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What's with those styrofoam cups on columnar cactus?

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If you live in the desert southwest, especially the Tucson and Green Valley areas, you’ve probably seen this method of frost protection for columnar cactus. But does this work?

The first thing I should point out is that if you are sticking with cold tolerant plants (hardy to 15 degrees in Tucson; 10 degrees in Green Valley) then it won’t matter if this works or not, because you won’t be doing it!

This fad came about during a cold snap a number of years ago, when the Tucson Botanical Gardens came up with the Styrofoam cup idea to protect the tips of their tender cactus. It showed up on the news, and now you see it everywhere. But back to the point – does it work?

While it may keep the tips from suffering from frost damage in a mild dip below freezing; this method is worthless in a hard freeze. What most people don’t realize, is the point of covering frost tender plants is not to protect the outer leaves or plant parts from the cold. What you are trying to do is capture the warmth coming up from the soil, creating a mini-greenhouse. When temperatures are below 20 – not uncommon for the desert southwest – putting Styrofoam cups on the tops won’t prevent the whole stem from freezing in very cold weather. You must cover the entire plant all the way to the ground (and not wrapped around the base of the plant) creating a tent. For very frost tender plants, you may need to add a drop light.

In this photo of a Mexican Fence Post cactus, only hardy to 25 degrees, these cups will not prevent damage from a hard freeze (below 20). While plants may eventually recover, it takes years, and who wants to look at a damaged plant for that long? If you stick with hardy species, your landscaping will always be in good form, and you can watch your neighbors struggle to keep covers on plants when an artic cold front blows through.

And about those covers and Styrofoam cups. Many people leave them on all winter to save them the hassle of taking them on and off. This is not advised, since preventing sunlight from reaching plants can result in permanent tissue damage, depending on the type of cover you are using. If you have floating row covers, not the green frost blankets, but the white version, you can leave them on all winter since this particular kind of cover is meant for gardening, and transmits light. The green versions do not. Keep in mind, however, that some of these products do not protect against temperatures below 22 degrees.

Aside from the hassle, who wants their yard to look like this all winter? Plant hardy species, and you can enjoy your landscaping all year long, not just when the weather is nice.

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