Growing NON-Hardy Cacti in Cold Climates
CactiGuide.com was created specifically as an identification resource. While this still serves as the primary function
of the site, many visitors are seeking cultivation information. In fact, many people seeking identification are doing
so ultimately to find more specific information on cultivation. The CactiForum will
always be the best source for personal and very specific answers to cultivation questions. The cultivation articles,
on the other hand, are written to give a broad overview of some aspect of cultivation. This should be kept in mind when
reading this article. It is not intended as a precise method for growing every possible species in every
possible climate. None-the-less, this article should prove very helpful for anyone trying to grow cactus plants outside their
native range - specifically in reference to "warm-blooded", cactus plants in pots being grown in areas with
cool to cold winters.
There are some epiphytic cacti that thrive as houseplants, such as the well known "Christmas Cactus". While these plants
are true cacti, this article's focus is on growing the desert plants whose thick, fleshy, spine-covered stems fit the
universal stereotype of the name "cactus".
Left: This "Jungle" Cactus Thrives as a Houseplant
Right: These "Desert" Cacti Need Strong Light to Grow
The challenge with growing the sun-loving desert cacti where it is too cold for year-round outdoor growing,
is actually not the cold temperatures, but the amount of available light. This might seem counter-intuitive at first, but consider
the various tropical plants that are grown as houseplants including the afore mentioned "Christmas Cactus". These
plants are even less cold tolerant than most desert-dwelling cacti. Therefore, it is their lower light requirements
that enable these plants to be grown indoors successfully. On the other hand, desert-dwelling cacti require a much
higher amount of light for proper growth and cannot be grown as a typical houseplant. With this in mind, we will
further examine lighting conditions and then discuss how to best work with these limitations.
Aside from the jungle-dwelling varieties, the majority of desert cacti require strong light in order to grow properly. As these plants
do not grow well in competition with other vegetation, they are found primarily in hot/dry habitats where other vegetation is sparse.
In these conditions, cacti are rarely found in the shade while at the same time they frequently grow at latitudes or altitudes with
It is not easy for people to detect the difference in light intensity using their senses. After all, to the person standing in a
south-facing window in some far-northern abode in January, the sun will still feel quite warm and the brightness will hurt
their eyes - even if the outside temperature is well below freezing! In reality, the light intensity in that south-facing window is
only a fraction of the intensity of outdoor (direct) sunlight in the summer. It may be helpful for readers to imagine themselves
standing outside all day in the hot desert sun with no shade from morning to night. Then think again about standing in that
south-facing window. Not only would the intensity of light be much less in the window, but the duration of light is also much
shorter in the window. This is partly because the length of day decreases in winter with distance from the equator. It is also
because the window frame and house provide shade on either side of the window. Thus, the amount of light available to a windowsill
plant is further reduced by the shorter interval of sunlight each day.
This Mammillaria flowers profusely after a period of winter rest
Maximizing Summer Conditions
Before discussing how to best overwinter your desert cacti, let's first consider making the most of the growing season. As days
are shorter in winter further from the equator, so the days are longer in summer. These nice long days are when you want your
cactus plants to be soaking in the sun and actively growing. If possible, put your plants outside in a location where they
can receive the most amount of direct sunlight available. Caution should be used to avoid sun burn (more on this below).
If your yard or patio is partly shaded, look for the part with the least shade. As your plants are actively growing,
this is when they will be taking in water and nutrients and may take more than expected. You may find that shaded
plants seem to grow faster, but if you compare growth between a shaded and non-shaded plant, you will find that the
growth is not the same. Plants which receive more light will have thicker growth and the spines will especially be more
robust and more numerous. This latter growth is more like that in habitat and so it is generally considered more desirable.
This C. bigelovii
shows thick spine growth below and thin growth on top due to being moved into lower levels of light
Even better than placing your plants in the brightest spot in your yard, would be inside a greenhouse. There are several advantages
to using a greenhouse. Increased heat is perhaps the most obvious of these. In higher latitudes or altitudes, cool spells in
summer are common as are cooler temperatures in spring and fall. A greenhouse will enable you to keep your plants consistently warmer
throughout the growing season. Ventilation is important as a greenhouse can overheat and cook even the most sun-loving of species, but
this should not be a problem with a proper construction planning. In addition to keeping consistently warmer temperatures in the
summer months, a greenhouse can also greatly extend your growing season by providing warmer temperatures in the spring and fall.
While the sun may be quite strong in early spring, overnight temps may still get too low to safely leave pots outside. A greenhouse can
be heated on cool nights. A greenhouse also provides further advantage due to controlled conditions that are optimized for the best
possible growth. This is one reason that greenhouses are used even where plants can survive the winter. Of course, plants in the
wild, do not live in such luxury and often bear the marks of environmental hazards.
A heater helps extend the growing season in this greenhouse
Getting Through the Winter
As we stated early on, it is the dramatically lower light levels in winter which are the primary concern during winter months in
higher latitudes. Even a fully heated greenhouse that receives direct sun from morning to night, will have too little light
to properly grow desert cactus species as a result of the shorter days. Likewise a window in an apartment or house will have
even less available light. If a cactus is actively growing in these conditions it will undoubtedly begin to etiolate.
