The easiest way to propagate columnar cacti (Cereus, Pachycereus, Cephalocereus, etc.) is by cuttings. Some cuttings are growing tips, while others are stem sections without growing tips. The same methods can be applied to both.
When you receive your cutting, it may be freshly cut, or it may have been cut a while ago. If you do not know how long it has been cut, then it is advisable to cure the ends by leaving your cutting lying on its side in a dry, shady (but well-lit) place for 2 to 3 weeks at least. Place it where it will not be damaged and will not be exposed to sun or rain. Once the ends are cured, they usually become quite hard and cave in a little. They may also start to flake some dry layers of tissue, and some bulging may start under the surface near the edges.
Depending on the time of year and the position the cactus was placed in, it may have already started putting out root nodes by this time. It does not matter where these are. If they are in the right place, they will be encouraged to keep growing in the next steps, but they will probably just dry off if they are not in the right place. This does no damage to the plant.
There are many ways to root cacti, but all of them should follow a few simple rules. The medium must be suitable for growing cacti, and it should be sterile for best results. Heat on a tray in the oven at over 392 °F (200 °C) for 1 hour (perlite is already quite sterile and does not need special treatment).
Planting of the cutting is best done when they are actively growing (not in winter). Do not overwater. Cacti will almost never die from a lack of water (they store several months' worth) but frequently from overwatering.
The most professional method is to root your cutting in perlite. This has the advantage that the cutting can be placed onto the dry perlite bed as soon as it is cut and can undergo the drying process there. Once the ends are well cured, the cutting can be partly submerged into the perlite in the position desired. Whether you choose to place your cutting sideways or upright is a matter of choice. A tip cutting, if placed sideways, will bend upwards at the tip and may also produce side-shoots. The reason why side rooting is preferable is that more root fibres develop, and the plant receives more nutrients in a shorter period of development, which increases the amount of new growth.
The cutting and the surrounding perlite should then be watered. Once watering is started, the tray or pot should be placed in a full sun position. The perlite should not contain any vermiculite (a popular hydroponic mixture) as this retains too much water, and it should be watered every few days. The tray or pot containing the perlite should never sit in a puddle of water and should be entirely free draining. After a few weeks, the cutting will have produced several roots and can be transplanted to a more nutritious medium, such as potting mix or cactus mix.
To transfer the cutting, do not shake off the perlite. Transfer the whole rootball with as much perlite attached to it as possible. Place the new medium in the bottom of the pot and also around the rootball, with only a thin layer over the top. This will ensure that growth is not interrupted and reduces the risk of damaged roots being affected by diseases. After repotting, do not water for 5 to 10 days and protect from rain. This is to reduce the risk of diseases.
Many of the columnar cacti respond well to fertilizing. However, cacti should never be fertilized in winter.
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