Cacti handle heat far better than cold. Many survive tissue temperatures that would cook most garden plants, yet a few degrees of frost can kill them. Which cactus you have matters enormously, because the toughest prickly pears survive winters colder than −40 °C.1Ecology. Freezing Tolerance and Water Relations of Opuntia Fragilis from Canada and the United States How quickly the temperature changes counts nearly as much as how extreme it gets: a cactus given weeks to adjust can survive conditions that would kill it if they arrived overnight.
How Hot Is Too Hot for a Cactus
In common cultivated cacti, half of the stem cells die only when the tissue reaches about 57 °C (135 °F).2Journal of Arid Environments. Temperature tolerances for stems and roots of two cultivated cacti, Nopalea cochenillifera and Opuntia robusta: Acclimation, light, and drought The pads of the prickly pear grown for food tolerate tissue temperatures up to 65 °C.3PubMed. Tolerances and acclimation to low and high temperatures for cladodes, fruits and roots of a widely cultivated cactus, Opuntia ficus-indica Even so, the plant’s history matters. An hour at 60 °C killed plants that had been growing in mild 30 °C days, over the following two months.4PubMed. Recovery of photosynthetic reactions after high-temperature treatments of a heat-tolerant cactus After an hour at 55 °C, the same kind of plant stopped growing for a while and then fully recovered within about 60 days.5PubMed. Recovery of photosynthetic reactions after high-temperature treatments of a heat-tolerant cactus
A 45 °C day doesn’t mean a 45 °C cactus. Direct sun heats the stem surface well above the air temperature, and a dark pot can heat the root zone even more than the air around it. Still air carries away less heat than a breeze does, and a spot that reflects heat, like a south wall, adds more. Bare desert soil can reach 70 °C at the surface.6PubMed. Extreme temperatures and thermal tolerances for seedlings of desert succulents
A cactus on a sunny windowsill can overheat behind glass, because the glass traps heat. Within one plant the damage is lopsided: the side facing the afternoon sun and the part touching a hot pot rim show harm first.
Long heat waves can weaken a plant without killing it. After two weeks at a constant 40 °C, prickly pear seedlings took in roughly a third to a half less carbon dioxide at night.7PubMed Central. Continuous high and low temperature induced a decrease of photosynthetic activity and changes in the diurnal fluctuations of organic acids in Opuntia streptacantha
A stressed plant pauses its growth but stays firm and green. Dead tissue turns soft, then brown or black. Because recovery after a heat shock can take about two months, don’t cut back pads that are stressed but still firm.
How Cold Can a Cactus Survive
Opuntia ficus-indica, the prickly pear grown for its fruit, has pads, its flat stem segments, that are damaged below about −6 °C (21 °F).8PubMed. Tolerances and acclimation to low and high temperatures for cladodes, fruits and roots of a widely cultivated cactus, Opuntia ficus-indica Other common cultivated cacti lose half their stem cells at about −7 °C.9Journal of Arid Environments. Temperature tolerances for stems and roots of two cultivated cacti, Nopalea cochenillifera and Opuntia robusta: Acclimation, light, and drought
Hardy species go far lower. Opuntia fragilis, the brittle prickly pear, can tolerate about −29 °C on average and is the most freeze-tolerant cactus known.10Ecology. Freezing Tolerance and Water Relations of Opuntia Fragilis from Canada and the United States Its wintering pads survived −50 °C without visible injury, and plains prickly pear (O. polyacantha) survived −40 °C.11Canadian Journal of Botany. Freezing and heat tolerance of Opuntia cacti native to the Canadian prairie provinces Populations from colder places are also more cold-hardy than those from milder ones.12Ecology. Freezing Tolerance and Water Relations of Opuntia Fragilis from Canada and the United States
Short chills can be survived. After one night at −10 °C, a hardy pincushion cactus’s growth dropped by 70% but fully recovered within 4 days.13PubMed. Influence of freezing temperatures on a cactus, Coryphantha vivipara
To find out where your own plant falls, start with the plant tag or look up the full Latin name, rather than trusting a shop label like “hardy cactus.” Species from the tropics, rainforests, or low, warm deserts are generally frost-tender and should be sheltered whenever freezing is forecast. The ones that tolerate hard freezes are mainly northern prickly pears native to cold regions and some hedgehog cacti, and even these do best when the soil is dry. If you can’t identify the plant, treat it as frost-tender.
