It’s that time of year – at least, for many of us in the Northern Hemisphere – when the weather gets colder, days get shorter, and it’s time to figure out what to do with your beloved cacti if you can’t keep them outside. I’ll preface this blog post with a bit of the obvious: I’m based in San Diego County, and as a result, I don’t really see a super cold winter. We don’t get snow, we don’t see a hard freeze, there’s even years when it’s been 80F around Christmas time. When I describe keeping a cactus outdoors year-round, or in my greenhouse, bear our local mild winters in mind.
Having said that, we do get the occasional cold snaps, with temperatures in the high 20s for a brief period early in the morning, and it’s more common for us to have weeks where the high is between 60 and 70F.
Note that what I consider winter conditions (i.e. when cacti are dormant) are temperatures where the daytime high is below 70F…if you run your AC constantly in summer, with it set to 68 or 72, you’re mimicking their coldest time of year!
Daylight signals
Someone out there is probably doing a doctorate study on how cacti know it’s the right time of year to bloom, or it’s already been published and I haven’t read it…but as days get shorter, the plants respond in their own ways.
Other cues, such as cooler nights, increased humidity from fog in the morning, clouds, or even rain can be triggers for new seasons.
In my experience, the first major cue they receive about winter’s arrival is the shortened daylight hours. Even in our current climate, where we have summer-like weather stretching way into October (we just had a week of 90F+!), my cacti are all responding like it’s fall. My Ariocarpus are some of the most consistent about time of year: they bloom in the last week of September at the absolute earliest.
But consistently, year after year, when I review my past photos of my Ariocarpus – they bloom once a year, within 5 to 10 days of the day they bloomed the year before.
In 2022 and 2023, they bloomed around October 20th and in the following couple weeks. This year, they started blooming quite early. Maybe the warmth from the long summer? Who knows.
But as a signal: Ariocarpus know what time of year it is, and they are precise.
Other genera are less rigid (more or less), but the shortened days in October and the accompanying Ariocarpus blooms are a signal to plants – and a signal for me. When I see the Ariocarpus blooming, it’s my signal to start readying the greenhouse and my cacti for winter.
The cacti were getting ready anyway, but I know for sure when I see those flowers.
Readying In-Ground Cacti
If you’re able to grow your cacti in-ground, you have a few options.
For species native to your area, there’s probably very little you have to do. If the soil is well-draining, you can probably just let them be and hope for the best.
I have oodles of cacti not native to my area, and while they are adaptable, most of them don’t actually like the cold that we see in my particular area of San Diego county.
Personally, my current in-ground cacti are all in unfinished beds – no formal landscaping or anything of yet, just letting them get some size first. To help them through winters, I have a few practices.
In-Ground Prep
Spray with a systemic pesticide about 2 to 3 weeks before the “last watering” of the season
I aim to have my cacti absorb the systemic (internal) pesticide ahead of winter dormancy, so they have it in their system to prevent infestations over the cooler months. They’re not so dormant that the systemic pesticide truly lasts all winter, but it’s enough to defend them against the first flush of pests that seem to emerge when summer’s heat has passed.
Clean up debris and dead leaves
Pests absolutely LOVE debris and dead leaves. By debris, I mean old bloom stalks, random dropped leaves, or other miscellaneous bits of odds and ends that end up collecting in the garden. This bit of tidying helps remove any hiding spots for pests, as well as any hidden areas they might have laid eggs or where larvae are hiding out.
Mulch around each cactus with a layer at least 1″ thick, and at least 6″ around the base of each plant
This protects from the roots getting too cold if there’s frost, minimizing the risk of root damage.
Check my frost-cloth and sizing as needed
I cover my particularly sensitive cacti when there’s a frost warning, notably my brazilian species – such as the Pilosocereus azureus. Rather than a giant sheet of cloth, I prefer to have each of my sensitive plants under their own section, which makes it significantly easier to cover and uncover them. Every season, though, they get a little beat up, a little dirty, a little torn. I replace anything that has significant tears or spines in it, or add to my frost cloth collection if I’ve added new plants to my area.
Potted Cacti – Outdoors
If you’re in an area where the weather only dips to 30F or lower for an hour or two in the wee hours of morning, you can probably leave your potted cacti outdoors, as long as they’re sheltered from rain or snow.
Having a little plastic greenhouse from Amazon can be a helpful winter setup if you’re in an area that’s borderline. It’s not going to protect your plants from a hard frost, snow, or nights that stay 30F for hours.
But…it does bump up your daytime highs a few degrees, keeps cold air from frosting your cacti, and basically makes a little bubble of protection for your collection if conditions get sketchy. I used one for two years before investing in my main greenhouse, and it was worth it for my treasured specimens!
