Some thoughts on Plant Hardiness...
"Hardiness" can be a mysterious subject. There are some absolutes, of course: Coconut Palms will never survive an Oklahoma winter outdoors, while Eastern Red Cedar trees always will. But in between the absolutes is a huge area of gray. Some plants will live for years, only to be damaged or killed in an extreme weather event (like Italian Cypress, which grew in many areas of Oklahoma City for decades before being wiped out in the record setting cold of the 1980s). Sometimes a particular species of plant will die in one garden, but survive in another next door. Sometimes a plant will survive an extremely low temperature one year, only to be killed by a milder temperature the next year. There are always reasons why a plant survives or is damaged or killed, but those reasons are not always readily apparent.
Temperature is not the whole story.
Frequently, in the hardy subtropical plant discussion groups, a new gardener will say something like "I heard that there was a windmill palm in North Carolina that survived -9 degrees. If I can get some seeds off that plant, I know they will grow for me here in Indiana, because we hardly ever get that low where I live" This gardener is making the assumption that if the plant can survive this extreme temperature once, then it could take similar temperatures every winter. The plant in North Carolina probably survived "by the skin of its teeth", losing all of its leaves, and taking a full season (or more) to begin to recover. And the -9 degree temperature was probably a record low, never experienced before or after that time. It does not follow that the same plant would have survived in an area where the temperature dropped below zero every year.
Duration of cold weather is another factor. El Paso has a number of plants which have survived below zero temperatures that would not grow well in Oklahoma City. Cold events are often very brief in the desert climate of El Paso, so plants are sometimes damaged, but not killed by very low temperatures. Imagine stepping into a walk-in freezer at a restaurant. The below-zero temperature is bearable for a few moments, even if you are wearing short sleeves. But if you were accidentally locked inside for 12 hours, the same temperature would affect you quite differently. The same thing holds true for plants. A large, well established plant can often take a very low temperature for a short period of time with little or no damage. If similar temperatures last for several hours (or days) then the results will be quite different.
"Hardening off" is another variable. Plants grown in colder climates stop growing (or slow down dramatically) during the winter. New growth is usually more susceptible to damage than old growth. Some species of hardy citrus, that have been documented to survive single digit temperatures in more northerly climates have been killed completely by temperatures in the high teens or low twenties in central Florida. The difference is that they were actively growing in Florida, so the plant tissues were more tender. For the same reason, a 20 degree freeze in early November will often do more damage than a 10 degree freeze in late January.
The Amazing Gardens Hardiness Ratings
We are using our own 5-step rating system, based on our experiences in central Oklahoma. These may or may not apply in other Zone 7 areas of the country. If there is a question mark (?) by a plant's rating, it means that we are using our best judgment, but don't yet have enough empirical data to be sure that it is in the right category. We are testing new plants every year, but each winter is different.
1-Super Hardy (Bulletproof) Should never be killed by cold
weather alone in central Oklahoma. Rarely even damaged by cold temperatures.
Examples: Most Native plants, Sabal minor, Needle Palms, Some cactus (Tree Cholla, Opuntia englemanni v. alta), many Yuccas (rostrata, rigida, faxonia, elata, arkansa, glauca) many forms of Bamboo (Phyllostachys aureosulcata, nuda, bissetti, Arundinaria gigantea). Some plants in this (and other) categories are "die-back perennials" in our climate. For example Giant Reed, Arundo donax, dies back to the ground most winters, but grows back from the roots every spring. The Hardy Banana, Musa basjoo, falls into this category, as do many Cannas.
2-Very Hardy Rarely, if ever, killed by cold weather here.
May be damaged in some colder winters, but recovers quickly.
Examples: Sabal louisiana, Some Bamboos (Golden, Black), Crape Myrtles, Photinias
3-Usually Hardy Will survive without protection most winters,
but may be damaged or even killed by extreme cold, or long-duration events.
Examples: Many Cannas, common Bananas,
4-Marginal Will need protection most winters. May
be heavily damaged or even killed by temperatures in the single digits.
Examples: Windmill Palm, Washingtonia filifera, Mediterranean Fan Palm
5-Not Hardy (House Plant) Will be killed in most Oklahoma
winters unless given active protection (wrapping, heat tapes, portable
Examples: Washingtonia robusta, Queen Palm, Chinese Fan Palm, Clumping Bamboos
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