Temperature, light and ventilation
Temperature
In general, cactus can stand high temperatures. Some species are
able to even resist until 45º C or even more, if the light, ventilation
and ground humidity are adequate. On the other hand, a night temperature
above 23º C can stop growing.
As to the minimum temperatures, while some species can even stand
-20º C, epiphytic species require temperatures over 8º C.
If cactus remain indoors during winter, they do not have to be in
very warm rooms, since 18º, 20º or 24º C, which is usually the temperature
on heated rooms, would avoid that winter rest took place.
A fresh atmosphere, 10º C average between day and night during the
cold months, keeping a minimum of security from 5 to 7º C, are the
best winter temperatures for cactus.
To help cactus to stand the cold ambient of winter, the relative
humidity of the air must be low, the lower the better.
When we talk about minimum of security, it must be understood like
a minimum temperature which a species can surely be kept without
suffering damage. If the plant is able to tolerate more cold, will
depend on health, age, and other factors, being one of the most
important, the humidity of atmosphere and ground.
Species that in some countries can tolerate temperatures below 3º
C during several hours, in others suffer damage at the same temperature,
since the ambient humidity is remarkably higher during winter, reason
why the minimum recommended for those countries will be several
degrees higher.
During winter rest the hardening of the parts that were born in
the previous spring and summer takes place. Many species produce
their flowers on these parts that were developed previously. These
hardened tissues will be able to stand lower temperatures even better,
but this will not take place if the plant has not been induced dormancy.
Light
A common mistake which occur with relative frequency in cactus growing,
is to assume that all cactus species will resist intense insolations.
Before this idea we must understand that many cactus live between
scrubs, shrubs and rocks that filter solar rays. From the many seeds
that are in the fruits of cactus, only a very small number of them
give rise in their habitat to plants that can reach adult size.
The reason for this is that not all the seeds fall in shaded places
and therefore, when rains arrive and germination takes place, small
plantlets quickly collapse because of the intensity of solar rays,
with the exception of those who fell under protection of which rocks
or scrubs, being these the only ones that prosper and give rise
to plants that continue growing under partial shade through their
youthful state, and in many species throughout all their life.
As a rule, we can say that the species provided with hairs, very
strong, dense or closed spines, require full sun. However, species
without or with few spines, require partial shade. In any case,
belonging the plants to the first or to the second group, the place
where they are located must be very illuminated. At this one point
we must mention that intense illumination and direct sun are different
things.
If the plants do not receive enough light, the growing will be weak
and the plant will etiolate. Plants will have a faded color and
will be extremely sensitive to parasitic diseases.
On the contrary, if the intensity of solar rays is excessive the
plants will look reddish. If we notice this, we must provide the
plant with some shade.
Cactus can also be growed under fluorescent tubes or halogen lamps.
Ventilation
Cactus like to grow in an atmosphere of little relative humidity.
This is impossible in certain zones during winter, reason why some
people protect the plant bringing them indoors and locating them
on the lower base of a window oriented towards equator, in a room
where nocturnal temperature are not higher than 10-12º C. Even in
these cases, the plants must be ventilated at noon or at morning,
whereas the outer temperature is not lower than 10º C., by opening
the window during a short period.
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