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Acclimating Indoor Cacti to the Outdoors

For those who do not have the luxury of keeping their cactus collection outdoors all year round, spring is usually the time when we put our plants outdoors.  We all want to take advantage of the growth spurt we get from the strong summer sun and heat.  The sun helps put on growth and wakes up the plant’s immune system.  And cacti will also benefit from the increased fresh air exchange that occurs outside.  But if you do not properly acclimate your cacti, it can ruin their aesthetic appeal.  And for serious collectors, that can potentially mean hundreds or even thousands of dollars in cacti damaged.

People commonly misconceive that cacti can handle the sun without issue. After all, they live in the desert where the strongest sun on earth occurs. How can springtime sun, when the sun is barely gaining strength, do any harm? Well, there are two things you’ll want to keep in mind. The first is that the sun outdoors, even in the shade, even in the spring or fall, is many times stronger than even bright lights. You might feel like you can see better in bright indoor light. However, the total output of energy is much greater outside. After all, we’re talking about the sun here. It powers the entire planet. So any move outdoors is a move to more light.

The other issue you want to keep in mind is that although cacti have an incredible ability to handle sun, it is a capability that has to be turned on. Wild cacti have been outside their entire lives, so the level of sun they are “tuned” to handle is pretty much the same all the time. They never reach the level of darkness that indoor cacti experience. There is variation between seasons, but the overall change is gradual. Wild cacti do scar, but not to the degree that will usually happen if you totally neglect acclimating your cacti before bringing them out.

When you have a cactus that has been indoors all winter, even if it was under bright artificial lights, it is like a person who has been indoors all winter. Did you ever notice how easy it is to get burned early in the summer? It’s not because the sun is stronger early on. It’s because your skin has not yet built up a tolerance to the sun. Cacti (and other plants) are the same way.

A cactus under too much sun can burn in only a matter of minutes. It will usually cause the cactus to quickly discolor. That discoloration will eventually dry out, shrivel and scar, leaving an unsightly rough texture where the nice cactus flesh color had been. Unlike human skin, these scars are permanent. So, how do we avoid this?

The concept of acclimation is very simple. You want to start out with a minimal exposure, both in time and intensity, and gradually increase it over time until your cacti reach their desired location. While conceptually it is simple, it can logistically be a big ordeal depending on the size of your cactus collection. If you have a lot of cacti, the best thing to do is put them in crates or bins so you can transport them more easily. Instead of moving back and forth with each plant, you can carry multiples at once.

t is best to start your acclimation early in the season when the sun is at a lower intensity. But you also want to make sure to do so after the nighttime temperatures are securely above freezing. Find a shady spot in your yard, preferably under a deck or bush. Keep an eye on your plants. If they start to lighten or change in color at all, bring them back indoors immediately. Sometimes it is already too late to prevent scarring once you notice any color change. Usually under a deck or bush will be dark enough that you do not have to keep moving them back and forth between indoors and outdoors. Keep the plants in that spot for about 2 weeks. Next, find another shady spot with slightly more light, and do the same thing. The more levels of shade you can graduate to before reaching your final spot, the better. Sometimes, even after the plants have been outside for a while, you can still do damage when you get to direct sun. So you cannot be too careful if you really want to avoid any harm.

If you lack the extreme shade of underneath a deck or bush, you may have to manipulate your sun exposure with time. In that case, start your cacti in the shadiest available spot for about an hour or so before moving it back inside. The following day, try two hours and continue to lengthen the time. There’s not a set formula in terms of how long to take to do this. It depends on a lot of factors. But the idea is to do it gradually enough for the plant to build up its immunity to the sun before the sun does its damage.

Another thing to consider when acclimating is rain. If your cacti have been in dormancy all winter, it is best for them to be “awake” before they are watered. If you know you are getting any substantial rain within the first week of acclimation, it might be a good idea to bring them in for those periods as well. Otherwise, try to find an overhang or somewhere they won’t get soaked. You might expect that they really need a drink at this point in the season. But let them wake up and get their immune systems in gear before you allow them to drink. Chances are they would still be fine. But you should always have a cautious mentality, especially if you have rare or expensive specimens. Anyone who’s experienced the loss of a few plants they love probably won’t find it hard to think that way. And if you can learn before you have to experience that, then you’re ahead of the game.

As you approach this process, think of it in terms of chemicals that have to build up in the skin of your cacti. It is ironic because you need the sun to activate the production of these chemicals. Yet, if you give them more sun then they are ready for, the very thing they need will harm them. But if you have patience and avoid shortcuts, the payoff will be that you will have the types of specimens that other collectors drool over.