Freezing is so harmful to a plant full of water because ice can form inside the cells themselves, and the sharp crystals can rupture the cell walls. That’s why thawed tissue turns to mush. A plump, well-watered stem holds more freezable water than a shrunken one, so a full plant going into frost is at greater risk. A sudden overnight plunge is more dangerous than a slow decline, because the cells have no time to adjust.
Why Gradual Change Makes Cacti Tougher
Hardening off is the plant’s slow adjustment to a new temperature over days to weeks, during which its cells change their chemistry to cope. Plants kept in hot conditions of 45 °C by day and 35 °C by night survived an hour at 60 °C without visible damage.14PubMed. Recovery of photosynthetic reactions after high-temperature treatments of a heat-tolerant cactus The reason is that plants grown warm produce more kinds of protective proteins.15Genome. The expression of temperature-stress proteins in a desert cactus (<i>Opuntia ficus indica</i>)
Cold works similarly. In cool weather, cacti build up sugars in their cells, which helps keep ice from forming inside them.16PubMed Central. Water Relations and Low-Temperature Acclimation for Cactus Species Varying in Freezing Tolerance They also make more of their natural gel, which slows how fast water leaves the cells during freezing.17PubMed. Water relations and mucopolysaccharide increases for a winter hardy cactus during acclimation to subzero temperatures18PubMed Central. Changes in Osmotic Pressure and Mucilage during Low-Temperature Acclimation of Opuntia ficus-indica Hardy prickly pears let their water content drop by about half in autumn and shrink about 30%.19Canadian Journal of Botany. Freezing and heat tolerance of Opuntia cacti native to the Canadian prairie provinces As a result, their freezing tolerance rose from −7 °C to −50 °C between September and October.20Canadian Journal of Botany. Freezing and heat tolerance of Opuntia cacti native to the Canadian prairie provinces But new pads formed in June could not survive even −3.5 °C.21Canadian Journal of Botany. Freezing and heat tolerance of Opuntia cacti native to the Canadian prairie provinces
The change isn’t instant. A plant just moved from a warm room to a cold porch hasn’t built any protection yet and is as vulnerable as before. The same holds for heat: a plant brought out of a cool house onto a hot, sunny patio needs time to toughen up.
Wrinkled, shrunken pads in autumn are the plant deliberately losing water to prepare for cold, not a sign of thirst or disease. Resist the urge to water heavily to plump them up, because that refills the cells with freezable water just as the cold arrives.
You can help the process along. Cut back watering from late summer onward, and leave plants outdoors through cool autumn nights before any frost. Avoid encouraging soft new growth late in the season. In spring, move indoor plants outside gradually.
The Weak Spots Roots Seedlings and Fruit
Prickly pear roots and fruit are damaged after an hour above 55 °C or below −7 °C, a narrower range than the pads.22PubMed. Tolerances and acclimation to low and high temperatures for cladodes, fruits and roots of a widely cultivated cactus, Opuntia ficus-indica Desert cactus roots grow best at a soil temperature of about 30 °C.23Annals of Botany. Thermal and Water Relations of Roots of Desert Succulents The top 2 cm of desert soil gets too hot for roots, which is why roots are missing there.24Annals of Botany. Thermal and Water Relations of Roots of Desert Succulents
In pots, this makes the container matter. A black plastic pot soaks up sun and heats the root zone, so light-coloured or clay pots generally stay cooler. Group pots together or shade their sides so the walls aren’t in direct afternoon sun. In winter, a pot sitting outdoors is exposed on all sides, so wrap it, insulate it, or set it against a wall, because roots can freeze before the stem does.
