Optimizing Conditions: Light, Water, Temperature

These cacti’s metabolism scales with the temperature. From 68-86F they grow full speed and need full light, water, and nutrients. Their metabolism stops and they enter what we call dormancy when the daily highs are below 60F for a couple days and require pretty much no water, light, and greatly reduced nutrients. When daily highs are above about 94F their growth slows way down and they will still require water but will appreciate shade and less nutrients.

Most of these cacti can handle full all day sun in all but the most UV intense climates, though they genetically vary in their preference of total amount of light per day. However all light changes must be made gradually or they will experience sunburn. A sudden swing in the weather from cloudy and 80F highs one week to sunny and 95F the next week is enough to cause sunburn. First they will appear yellow on the side facing the sun and then they will burn, which eventually will turn to a permanent scar. Throwing up some 30-40% shadecloth that covers them during the hottest hours of the day, 2:30-5:30p, should be sufficient to prevent sunburn. Morning light is more blue spectrum and evening light is more red spectrum. These cacti appreciate blue spectrum more - it promotes wider growth, while red spectrum promotes taller growth. UV spectrum promotes alkaloid production. More shaded conditions or shorter light hours in spring and fall will result in more blue farina production. Insufficient light results in skinny new growth at the tip called etiolation, which is permanent.

If you are growing mature cacti indoors, cheapo grow lights or windows will not suffice. It is a requirement to use a quality LED panel grow light with a published PPFD map. These plants need 750 ppfd bare minimum, 1000-1250 ppfd would be much better. I would also recommend understanding the spectrum and checking the amount of UV the light puts out. Check out Viparspectra, Mars Hydro, and Spiderfarmer lights.

When watering you should always thoroughly soak the whole pot, unless you're dealing with freshly potted transplants or cuttings. This encourages the roots to thoroughly penetrate all the substrate in the pot. Bottom watering is the preferred method, but is not possible for large pots or large collections. Try not to splash soil up onto the columns, as this can encourage fungal activity.

Judge soil moisture by sticking a finger a couple knuckles deep down into the substrate. If it's dry beneath the surface, it's time to water. When it's really hot it's fine to water before the soil is thoroughly bone dry. When daily highs are below 68F but above 60F it's fine to let the soil sit dry for a few days before watering again. When daily highs are forecasted to drop below 60F for a few days or when nightly lows are forecasted to dip below 35F, I recommend you let the soil dry out before these conditions occur to prevent issues. If frost is in the forecast, cover column tips with styrofoam cups. If the soil is dry, most varieties of Trichocereus will survive nightly lows down to 25F or lower, as long as the temperature rises back above freezing when the sun comes up, but each variety has its own limitations. For more information on how to care for these cacti when temperatures are low, see dormancy.

The roots wick up moisture from the soil into the column. The plant has pores in its green skin that it breathes and exhales extra water through, but those pores close during the day and open at night. When the plant can't exhale moisture as quickly as its roots are taking up moisture, you will see oedema - also known as “humidity spots”. These are raised blisters that turn black caused by excess moisture buildup in the column. Make sure the plants have airflow, sufficient root zone aeration, soil drying between waterings, the skins pores are not blocked by products such as oils, and the air humidity isn't too high. When it's really hot for consecutive days you may see the opposite occur - wrinkling of the skin, indicating the columns moisture reserves are empty. Make sure to water asap when you see this happen, and don't fertilize or add anything else to the water that may impact how the roots take up the moisture.