Cactus (Cactaceae) plants have long been used as anchor plants in desert gardens, but as low-water gardening gains popularity, these spiny plants are gaining a following as landscape plants throughout U.S. Department of Agriculture plant hardiness zones 3b to 11, depending on species. Freezing is a common problem with cactuses, especially when they've been planted in the landscape without careful investigation of the cold tolerance of a particular species. When cactuses freeze, they typically turn yellow. The damage can be considerable, but it's hard to know for certain how badly your plant is injured simply from the color of your cactus.
What Happens When Cactuses Freeze
As the temperatures drop, the large water stores inside of a cactus begin to freeze. Their cells swell and stretch to accommodate the forming ice crystals, but soon these pointed formations burst through cell walls, causing the contents to spill out into other cells. With the integrity of the internal cell walls breached and chlorophyll-producing structures spilling out, plants take on a yellow appearance. Depending on the depth of freeze damage, parts of the plant may become squishy or ooze if the outer skin bursts.
Dealing With Cactus Freeze Damage
When your cactus freezes, there's not a lot you can do but wait. Some gardeners may be tempted to cut out the freeze damage, but stay your hand and allow the cactus to show you how badly it has been injured. If your cactus is cold-hardy, there's a good chance that the damage is superficial and the plant will grow out of it in a few years. It will be hard for you to know how extensive the damage is until the yellow tissue that has been killed has completely blackened and dried up, however. Wait until after all threat of frost has passed to trim out damaged tissues, so that your plant only has to go through the stressful process once.
Preventing Further Freeze Problems
Cold-hardy cactus plants have a trick for protecting themselves from freeze damage -- they unload some of their water and allow themselves to shrivel a little so that their cells have more room to expand. You can help your cactus through the winter by watering sparingly once fall arrives and removing any mulch you may have spread around the plant. Even the most cold hardy of cactus can do with a little protection when cold snaps threaten; so, have a fabric tent or cardboard box big enough to fit over your cactus on hand to help prevent excessive heat loss on cold nights.
Not all cactuses are cold-hardy. If you intend to overwinter your plants in the landscape, it's important to choose species that can take the winter your area dishes out. Many of these species originate in mountainous climates, where cold weather is common. Many species of prickly pear (Opuntia spp.; USDA zones 3b through 11) are extremely winter hardy, but a cactus garden can't be built on prickly pears alone. Companions such as the green-flowered hedgehog cactus (Echinocereus viridiflorus; USDA zones 4 through 9), spinystar cactus (Escobaria vivipara; USDA zones 4 through 9) and mountain ball cactus (Pediocactus simpsonii; USDA zones 4 through 7) are easy-care choices for the cold-hardy cactus garden.
Other Causes of Seasonal Yellowing
If your cactus begin turning yellow, don't panic. As the long days of summer disappear and fall begins, many cactus varieties change colors just like the leaves on your favorite bushes and trees. As the chlorophyll in the cactus stems fades, the bright pigments underneath are revealed, making some species appear yellow. If your cactus remains yellow and rigid, without outer tissues turning black and collapsing, it's probably showing a seasonal shift in pigmentation. Unless the yellow coloration remains through the spring, there's no reason to worry about your plant.
- University of Florida IFAS Extension: Opuntia Spp.
- Arizona Cooperative Extension: Problems and Pests of Agave, Aloe, Cactus and Yucca
- Planttalk Colorado: Winter-Hardy Cacti
- Arizona Cooperative Extension: Identification and Prevention of Frost or Freeze Damage
- Arizona Cooperative Extension: Frost Protection
- University of Nevada Cooperative Extension: Freeze Damage to Plants in Lower Elevations of Southern Nevada
- High Country Gardens: Gardening With Cold Hardy Cacti -- Part I
- Lady Bird Johnson Wildflower Center: Pediocactus Simpsonii
- Lady Bird Johnson Wildflower Center: Escobaria Vivipara
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