Seedlings get only slightly more heat-tolerant as they age.25PubMed. Extreme temperatures and thermal tolerances for seedlings of desert succulents In the wild, young cacti survive mostly in shade under shrubs and rocks.26PubMed. Extreme temperatures and thermal tolerances for seedlings of desert succulents At home, keep seedlings in bright shade rather than full sun, and never leave them in hot afternoon sun. Harden them off slowly before giving them more light.
How Spines and Ribs Help Cacti Handle Heat
In a barrel cactus, spines cut the daily summer swing in stem-surface temperature from 41 °C to 25 °C.27PubMed Central. Thermal Energy Exchange Model and Water Loss of a Barrel Cactus, Ferocactus acanthodes Ribs give the stem 54% more surface for shedding heat than a smooth stem would have.28PubMed Central. Thermal Energy Exchange Model and Water Loss of a Barrel Cactus, Ferocactus acanthodes Without spines and ribs, the stem would average 5 °C hotter during the day.29PubMed Central. Thermal Energy Exchange Model and Water Loss of a Barrel Cactus, Ferocactus acanthodes
Spines cast tiny shadows over the stem surface and break up the sun’s direct hit. Ribs also create shaded grooves between the ridges.
So don’t remove a cactus’s spines. Densely spined species usually handle sun better, while a smooth, green species may need more shade.
Sunburn Versus Heat Damage
Strong sun combined with heat yellows dragon fruit stems.30PubMed Central. Sunburn mitigation in dragon fruit (Hylocereus spp.): unravelling genotype-specific physiological and biochemical responses About 80% of unprotected dragon fruit plants grown in the open got sunburn.31PubMed Central. Sunburn mitigation in dragon fruit (Hylocereus spp.): unravelling genotype-specific physiological and biochemical responses A kaolin clay spray plus 30% green shade net cut sunburn to about 19%.32PubMed Central. Sunburn mitigation in dragon fruit (Hylocereus spp.): unravelling genotype-specific physiological and biochemical responses
Sunburn shows first as yellow or white patches on the side facing the sun. Over time the patches can turn tan and corky as the damaged skin dries and hardens. Those areas stay scarred, but the damage is usually cosmetic and not fatal if the rest of the plant is healthy. Plants from a shop or from indoors burn easily. Increase their sun exposure over two to three weeks, and shade them in the afternoon during a heat wave.
Protecting Your Cactus From Frost and Heat Waves
Nearby objects can do a lot of the work. In winter, spots next to rocks stay about 2 °C warmer than open ground.33Journal of Arid Environments. Nurse rock microclimates significantly buffer exposure to freezing temperature and moderate summer temperature That protection gets stronger as nights get colder and lasts through the whole night.34Journal of Arid Environments. Nurse rock microclimates significantly buffer exposure to freezing temperature and moderate summer temperature Under a mesquite tree, 95% of young cacti survived simulated warming, while all those in the open died.35Journal of Arid Environments. Do nurse plants enhance cactus survival under global warming? Experimental evidence from Coryphantha maiz-tablasensis, a threatened species For new plantings, put cacti next to boulders or walls for winter warmth, and choose an east-facing spot to avoid harsh afternoon sun.
On a frost night:
- Cover the plant with frost cloth or an old sheet rather than plastic that touches the stem, since plastic transmits cold to wherever it touches.
- Take the cover off in the morning so the plant doesn’t overheat or stay damp.
- Keep the soil dry before a freeze, since wet soil and a full plant hold more freezable water.
- Move pots against a house wall or indoors, where the building gives off a bit of warmth.
In a heat wave, provide afternoon shade and water established plants deeply but not often. Hold off on repotting and fertilizing until it passes.
Will a Damaged Cactus Recover
Firm green tissue usually comes back over days to months, while mushy or black tissue is dead.36PubMed. Recovery of photosynthetic reactions after high-temperature treatments of a heat-tolerant cactus After a frost, wait about a week before judging, since the damage often takes days to show. Soft, translucent tissue is dead and won’t recover, while firm tissue is likely alive. Once the weather warms, cut back to firm tissue with a clean knife. Let the cut surface dry for several days before watering, so the wound seals rather than rots.
Rot at the base usually spreads through the whole plant. In that case, take healthy pads or tops as cuttings to start new plants.