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how many days can it go without water?
Hi Jim, cacti generally can withstand a considerable time without water - in some cases, years. They have to ability to go dormant, shriveling up as they use up what moisture is in their tissues, until such a time as the rains start again.
Never worry about underwatering a cactus; the most common cause of their dying is too much water.
This is something that novice growers have a hard time wrapping their heads around; most houseplants, like Ficus, ferns or other commonly grown jungle type plants, will die instantly if they go even a few days without adequate moisture.
Cacti have originated in much different climates, many in extremely dry air in a desert situation in very poor soil, not the high humidity, deep shade and rich compost of a jungle.
They have evolved the adaptations of waxy skin for protection against high light intensity and heat, no leaves (these have changed into spines for protection) and the knack of swelling up with water for long term storage.
They also have distinctly different root systems; in many cases a deep taproot to seek out water from the lower levels of soil and also to anchor the plant, and a fine network close to the surface to obtain moisture from a quick rain shower or even dew.
Hope that helps,
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