A cactus gets through winter by resting. It stops growing, needs little water, and stands cold best when it is dry.
The short version
- When nights fall below about 60 F in autumn, stop regular watering and stop feeding, so the plant goes into winter dry and no longer growing.
- Cold is harder on a wet cactus. In a rainy winter, keep the plant dry as well as protected.
- A brief frost is less dangerous than a freeze that lasts many hours.
- For San Pedro and the other Trichocereus, often sold as torch cacti: start protecting at 28 to 30 F, earlier for pots and in wet winters.
- Cover with frost cloth, a cotton sheet or burlap, never plastic. On a columnar cactus, the tall upright kind, protect the tip first.
- A plant wintered indoors wants a cool, bright place, with nights around 46 to 50 F if you can manage it, and little or no water.
- After a frost, wait before you cut anything. Damage that dries may be only cosmetic. Cut out damage that turns soft and wet and keeps spreading.
Dormancy
A cactus grows in the warm, bright months and rests in the cool, dark ones. That rest is called dormancy. A dormant plant grows little or not at all and needs little water. San Pedro (a Trichocereus) and the other Trichocereus slow down and stop growing as the weather cools, and start again in spring as the nights warm up.
A plant that looks unchanged from November to March is doing what it should. Do not try to wake it with water or fertilizer. A cool, dry winter rest also encourages many cacti to flower the following year.
A dormant plant still needs light. Give it as much as you can all winter.
Why a dry plant stands cold better
Water or fertilizer given in autumn pushes soft new growth, and that growth is the most easily injured by a freeze.
Wet soil is also most likely to cause rot in the cool season, when the plant is dormant. Rot is decay that turns tissue soft, dark and wet. If your winters are rainy, keeping the plant dry is as much a part of winter care as covering it.
Frost and freeze
A frost is a brief drop below 32 F. A freeze stays below 32 F for 3 hours or more, sometimes longer than a day. Many desert cacti come through a brief frost with little damage. Long freezes do the most damage.
When you read a forecast, look at the hours as well as the low. A night that touches 27 F and is back above freezing by mid-morning is a frost. 2 days that never rise above 30 F are a freeze, and a potted plant should come indoors for it.
Preparing in autumn
- Stop regular watering once nights fall below about 60 F. Where winters are cold or rainy, give no water until spring. Where winters are mild and dry, water lightly only if the plant starts to shrivel. A shriveling cactus has wrinkled skin and thinner ribs, the raised ridges that run down the stem.
- Stop feeding. Give no fertilizer from autumn until growth starts again in spring.
- Decide where each plant will spend the winter. In the ground, in a pot under cover, or indoors. Decide before the first frost is forecast.
- Get pots out of the rain if your winters are wet. Move them under an eave or a covered porch.
- Have covers ready. Frost cloth is a light, breathable fabric sold for protecting plants. Old cotton sheets and burlap do the same job. For tall columns, keep a few foam cups to set over the tips.
When to protect
For San Pedro and the other Trichocereus, this is the guide to work from.
Start protecting at 28 to 30 F, earlier for pots and in wet winters.
Some growers report that their mature, well-established plants came through brief dips into the low 20s F unharmed. Established means the plant has been growing in place for at least a full season. However, it is always wise to stay on the conservative side.
If your plant is in a pot, or wet, protect it whenever frost is forecast. Newly planted and unrooted plants are more vulnerable, so protect them from any frost as well. That includes a plant or cutting you received this year.
How to cover a plant
- Use fabric. Frost cloth, a cotton sheet or burlap all work. Wrap or drape it loosely.
- Never use plastic. It is a poor insulator and passes the cold to the plant.
- Cover the tip first. The tip is the top of the stem, where new growth forms, and on many columnar cacti it is the part most easily damaged by cold. Set a foam cup over it and tie or tape the cup down so the wind does not take it. If the tip is too thick for a cup, or the plant is short enough to reach all of it, wrap the stem loosely in frost cloth.
- Take covers off each morning once the air under them warms to about 50 F. Put them back on before the next cold night.
- Watch calm, clear nights. They are the coldest for plants, because no cloud holds the day's warmth in and no wind stirs the air.
A fabric cover helps on nights at or a little below freezing. For colder or longer freezes, move potted plants indoors. Where long freezes come most winters, grow the plant in a pot so that it can be moved.
Cold air flows downhill and settles in low spots, so those places freeze first. A spot against a sun-warmed wall stays warmer at night. For more on choosing a spot, see growing cactus for your conditions.
Overwintering in pots and indoors
Roots in a pot sit closer to the cold air than roots in the ground, so a potted cactus is less cold-tolerant than the same plant in the ground. A pot has 3 places to spend the winter.
Outdoors under cover. In a mild climate, a pot moved against a sun-warmed house wall gets some extra warmth, and an eave above it keeps the rain off. Bring it inside when a freeze is forecast.
In a greenhouse, or a garage or shed with a window. A cool, bright space that stays above freezing lets the plant rest. Keep the plant dry.
Indoors. The best spot is cool and bright: a sunny window in an unheated or lightly heated room, with nights around 46 to 50 F if you can manage it. Give little or no water, and water lightly only if the plant starts to shrivel. Too little light gives weak, misshapen growth, so a plant in a warm room needs the brightest window you have. Keep it away from radiators.
Going back out in spring. Wait until your last frost has passed. A plant that has spent winter indoors is no longer used to direct sun and can burn. Start it in light shade, such as under a tree, and move it into more sun a little at a time over a period of weeks. Begin watering again once the nights are back above about 60 F.
After a frost
A hard enough freeze kills cactus tissue. The damage shows on the exposed parts of the plant. Often the tissue turns pale and soft first. It then blackens, and over the next few weeks the blackened area dries out and turns crisp. In a severe freeze, sections of a columnar cactus turn yellow and then die.
What to do first. Do not cut anything yet. Keep the plant dry, protect it from further frost, and look at it every few days. The full extent of the damage takes time to show.
If the damage dries. Freeze damage that has dried and stopped spreading may be only cosmetic. Once the weather warms, you can leave it or cut the stem back to clean, green tissue.
If the damage turns soft. Tissue that is mushy, wet and dark, and that has spread further each time you look, is rotting. Do not wait for spring. Cut it out with a clean, sharp knife, taking a small margin of firm, healthy tissue with it, and keep the plant dry afterward.
If you are unsure what you are looking at, email support@thoughtfulpricks.com.
Sources
Cactus, Agave, Yucca and Ocotillo. University of Arizona Cooperative Extension, 2009. https://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1225.pdf
Problems and Pests of Agave, Aloe, Cactus and Yucca. University of Arizona Cooperative Extension, 2011. https://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1399.pdf
Frost Protection. University of Arizona Cooperative Extension, 1998. https://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1002.pdf
Frost Freeze. Zann Wilson, SaddleBrooke and SaddleBrooke Ranch Master Gardeners, University of Arizona Cooperative Extension. https://extension.arizona.edu/sites/extension.arizona.edu/files/attachment/Frost-freeze.%20prelim.%20SBMG%20docx.pdf
The Desert Garden Buzz, Winter 2023. Southeast Pinal County Master Gardeners, University of Arizona Cooperative Extension. https://extension.arizona.edu/sites/extension.arizona.edu/files/attachment/The%20Desert%20Garden%20Buzz%202023.3%20Winter.pdf
How to grow houseplant cacti and succulents. Royal Horticultural Society. https://www.rhs.org.uk/plants/types/cacti-succulents/houseplants/growing-guide