Why can a cactus in the desert live without water for 2+ years, but the cactus on my desk needs to be watered regularly?

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I’ve heard it has something to do with conditions, but this seems counterintuitive since the desert conditions are much harsher (hotter, drier) than those in my house, so if anything you’d think it would require less water.

In: Biology
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Might be a different kind of cactus. Not all plants that look like that live in deserts, although most of them do. And even fairly dry deserts usually don’t go *years* without rain (those that do, like the Gobi or the Atacama, usually don’t have very many plants).

Saguaro cactus are the only species of cactus that can go for more than a few months without water. Very old, very large saguaro *can* live for years without water. That doesn’t mean that all old saguaro will survive – just that some will. It also doesn’t mean that younger, smaller saguaro will survive those conditions.

Most indoor cactus are not desert species – they come from the western regions of the US and Mexico that have a fairly unique climate. Those regions get tremendous amounts of rain between October and April and 0 rain for the rest of the year. But just because there is 0 rain doesn’t mean there is 0 water in the soil. The soil only totally dries out for a few weeks in August and September and those cacti store water all year in preparation for that.

Prior to August there is still *some* amount of moisture in the soil and one of the ways that cacti have evolved to deal with that climate is that they will develop root systems that are dozens of times the size of the cactus itself. Those gigantic root systems allow the cactus to extract enough water from the soil to survive as long as there is some moisture in it.

When people grow cacti they typically give them the exact same amount of space as they would any other plant. This prevents the cactus from growing its root system enough to survive on soil with a minimal moisture level like it could in the wild.

If you planted a cactus in an area where it had 200 square feet of soil all to itself then you would be able to just dump few feet of water on that area once a year and forget about it. If, however, you treat a cactus like you would any other plant – IE, you plant it in a small area and give it small amounts of water at a time – then its going to have needs that are similar to other plants.

Part of it has to do with how humidity and heat work when the sand/soil is in a huge mass. Deserts get very hot and dry, but it’s only the top layer which really catches the worst of the heat. A few meters below the surface, temperatures stay much more moderate through the day and night cycle, and the dryness is not so extreme. Residual water from rainfalls is retained down here a lot longer, and there may also be some groundwater reserves that it seeps up from. Most of the time even in a desert, there is always a *little bit* of moisture working its way up from a more humid layer towards the surface thanks to capillary action and evaporative cycles. It’s not *very* moist, but there’s a whole ton of it by mass, so it takes a long time to exhaust.

Your potted plant is just in a little isolated clump of soil, surrounded on all sides by air. [And the smaller the pot is, then the greater its surface area is, relative to its volume.](https://en.wikipedia.org/wiki/Square%E2%80%93cube_law) When the water in that clump of soil is exhausted, that’s it, it’s gone, it doesn’t have any deeper, moister neighbouring soil to sponge from. But if you tripled the size of the pot your plant was in, you would have to water it 3 times as heavily to reach the same soil moisture level, but thanks to the squared-cubed law linked above, it would take a lot longer to dry out.

Cacti are native to the Americas, most of them to Mexico and the Southwestern United States.

Desserts in these areas aren’t sandy dune deserts like the ones we see in movies, but rather rocky deserts. The dirt is dry or hard packed, and there are extensive areas of above ground exposed rock with no soil covering it.

These conditions already make it hard for most plants to grow and ensure there is very little water ever in the topsoil, making it all but impossible for plants that aren’t adapted just for the task. But they still get water pretty frequently through rain, it just doesn’t stay in the soil where plants can reach it.