Etiolation is a pale, elongated growth that is weak and on a cactus plant often features little or no spine growth.
(More Here). If a plant is kept growing all year with good light in
summer and poor light in winter, the resulting growth will be alternate from thick to thin and the heavier summer
growth will often cause the weaker etiolated growth to collapse. Obviously, this should be avoided.
To avoid etiolated growth, the plants should be forced (coaxed? allowed? soothed?) into dormancy. To do this, the grower
must first stop watering the plants entirely. Seedlings, "jungle" cacti, and a handful of other species do not
like to go completely dry for long periods. However, the vast majority of cactus species will have no trouble being
completely dry for many months at a time - some even last years without water. Along with dry conditions, lower
temperatures will further help sustain a dormant period. However, it is imperative that the plants are allowed to
dry completely before exposing them to the lower temperatures. A temperature of around 45F (7C)
is ideal. A few degrees either warmer or cooler is acceptable. If temperatures are above 60F (15C), then some plants
may need a little winter water and without light, etiolating is very possible.
Some cactus species naturally shrivel up when going dormant - Do not water them during this time!
When dormant, the plants can be kept in total darkness. This does not mean that the darkness is necessary or even beneficial,
but only that the lack of light during dormancy doesn't adversely affect them. That said, species of some genera
(Mammillaria and Rebutia for example) will appreciate a winter rest period and often flower quite profusely in the
next growing season.
While it is easy to stop watering, getting the right temperature can be more challenging. Some people can heat their
greenhouse to a minimum temperature over winter and let their plants sit dormant in place. In most cases, heating a greenhouse
can be prohibitively expensive and they will need to be moved to a minimally heated space. Some good over-winter places
include partially heated garages or basement rooms where the main house heating is closed off or has a reduced effect.
Some growers in apartments build a box with shelves around the inside of a window. This shields plants inside from the heating and
the cool window keeps temperatures down. In this case, the sun can quickly heat up such a space and a window away from the sun
is preferred. It is even possible to keep a cactus plant dry and dormant in a typical kitchen refrigerator with no ill effects!
Again, in all these cases, the key is that the plants are kept completely dry.
Cacti in a cool basement room where they spend winter completely dry.
Those closer to the equator or in areas warmed by ocean currents can often get by with a shorter dormant period of 4 or 5 months,
while those in colder or more extreme altitudes may have plants dormant for 6 to 7 months. During this entire time, the plants are
kept cool, given no water, and may or may not have any light available. In fact, since light is associated with heat, trying to
supplement with artificial lights will increase the temperature around the plants and prompt growth. For this reason,
artificial lighting should be avoided. On the other hand there is no need to try maintain darkness and light from a window or
lamp is not a problem if it isn't heating the space around the plants.
Surviving the Transition
Some care must be given when transitioning plants into and out of dormancy. It cannot be stressed enough that allowing the plants
- that is the soil in the pots - to dry out completely before moving them into a cool/dark space is essential. This means
that all watering should be stopped from 3-4 weeks before moving them into dormancy. Temperature, humidity, and pot size can all
affect the time it takes a pot to dry out, so be mindful of this. If the plants are moved into a cool/dark place while wet,
they may rot, lose their roots, or etiolate as they continue trying to grow. Even when dry some plants try to keep growing
and a little etiolation in fall is almost inevitable on a larger collection. If the pots are dry, however, the plants should
slip right into dormancy in the cooler conditions and patiently wait out the winter.
When moving the plants out in the spring, it is possible for them to get sun-burned. This is especially the case if growth
occurred in late fall when light light levels were lower than late spring. Since the days get shorter before it gets cold
in fall and the days get longer before it gets warm in spring, most often, the plants will have more light in
spring than they did in fall. This difference could cause some sun burn on the plants. To avoid this, use some shade
cloth or place the plants in a partially shaded spot and gradually introduce them to more direct sunlight.
Left: These plants have just come out of winter storage
Right: Some etiolation is unavoidable
Also in the spring, there should be no rush to water the plants. Even though the plants have been dry for many months, it is
important to allow them to "wake up", before watering. Allow approximately one week for this an then water only lightly.
Keep an eye out for signs of growth and continue to water sparingly until you are sure that the plants are actively growing
and using the water you are giving them.
Keeping non-hardy (potted) cactus plants in cold climates is best done by keeping them dormant during the darker winter
months and taking advantage of the growing season in summer. Care should be given when making the transition from dormancy
to active growth and back again. Since dormant plants require essentially no maintenance, this method is preferred
over attempts to use artificial lighting, a fully-heated greenhouse, or warm windowsills which promote growth without
adequate lighting. Finally, as a reminder, this article is intended as a general guideline for most cactus species
and exceptions do exist. Some other types of succulent plants can be grown this way, but many cannot. For
help with specific types of plants and advice, be sure to consult the friendly members at
Author: Daiv Freeman