A columnar cactus, one that grows as an upright column, such as San Pedro (a Trichocereus), changes as it gets older. A single stem grows taller, may send up side shoots, and slowly turns brown and bark-like at the base. Most of these changes come with age. A few are marks left by the conditions the plant has lived through.
The short version
- A cactus grows from the tip of each stem, mostly in spring and summer. In the low light of winter it grows less.
- In warm weather, with regular water and good light, San Pedro may add up to about 12 inches of stem in a year, growers report.
- Mature San Pedro and Peruvian torch, a close relative with thicker stems, grow to about 7 to 20 feet tall. Torch hybrids stay lower: their stems cluster to about 2 feet and then sprawl.
- Pups are new shoots that grow from the base. Leave one if you want a clump of stems, or cut it off and root it if you want a separate plant.
- A column that loses its tip often branches below the wound.
- Brown, hard, bark-like skin at the base of an old stem is corking, a part of aging.
- Sunburn scars are permanent, and a section that stretched in poor light generally stays narrow. Growth made in good light above them comes in at normal width. Light frost marks on the skin grow out over several years.
- A rooted cutting or pup is genetically the same plant, so rooting a healthy piece carries an old plant on.
How a cactus grows
A column adds new stem at its tip. Each stem lengthens from the top, which is why a column's history reads from the base up: the oldest tissue is at the bottom.
Most cacti grow actively in spring and summer and grow less in the low light of winter. Many growers report that San Pedro can add up to about 12 inches of stem a year in warm weather, depending on the plant's age and conditions. Four things limit how much it adds.
- Water. Many cacti in pots can sit through long dry spells, but they grow less while they do. See watering cactus.
- Light. Too little light gives thin, pale, stretched growth. See cactus etiolation.
- Root room. Some hybrids, including many torch hybrids, have vigorous roots that can fill a pot within a year or two. See cactus pots and repotting.
- Food. Cacti need little fertilizer. Once or twice a year, in late spring or summer while the plant is growing, at half the strength on the label, is enough. See fertilizing cactus.
How tall it gets
Mature San Pedro and Peruvian torch plants grow to about 7 to 20 feet tall. San Pedro stems are about 2.5 to 4 inches across, occasionally up to 6 inches. Peruvian torch has stouter stems, about 6 to 8 inches across. At up to about 12 inches a year, reaching full height takes many years.
The torch hybrids (Trichocereus hybrids) grow differently. Their stems form clusters about 2 feet tall and then sprawl along the ground. If your plant is a low, spreading clump rather than a tall column, it is most likely one of these.
Pups and branching
A column carries ribs, the raised ridges that run its length. Along each rib sit the areoles, the small pads that carry the spines. Areoles are also where flowers and new branches appear.
Pups. Many cacti form new shoots at their base, called pups or offsets. San Pedro can send up pups, and over the years a single column can become a clump of upright stems standing side by side. A pup can stay where it is and grow into another stem of the same plant, or it can be cut away and rooted as a new plant. Some pups already have roots of their own when they come off. Which to do depends on whether you want a clump or another plant. For how to remove one, see taking cactus cuttings.
Branches. The growing tip of a stem holds back the buds in the areoles below it. When the tip is cut off or damaged, those buds are free to grow. Many growers report that a column that loses its tip sends out new shoots at or below the wound, sometimes more than one, and becomes a branched plant. Taking cactus cuttings covers where to cut.
For a grafted cactus, one plant joined on top of another, see caring for grafted cacti.
What age does to the stem
Corking. As a stem ages, its oldest skin, near the base, loses its green, turns brown and hardens into a bark-like layer. This is corking. At the base of an old stem it is part of aging. Corking that appears high on young growth can be a sign that the plant's growing conditions are off. To tell corking from sunburn, frost marks and rot, and to see what to check, go to corking, sunburn or rot.
Marks on the stem. Some marks stay and some fade.
- A sunburn scar is permanent, though the plant usually lives.
- A section that stretched in poor light generally stays narrow. Growth made once the light improves comes in at normal width, so the column ends up with a narrow waist between wider sections.
- Light frost damage that only marks the skin is something the plant grows out of over several years.
Spines. How many spines a stem grows, and how long they are, depends on light and other conditions. One stem can carry areoles with no spines and others with as many as 10. A plant that has been moved, for example from shade into more sun, may show older and newer sections with different spines. The difference comes from the change in conditions.
Carrying an old plant on
A cutting or a pup is, with few exceptions, genetically identical to the plant it came from. Once it has rooted, it carries the same plant on. To keep an old plant going, root a healthy piece of it: start with taking cactus cuttings, then how to root a cactus cutting.
Sources
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