Trichocereus cacti will go into a state of “sleep”, or dormancy, when temperatures cool down, water becomes scarce, nutrients dwindle, and light is reduced. During this dormancy state they can survive for over a year without light or water. The primary trigger of dormancy is temperature remaining below about 55℉, but the other factors mentioned do play a minor role as well.
For growers who live in areas that get too cold for their Trichocereus to survive outdoors, action is required.
Trichos generally will be fine with substrate experiencing wet and dry cycles down to nightly lows of about 35℉ and daily highs at or above 60℉. To prevent issues like root rot and fungus, its recommended to keep substrate dry when nightly lows dip below that. They should be OK with dry substrate when nightly lows get down to as low as 25℉ as long as temperatures rise back above freezing fairly quickly in the mornings, but the exact temperature that cold damage occurs will vary between different varieties. Some are significantly more cold hardy than others.
There are many methods that can be used to keep outdoor plants dry. Which method is right for you will depend on your weather conditions like how much wind and rain you will receive, the location and number of plants, your budget, and how long you plan on protecting them from rain/snow. Simple shelters can be constructed like a lean to or frame used to support clear plastic sheeting, all the way up to fully enclosed greenhouses with removable panels, ventilation, and heating.
Frost can damage tips. You can protect outdoor plants by placing Styrofoam cups over their tips.
Potted plants can be moved into a sheltered area to protect them when outdoor temperatures will be cold enough to damage or kill them. A proper overwinter shelter should remain in the 35-55℉ range, which will allow the cacti to remain dormant, will not require lighting, and the plants should remain dry. Some ventilation and considering humidity levels is still a good idea to prevent pest, fungal, or other issues. Common places to use as overwintering shelters include a garage, shed, basement, greenhouse, or enclosed porch. If the temporary shelter space will not have adequate grow lighting, wait to move plants into shelter until after they have gone dormant, if possible. This will prevent etiolation. If the space is subject to temperature swings outside of the acceptable range, monitoring the temperature with a wifi thermometer or hygrometer is a good idea, and having a heating method on hand to keep the space above freezing temperatures but below 55℉ may be required.
Alternatively, you can keep your cacti growing year round. That's right - dormancy is not required at all, though there is some evidence that it does help promote flowering during the following season, and maybe alkaloid production. Set up an indoor grow area
There is another type of dormancy where these plants will stop growing due to high temperature referred to as heat dormancy. Remember, these cacti are native to the Andes mountains, not to scorching sandy deserts. When temperatures exceed daily highs of around 95℉ for extended periods, typically coupled with very high nightly low temperatures, Trichocereus will slow or stop growing. They do this to conserve moisture and energy to better withstand the thermal stress. The mechanism they use to do this works through CAM photosynthesis. When nightly VPD is not sufficient for the plants areoles to open, the plant does not breathe and exchange carbon dioxide and oxygen which stops growth metabolism. They also will not exhale water vapor which halts uptake of water and nutrients from the roots. Due to this, it is best practice to stop fertilization at excessively high temperatures. In some cases when heat dormancy is fully in effect it can even be best to reduce or stop watering, since the roots will not be up taking moisture. If the plants take on a shriveled appearance on the columns this is an indicator that their internal water reserves are depleted, they have opened their areoles at some point, and they should be watered as soon as possible. Its also helpful to spray columns with a fine mist on extremely hot days to cool or nights to help them open areoles. Though some varieties can handle full sun in extreme temperatures without severe damage, almost all will appreciate shade during the hottest hours of the day for cooling and times with extreme UV to prevent sunburn.