1 - When trimming indoor (or outdoor) cacti, be careful. You can start new plants from the trimmings.
Q. My question is about a cactus growing indoors that has grown to the ceiling. Can it be can cut back without killing it. Miguel Antonio S.
A. The answer may depend to some degree on the kind of cactus, however, in most cacti, it can be cut. The lower portion will probably develop new branches and upper (part that was cut off) may very likely be induced to produce roots. I am assuming this is a tree-form (caulescent) cactus rather than a pad-type cactus since you said it reached the ceiling. First and foremost I advise caution. Handling a cactus can create a hazard for eyes and flesh. I would suggest you have large kitchen tongs (appropriate to the size of the cactus) available. It may also be wise to take some newspaper rolled diagonally to create a long, flexible tube that can be wrapped around the cactus stem and provide a handle (the handle is the portion beyond that wrapped around the cactus - the spines may easily pierce the wrapped portion and spear the gardener). A short length of old garden hose may also be used for this purpose. Goggles or safety glasses (or at the very glasses) should provide eye protection. Take the cactus outside or to another area where you can lay the pot and cactus on its side (not essential, but makes this much easier). If indoors, lay a sheet of plastic under the pot and the region of the cactus where you will make the cut. Several thickness of newspaper (not rolled in this case) may also be put on top and/or under the plastic for additional protection for carpets and flooring and to facilitate cleanup. Have a large knife and a saw handy (the inside of the cactus stem may be quite woody and require a pruning saw). An assistant will be useful to hold the cactus steady (with rolled newspapers and tongs) while the cactus is being cut. Cut the required amount from the top. You may cut several segments if necessary to reduce the size of the original plant (so that new branches will not too quickly reach the ceiling again). Each segment that was cut from the original plant may then be set aside in a cool, shady location for a few days to allow the cut surface to close the wound (suberize - create a layer of suberin). These pieces may then be put into a moist (NOT SOGGY), well-drained potting soil and allowed to form roots. Do not put them in direct sunlight, but in a bright location until roots form. As the soil dries, add water, but high moisture is not needed for most cacti. The base of the cactus, from which the top has been cut, may be returned to its usual growing location and managed as before. If the base is healthy and receives adequate light, it should produce branches from one of the nodes (locations where the spines are produced). This may take months. The top should suberize naturally in this situation. Watering can be as before (proper watering for a cactus) and nutrients may be provided occasionally. Don't try to "push" growth by fertilizing, but do provide some nutrients. Cacti are not heavy feeders.
Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: email@example.com, office: 505-865-7340, ext. 113.
For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms.
Please copy your County Extension Agent and indicate your county of residence when you submit your question!