Understanding cold and heat zones for gardening can make the difference between having plants that thrive or ones that fail. Learn more about cold and heat tolerance for your area.
Plants can be fussy, can’t they? They need the right amount of water and light, and the right kind of soil. They also have requirements when it comes to their abilities to tolerate temperature lows and highs. To have a really successful garden, plants’ particular heat and cold tolerances must be attended to.
Cold Versus Heat Tolerance in Plants
As much as I would wish it otherwise, nature has tried to teach me this lesson many times, but, in my craving for certain plants, I have been a stubborn student.
One example, is the dwarf Mugo pine. I drooled over them in Kansas and toted them home to west Texas. As a result, I killed them by not knowing what I needed to pay attention to. I saw that they grow in full sun and are cold tolerant in zones 2 to 7. At the time, I was on the edge of USDA cold zone 7, closer to 8.
The reality is, full sun in zone 7 or 8 is not the same as full sun in zone 2. Mugo pines would appreciate some protection from a blast of afternoon sun in West Texas. I planted them in the wrong place without realizing it.
Water needs, soil amendments, fertilization, and more or less sun exposure can be manipulated by the gardener. Temperature needs are also important, but less easy to address.
COLD temperatures can be dealt with to a degree using wind breaks, blankets, mulch, etc. HEAT can be more problematic, however.
Cold Tolerance
Many gardeners are familiar with the USDA plant hardiness map. For beginners, hardiness means cold tolerance. These zones indicate the average minimum low temperatures that occur in that geographic location. (Incidentally, these do not match Canadian zones)
For instance, gardeners living in zone 4b, can expect minimum temperatures of -25 to -20. 0n the other hand, gardeners in zone 9a, can expect 20 to 25 as a minimum low, on the average. Quite a difference! You may be able to see from looking at the map, what zone you live in.
How Does This Help?
Knowing your plant hardiness zone is extremely useful. Imagine! You bring home a perennial plant and plop it into your garden with the idea that it will stick around until next year, only to find out that it froze over the winter! How disappointing!
This is where the plant hardiness zone map comes in.
Growers/experimenters/gardeners have worked to categorize thousands of plants indicating, among other things, their ability to withstand cold.
In addition, experience with a plant, or seeing it in your neighborhood or nearby botanical garden can let you know the same thing. But what if it’s a plant you’ve not seen or used before?
Plant Tags
Enter plant tags. For the uninitiated, when you go to the nursery, you will often find a plastic tag in the dirt or attached to the pot, that shows the range of cold zones in which a plant can survive. Compare this information with your knowledge of the zone in which you live. BE SURE TO CHECK!
Here’s a Real Plant
Take the beloved red geranium (Pelargonium) as an example. If you look at the tag that is in the pot, you should see an indication of the hardiness zone. It will read something like “USDA zones 9-10.” This means that although there are ways to overwinter geraniums, if they’re left in the ground or outside in a pot in zones 1-8, which are colder, they will die. Zone 9-10 means they should be ok depending on what the weather actually does. In zones 11 through 13, which are warmer, they will be fine. The upshot is, geraniums are not tolerant of freezing temperatures.
If there isn’t a tag, you can look up the plant on the internet, and you will find its hardiness zone. Sometimes, all you need is a temperature range, without a zone label.
Heat Tolerance
When we plan gardens, we think about sun vs. shade exposure. They’re opposites. But what about cold versus hot? We worry about whether a plant will freeze, but can it also get too hot?
To my mind, the heat zone map is useful for raising a gardener’s awareness that heat is indeed a factor in the care of plants. This is especially true if you live in a warmer climate.
The AHS Heat Zone Map
The AHS (American Horticultural Society) heat zone map, like the cold hardiness map, divides the U.S. map into geographical zones that relate to temperature.
But the heat zone map is different.
