tips from Garden Horticulture Staff. Desert Botanical Garden. https://dbg.org/cactus-care-tips-from-garden-horticulture-staff/