Potted cacti can’t get as cold as the ones in the ground. The ground evens out the temperature, making it less likely to fully freeze over (unless you’re in an area that gets a hard freeze, which I don’t), and less susceptible to sudden changes in temperature – either hot or cold.
Pots don’t have that luxury! Plastic pots are the least insulated, but show pots can also be risky, holding onto cooler temperatures compared to terra cotta.
The more sensitive your plants are to cold (brazilian species come to mind), the more protection you should expect to provide them.
When Nights Below 30F Are Less Than 5 In A Row
If your coldest nighttime drops are rarely below 30F, and rarely for more than a few days at a time, keep your potted cacti outside. Move them under shelter so they don’t get rained on, and don’t water them either. If frost is a concern, simply moving them next to your house walls is often enough to keep frost from landing. I keep Monstera deliciosa plants outdoors with nights that get to 28F, and keeping them next to the house has kept them healthy and happy. Cacti respond equally well!
The cold weather, cold days, and short daylight hours should keep the cacti dormant enough that being under cover and lacking access to full sunlight are…negligible in terms of stress. Not ideal, to be clear, but for the worst of winter they should survive.
Keep in mind these recommendations are to help your cacti survive.
Remember that they’re hunkering down for winter! They are holding on until weather improves. They’re adapted for this. They expect it. A little suffering is actually ideal.
When You’re Somewhere That Might Get Snow
Areas that get cold enough for snow to stay on the ground are poor candidates for cacti stay outdoors all winter.
If it’s only two or three weeks that your area is cold enough that snow is likely, or daytime highs aren’t above 50F, you can follow the same recommendations as above: keep the cacti under cover, close to your house, and move them back out as soon as the cold snap has passed.
Longer periods, or where it’ll fluctuate between “okay” and “not okay” for a few months, just bring them inside.
South-facing windows are ideal, and it’s extremely likely you’ll need supplemental lighting.
The consistent mild temperature of your house (60F – 72F) is warm enough that your cacti won’t be fully dormant – so they’ll be warm enough to grow even while you’re trying to keep them dormant.
I recommend supplemental lighting because cloudy, dreary winter weather is not enough light for a cactus, even if it was outdoors. Indoors and next to a window, it’s even less ideal, so supplemental lighting is needed.
For just one or two specimens, those little spot-light style lamps are probably adequate to get you through. If you have more than one or two cacti that need a light, though, you’ll need a brighter and wider spread.
Consider something like this for a series of shelves by your window, although again, remember this is temporary. If you look at the reviews for those lights, nearly everything on the succulent shelves pictured is etiolated, so…they’re acceptable supplementation, but woefully inadequate as a primary light source.
If your cacti need to be fully indoors, in a room, closet, or basement, take a page from the cannibis growers’ playbook: you need the big boy lights. The lights are adequately bright to probably prevent etiolation, but for more than a handful of cacti, you’ll need a larger light or multiple covering the same space.
I’ve found that my cacti do better if I can keep them outdoors as much as possible – and while I moved a few species inside the first winter I had them, I no longer do so.
Keep Them Dry
The biggest mistake you can make is giving your cactus too much water when it should be dormant.
If they’re indoors for winter, no water. I promise you, 3 or 4 months without water when they are in less than ideal conditions is perfectly fine. They may wrinkle, shrivel, or look less than optimal – that’s a normal response!
The real trick is in waking them up slowly in springtime, once they’re back outdoors.
For winter, however, resist the urge to give them water. If they’re indoors with the heater running, a little dribble to keep the soil from drying out enough to completely dessicate the roots can be helpful. I’m talking a little cup of water, dribbled on the soil, not enough to really even drain through – and only once a month, at most. The goal of that water is just to keep the soil from turning into a dried out cake, not to actually feed the plant moisture.
When to Wake Up Your Cacti
The hardest part about overwintering your cacti isn’t the prep.
It’s when to start waking them up.
I time mine with temperatures. When nights are consistently at least 50F, I start looking for at least a few days where the daytime highs are projected to be at least 75F for at least the next 3 days, preferably longer.
That’s when I water everything for the first time, but not deeply – only a splash.
You’ll see them start to plump, and signs of new growth over the course of a couple weeks. You want to water cautiously, only when the daytime highs are warm enough that the soil will dry out. When your daytime highs start drifting up into the low 80s, that’s when you can start being more relaxed about frequency and deep-watering.
The important thing is to take your time. If you give them too much water, too quickly, they’ll crack. Certain Copiapoa are particularly at risk for this (my Copiapoa hypogea seem to crack every spring no matter what I do), and even one of my Astrophytum capricorne cracked this year.
Good luck!
Winter is coming – and it’s not easy. I’d love to see pictures of your winter setup! Before the start of winter and at the end, and what you set up. Next winter, I’ll happily share a crowd sourced “this is what others like you have done” if I get enough photos and descriptions! Email me at [email protected] 🙂
