A lot of people get hung up pretty tightly on hardiness zones. As I spoke about in my article about the zone system, it’s certainly a good place to start, but it’s not the supreme court of botany. Even if something you want to plant is rated at a half zone or zone warmer than where you live, or even just barely fits into your zone, but the rest of the plant’s requirements such as summer heat, water, and soil, are all good, then by all means, plant what you want.
With that being said, we and other places in the US, because of the open plains from Canada, are going to experience colder than average winters sometimes. Occasionally, once every quarter century or so, we will get a seriously bad winter, even two zones colder than we are rated. So even plants that are normally entirely hardy will face damage or destruction during such an event.
One notable example is December of 1989. Memphis TN fell to -4F. That’s a zone 6b winter. It’s a solid zone and a half colder than normal. Many plants easily rated for zone 7 struggled or died during that event. Take other cities during the same event, such as New Orleans LA and Jacksonville FL. New Orleans is zone 9b, meaning it usually doesn’t fall below 25F. They hit 11F, even though it is surrounded by water on three sides. Jacksonville hit 7F (a zone 7b winter!). That city is also normally 9a, so they experienced a bone crushing winter. I have read in many places that most trunking palms other than Sabal palmettos were destroyed. Queen palms, Washingtonia robusta, date palms, and various others were almost all wiped out in both cities. These are considered “normal” and “safe” plants to put in New Orleans and Jacksonville. Yet they lost an untold amount of plant biomass. Further south toward Orlando and the rest of central Florida, the orange industry was largely wiped out and had to start over further south. They had already experienced several other beatings during the early and mid 1980s, but Dec 1989 finished it off.
With all of this being said, if you ever feel like you are cheating by protecting a palm or other subtropical/tropical plant once every few winters, or even a few nights each winter if it is really out of your zone, don’t worry about it! That’s right, don’t get hung up on showing the world how your yard is a secret zone 9 while the rest of your region is zone 7. If you have a good microclimate, good. Take advantage of those. But if a forecast shows up with horribly below average extreme temperatures, suit up, go outside, and take whatever measures you need in order to get your plants to spring safely.
There is simply no sense in letting a plant suffer and limp through winter just to “prove” that it can handle your zone. I have a pair of decade old Brazoria Palmettos in my yard. I’ve never added any heat as they are zone 7b palms. But what I have done once or twice is put a cover over them to keep ice events from wrecking the crown of the palm. They would survive such events, I’m sure, but why ruin the plant by pushing it to the limit? I’d rather have them look amazing by May rather than having the palms recover all summer only to just start looking halfway decent as we enter another winter. I’ve paid too much money for all of my plants and spent too much time waiting for them to grow over the years and decades to let them die from a once in a decade arctic event. That’s nonsense.
If citrus growers in central and south Florida are willing to spend the effort to run sprinkler systems for a few nights in the 28F range each winter, then I can spend a few nights a winter throwing a tarp on a palm or wrapping Christmas lights around a plant. It’s not cheating. It’s just smart. Don’t waste money on showing up anyone else. I guarantee you, no matter how good someone’s yard looks in another state or a warmer zone, they will eventually get a zone busting winter that causes them to secretly run outside and protect their plants. I guarantee it.
As a horticulturist, I not only say it’s alright to protect sensitive plants, I recommend and encourage it. You’ll feel better for having done that by the time March and April rolls around as you look at a garden full of green plants that you don’t have to replace. They do it in Florida. We can do it in Memphis and other further northern climates.