Cactus are succulent plants, almost all of which live in desert climates. The number one cause of  death in cacti is overwatering!  How do you determine when to water? The usual method of feeling the soil just won't work; you would have to be able to feel the middle of the rootball, plus you don't want to disturb the surface roots. (The number two cause of cactus loss is fungus; disturbing the roots causes cracks, which is the entry point for the infection.)
The easy method: Wait until you know you absolutely must water...then wait a few more days. This will keep your cactus alive and fairly healthy. However, there is a better way that doesn't involve watching the Weather Channel for rain in Albuquerque. The cactus family has adapted to the specifics of the climate they evolved in.
The following can be used as a guide, but you'll need to customize depending on temperatures, humidity levels, amount and strength of sunlight, soil mix, and whether or not you have a cat.

Column cactus --- Also called organ pipe cactus. Column cactus are generally more
tolerant of water. When its cold, they should be left alone; only water if the skin
becomes wrinkled. In the spring, you'll notice fresh green growth at the top of
the plant. Start watering then, and treat them like an ordinary houseplant, allowing
the surface to dry, then watering thoroughly.
Barrel cactus -- Barrels are the hardiest cactus. They evolved under strong sunlight and months
between rains. Only water when you see a noticeable wrinkling of the skin, and give them
as much sun as you can.

Spine patterns -- One of the many purposes of spines on a cactus is shade. Sure, it's
a cactus...but it's still just a plant. If your cactus has thick, interlocking spines, give it as
much light as you can. If it has smaller spines that don't seem to cast much
may need less sun. Or it may have just shed them because it wasn't getting enough light.

Spine and body color -- Brighter colored cactus generally need more light. White and/or
yellow spine colors take the most sunlight. Don't be fooled by new growth, though...often,
new spines will be soft and a different color.
Almost all cactus have green bodies, but some are darker, almost black. Give them
protection from the midday and afternoon sun.

Soil -- Coarse! That's the word of the decade for cactus. If you buy soil mixed for cactus,
it will be adequate, but you should add course sand to it. Never add peat moss to it; most
cactus like alkaline soil, and peat moss will acidify it.

Potting -- Cactus like to be rootbound; that's how they do it in the desert. There should
be no more than one inch of space between the plant and the edge of the pot. Avoid
plastic pots if you can...but lets face it, plastic is sooo much better than those heavy, space-
hogging, and always cracking terra-cottas. I use square plastic pots. Just add a whole lot
more drainage holes in  the bottom, and to play it safe, a few around the side towards the
bottom. With any pot type, the bottom third of the pot should be drainage material (break
up those terra-cottas!).

General notes
*even though it's a cactus, it can still be sunburned; potted cactus in the midday sun isn't a good idea
*if you have potted cactus in the sun (any kind of plant, actually) shade the pot; aluminum foil works great;
heat will kill the roots on the sunny side of the pot; if you rotate the pots, you'll just kill the outside
roots all the way around
*you can take cuttings from any type of cactus; cut the stem or offshoot at the thinnest point to
minimize exposed tissue, and air dry the cutting in a brightly lit and very well ventilated
area for one week minimum, or until it has a thick, dry callous; after planting, don't water
for a minimum of two weeks, even in hot climates, unless you see pronounced wrinkling
of the skin
*these are general guidelines only

Older pictures of my plants, with more information
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