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Plant Hardiness Zones and Why They Are Important

If you are considering buying some plants for your yard, it would be nice to know whether or not they are likely to survive, wouldn’t it? This is why the United States Department of Agriculture has developed the plant hardiness zones map and why this map is so important to our landscapes.

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Here’s the Scenario

Okay, so let’s say you plan to revamp the landscape at your home. You’ve spent a few days on the internet looking at plant pictures, and you’ve decided that you love the look of palm trees. You have always loved Palms; you watched Beverly Hills 90210 and Californication, and you love the look of palm-lined streets. You’ve decided that you want to have them along the walk to your porch, on both sides. You have found them on google, and you are pretty sure that they are called the Washingtonia Filifera. You found a place that will ship them to you, and you are all set to get them delivered to your home; when your significant other walks in, realizes what you are doing, and says, “What are you nuts? Those will never grow in our zone!”

Our Zone?

We live in Denver, Colorado, and it seems like it would be sunny enough. What’s the big deal? A plant is a plant, right?

Right, but wrong! A plant is a plant, and before men started digging them up and moving them around, the plants didn’t know anything about zones. But, men in their infinite knowledge have decided that there should be some sort of a rating system for plants and locations so that we can all figure out where those palms will and will not grow.

So, What Happened?

Like everything else in nature, plants have evolved and adapted to their specific climatic conditions of their particular area where they grow. We humans love a moderate temperature with humidity. If we spent our entire lives outside in the elements the way the plants do, we would only be able to survive in a small portion of the world. We can’t just take a plant and stick it in the ground wherever we please and expect it to grow.

We have evolved to create fire, shelter, and air conditioning, which allows us to live in climates way outside of our outdoor comfort zone. Plants do not have this luxury. They are pretty much stuck where we put them or where they have grown.

Plants have evolved and adapted to the areas in which they have always grown. Just like in the days before horses, buses, cars, and planes, if we were born in an area, we were likely to stay in that area. Well, plants have always stayed where they were. Until, of course, we came along and decided to dig them up and move them around the world to make our surroundings look just the way we like them.

The Plant Hardiness Zone Map

We have been moving plants around for so long and pushing the boundaries of where they will and will not survive for so long; we have developed a system so that we can tell whether or not any particular plant is likely to grow in our area long before we decide to transplant it.

At least, that is the premise.

This system of classifying plants and where they are likely to survive is called the Plant Hardiness Zone Map, which is comprised of plant hardiness zones. This map is now updated and controlled by the United States Department of Agriculture (USDA)

The USDA Plant Hardiness Zone Map

Once we determined a plant’s ability to bear cold and heat, we created a map. The map uses the minimum temperature of a place in the last 30 years and averages it.

Each zone has a number and sometimes a letter for a sub-zone. The zones in the United States run from Zone 1 being the coldest and Zone 13 being the warmest. In Southeastern Wisconsin, we are in zone 5b, which is the same zone as Denver, Colorado. I can tell you from experience that if you want a plant to grow really well in my area, you will pick a plant that is zone 4 or less. Sure, a zone 5 might make it, but select a lower zone if you want to be sure.

How Can I Use Plant Hardiness Zones?

Now that the lowest temperatures of any area in the United States have been allocated a zone, it is easier to figure out which plants can grow in your area. Keep in mind that just about any plant will grow in the hot summer months in most zones, so while it won’t make it through the winter, I could enjoy my California Palm tree for one summer.

Pushing the Limits

Many people love to push the limits and plant more exotic plants in their yard than what will survive. This is fine, as long as you understand that you need to stick to your zone, or maybe a bit colder if you really want that plant to live for years to come.

Being a landscaper for most of my life, I have become very familiar with the plants in my zone and have used this map extensively to help plan out successful landscapes and gardens. What I’ve learned about this map and about contracting, in general, is that it is all about expectations.

More often than not, homeowners want pretty, blooming plants in their yard, and they want something to be blooming all year round.

I can’t tell you how many times I have been shown web pages or magazine articles with pictures of what appears to be zone 6-10 and asked,” can you make my yard look like this?” The simple answer is no; I cannot.

Set Your Expectations

If you are choosing plants for your landscape or someone else’s landscape, you need to have clear expectations. The hardiness zone map can help you know what to expect, but that doesn’t stop plants from being classified incorrectly.

You must always use your judgment and ask yourself if you’ve seen that plant growing in your area.

You are stuck in your zone, so unless you are going to move, don’t expect to have plants rated for the next zone higher to survive in your yard. Again, it’s about expectations. If you love that zone 6 plant and you must have it at all cost, then feel free to plant it in the most protected, sunny corner of your yard where the sun shines bright and hot, and the wind is entirely blocked. Then, before winter comes, go out there and mound the entire area thick with mulch and leaves and blankets. Then build a structure around it to protect the delicate branches and leaves from freezing, and you might have a chance to keep that plant alive.