The cold hardiness map reports an actual range of temperatures, ie. the plant will freeze. The heat zone map reports the average number of days per year that an area may experience temperatures over 86 F. This is the temperature at which plants begin to experience stress.
Examine the map. You will see that AHS HEAT zone 4, experiences 14 to 30 days a year, on average, above 86F. HEAT zone 9 experiences on average, 120 to 150 days a year over 86F. My plants and I should be so lucky to have days of 86F temperatures in the summer. Last year our temps reached 114!
So the heat zone map, while useful, is not a complete picture. Within the map, temperature can vary wildly. You may peak out in the mid-80s in heat zone 4 or even as high as 110F+ in heat zone 9.
What About Plant Tags? Heat Vs. Sun
So, what do plant tags tell us about heat? Pretty much nothing.
The plastic plant tag very rarely includes the heat zone. Nor is the heat zone indicated if you look up a plant on the internet. What you will see on the tag is the recommended amount of Sun a plant “needs.”
Commercial plant growers and retailers will show this either on the tag or on a sign at the nursery; full sun, part sun, or part shade, are common designations. Unfortunately, while it is a place to start and is really important, it doesn’t actually mean how much HEAT a plant can take.
Shade Vs Sun; Cold Vs ???
Getting sun exposure right is only part of the equation. Plants don’t like to be too cold, and THEY DON’T LIKE TO BE TOO HOT!
We love shade gardens and sun gardens. Reams are written about plants that can happily live in these locations.
But what about a shade plant that struggles with heat?
A plant’s negative response to heat, is not as drastically and suddenly and irreversibly observable as a plant’s sudden death from freezing. Gardeners often attribute a plant’s sad appearance in summer to causes other than heat: lack of water, not enough shade, disease… we blame ourselves for the gradual decline. I know I have.
Here’s a Real Plant
I overwatered a favorite hanging basket, an artillery fern, to death!
The scientific name is Pilea microphylla. I know. It’s a weed in Florida. But I fell in love with it in the late 1970’s in Austin, Texas.
It lived happily in a sunny window in my house in Abilene, Texas in the winter, and outside under a red oak tree in the summer for several years. Sadly, it Died, stem by stem, after we moved to another, more humid city, further south. It wilted every day!
Since it was wilted, I just kept watering since you can often counter plant stress with additional water. Unfortunately, I wasn’t checking moisture in the soil!
I had to give my artillery fern a funeral.
I shared my sad story with a local horticulturist, and she told me my artillery fern had been suffering from heat stress in the afternoon. That didn’t even occur to me!
It would have been fine if I had just let it go through the wilting in the afternoon: it would have rebounded by morning. When she told me, I realized, in retrospect, that I had overwatered it!
Pileas do not suffer overwatering gladly. I didn’t correctly diagnose the problem as HEAT.
The Moral of the Story.
If there are plants you want to grow, sometimes you can cheat a little bit.
A plant that can take full sun in the north, can often be grown further south in with some afternoon shade. A plant that needs more humidity, might get by with an extra drink and some shade.(Check the soil for moisture though!)
If it’s really just a bit cold for a plant, maybe some extra protection will let you have that coveted specimen.
By knowing a plant’s preferred conditions, sometimes, you can tweak it’s care and still have a plant that, although it may not be ideal for your location, is still doable. Or you can choose those that are more ideally suited to your location, and wind up with less work and guesswork.
Some plants can in grown in multiple places. As long as their needs are met, they can be perfectly happy! Take roses, for instance.
Roses are one of my favorite plants. They are very common in the south. Yet sometimes, when I am looking at online specimens, the description “brags” that this rose is hardy to zone 5, or 4 or even zone 3! I thought roses could grow everywhere!
I grew roses in Austin, Texas in full sun. My sister told me roses need morning sun, and afternoon shade: she lived in Phoenix, Arizona!
Plants can be complicated. Even common, “simple” ones, have conditions that need to be met to present you with their best selves.
Happy Gardening
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