The USDA zone map is based on the “average annual extreme minimum temperature”.
I’m in 5b/6a, which goes down to -15 F, but obviously with an average, we’ll get below that in some years (to -25 F for three days in 2017 most recently).
Is a fruit tree rated at approximately that minimum zone number, i.e. a 6a is likely to die below -10 F, or can they withstand some short swings below it for a day or two? I suspect it varies by species…
If so, presumably you’d want to be a 1/2 zone (or more) up to be more sure about year-to-year deviations from the USDA average?
BTW, the federal USDA zone webpage hasn’t been working for several weeks (months?) - a web security certificate issue that can’t be overridden on any browser I’ve tried on my computer or phone.
My area is interesting with the mountains - about 20 miles away as the crow flies in a remote, uninhabited area 3500 feet higher it can get down to -65 F (but “only be” -25 F here in town). The USDA map puts that area at 4b -25 F, which doesn’t seem quite right.
Zones are a good guideline, but only really a starting point. In town on the south side of a house will be significantly warmer than out of town at a high elevation or local low spot. Also, as @Drew51 mentions, there is quite a lot of variability in what a winter in the same zone, different places will mean. For example, I’ve lived in z6 in Massachusetts and Kansas. They both have the same average minimum temperature, but the winters are very different. The overall winter in KS was more like Baltimore than Boston, and it would start later and end sooner. The thing to remember for hardiness is that it’s time and temperature combined that does the damage. You could probably stick a tropical houseplant out the door for a couple seconds at -20 with it being fine, but prolonged exposure at +32 would kill it. On a less extreme example, I’ve seen many claims of marginally hardy plants surviving a freeze of X temperature unscathed. Now, was that just for a few hours before the sun came up? Or was that lows of that temp for several nights in a row? Makes a big difference. Might it be worth trying that plant if temp X is within your zone range? Maybe, but it would be far from a guarantee.
The other thing to consider is that it’s not always the cold that kills. Many plants will be done in by poor drainage at a higher temp than they would otherwise, or drying winds. In these cases, it’s the root rot or the dessication that does it, but not the freeze. Also, some plants might be just fine if you planted them in the spring but not if they’re fall planted, or might need to be protected for the first few years. All of these factors (and how much wiggle room you have) will depend on the species and sometimes variety of plant.
When you’re trying to figure out what will work in your location, you’ll ideally learn about your cold hardiness zone, heat zone, rainfall/humidity (how much and how is it spread out across the year), chill hours, growing degree days, etc. Then, get and idea of how your site deviates from the local averages. You can do a fair amount to improve your odds if you understand your limiting factors.
I agree with what you said. I don’t really zone push anymore, but that doesn’t mean I don’t grow tropical/subtropical plants. Most in ground plants can live in my zone. I grow oranges, pomegranates , figs, and Guava. I also grow western blackberries that are not hardy here. I use lights, my garage. leaves and other things to get my plants through the winter. I have been doing it for about 7 years now.
My house is full or herb plants too. I love southwestern and Mexican foods. So I grow my own Mexican oregano. I took a photo of the plant yesterday. That’s Rosemary in the background right. An orange tree background left.
Just in the window, no lights for them.
I really enjoy figuring out how to grow each type of plant. I always add new plant species almost every year. Many I can grow here but they are too much trouble. Figs, poms, nigra mulberry, and western blackberries are super easy for me. All deciduous, thus need no light in winter. Like the temps between 25-40F in winter. Which exactly matches the environment in my garage.
What’s great about southern Michigan is the Great Lakes buffer the temps by a lot. It keeps us warmer in fall, and cooler in Spring. Thus most plants flower with no freezes, everything is always gradual. It makes it easy to grow fruit here that is just hardy enough to make it.
Major climate data are mostly based on a 30-year average.
The most recent reference I could quickly find for the 2012 hardiness map used a dataset for the years 1976-2005. Perhaps there is a more recent one. These 30-year datasets can become outdated by 10 years.
The certificate issue mentioned above can be bypassed by clicking on the warning’s “Advanced” link and allowing the browser to display the page. Not recommended in general but probably OK for .gov hardiness maps.
Amen to all of this. I’m beginning my yearslong testing of allegedly cold-hardy avocados and their various crossed seedlings in Seattle, and while we don’t have cold snaps as severe as some of these varieties have survived, we have months on end where the lows hover right above freezing and the highs rarely go above 50°F, not to mention the ground getting saturated with rain and the day length and available sunlight being almost nil for midwinter. I’m expecting rain protection will be at least as important as cold protection, and root rot will be the main killer. But we shall see!
I am trying Morus Nigra mulberries in zone 5. We shall see how they overwinter in zone 5 in a garage. Main issue I have had is understanding how much water to get them in the garage. I have about half a saucer of water the last time I checked. I have successfully grown things that don’t require chill hours inside such as peppers. I found I did not get as much of a harvest as if I was in Florida growing in ground but I still got a couple servings of peppers. In fact I had more problems when I brought the peppers outside than when they were under grow lights inside.