That is, until the first year that you forget to do all of these things.

The Easy Way

Suppose you want a vibrant, healthy landscape that is low maintenance and pretty all year round that doesn’t need much help from you. In that case, plant only plants well suited for your zone and plant a wide variety of plants so that if any one type of plant gets a disease or ends up dying, it doesn’t affect your whole yard.

Humans always want to push their limits, and gardeners are no different. This is why so many people struggle and fight with their landscapes, trying to keep things alive that shouldn’t survive here. This is why people are fertilizing, watering, pruning, and protecting their plants like crazy people because they are trying to push the envelope.

Take my advice and keep it simple. Pick plants a zone lower and enjoy your yard rather than struggle with it.


So, you check the map carefully, and you see that the California Palm (Washingtonia) will not survive the winter in Denver, Colorado. The California palm grows in the Hardiness Zone 10 while Denver is in Zone 5a with a lowest temperature difference of 40 degrees (give or take).

So your dreams are shattered, but with a bit more searching, you find the Needle palm. Now the Needle palm doesn’t look much like the California Palm, but it is a palm tree, and low and behold, it says that it is cold hardy, and will grow outdoors in zones 5-10.

Heck, they even have a picture of it with snow on its leaves! You think to yourself, “Here it is; this is the palm for me!” You order up that palm tree, and you plant it in spring. It does fine all summer, but it isn’t looking so good next spring. Maybe you had a colder than usual winter? Perhaps you should have watered it again before winter hit? Or maybe, the people selling these plants realize that if they call it zone 5, they will sell more of them than if they call it zone 6 or 7, which is probably closer to what it really should be.

Lesson Learned

Either way, you might get some life out of it this second summer if you’re lucky, but chances are better that it will just slowly fade away, and you will have hopefully learned a lesson on plant zones.

Look at the map, but use some common sense. If you see a picture of that palm and think, “strange, this is a zone 5, yet I’ve never seen anything like this growing around here,” stick with your instincts.

If it is a neat-looking plant and it will grow in your area, chances are you would see it growing in your area, and chances are, the local landscapers would be planting it. There is a reason that you see the same sort of plants growing in every strip mall parking lot in your area. Because that is what survives the abuse of a barren cold parking lot full of ice, snow, and salt all winter or whatever the conditions are in your zone.

By the way, a quick search will show that the needle palm is native to coastal margins of the subtropical eastern Gulf and south Atlantic states of the United States. Yet they call it zone 5? As far as I know, palms are mostly in the Florida area and southern California; that’s about it. Why they would ever sell this palm as a zone 5 is beyond me.

Check out this map of where palms grow in the US: http://bonap.org/2015_SpecialtyMaps/Density%20Gradient%202015/original/2tax22_Palms.png

Now compare that to the hardiness zone map above. It looks to me like if that needle palm is the hardiest palm around, it should probably be rated down to maybe zone 7.

So, use your common sense when picking plants and realize that the less you push the limits, the happier you will be next spring when you take that first walk around the yard to see what survived.

California Palm (Zone 10)

Needle Palm (Zone 5?)

Are Plant Hardiness Zones Enough?

Here are a few things to note about hardiness zones. They are based on the lowest temperatures only. And as we discussed, it is not just the lowest winter temperature that affects plant growth, but also the soil, the highest heat temperature, the rainfall patterns, amount of sunlight, etc. So, while the hardiness zones give us an idea of the plant’s hardiness, there are many other elements to consider. Use your common sense and consider what is important to this plant you are choosing. How many hours of sun per day does it prefer? Does it prefer a wet or dry location?

Even if you choose a zone 4 plant for your zone 5 location, you still need to plant it in a spot in your yard where it will be happy. Too much sun will kill a shade-loving plant, and too much shade will kill a sun lover. The same holds true for soil moisture. If you plant in the proper zone and the appropriate location in your yard, you will likely have no problems and a happy plant.

Keep in mind that you can always press your luck and bring those tropical plants into the house for the winter, but that entails quite a bit of work, and it doesn’t always work out as planned. The safest way to do it is to have a large heated sunroom or greenhouse. Then you can probably keep them happy. Otherwise, in the corner of your house with a small window for light is not likely to cut it. At least in my house, light sunlight becomes the limiting factor.

What is the Plant Hardiness Zone of My Area?

USDA has provided a simple tool to punch in your ZIP Code and get your Hardiness Zone.

Try the Tool here.


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