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Zoning in on Hardiness

Learn how to determine if a plant will act as an annual or perennial in your area. Learn what USDA hardiness zones are and how they can help you have a better garden.

Contributors: Kerry Meyer

Learn how to determine if a plant will act as an annual or perennial in your area. Learn what USDA hardiness zones are and how they can help you have a better garden.

One of the most commonly asked questions by gardeners has to be “Is this plant an annual or a perennial?”  This deceptively simple question is a bit more complicated than it sounds.  Perennials are generally described as plants that return year after year without replanting.  Whether a plant is a perennial or an annual is based on whether that plant can withstand the cold temperatures of winter. 

This sounds simple and in some ways it is simple.  The main factor complicating things is that winter temperatures vary widely across North America.  In an effort to help gardeners understand which plants are hardy in which areas the USDA developed and published a plant hardiness zone map.  This map was first published in 1960.  It was updated in 1965 and again in 1990.


This USDA Plant Hardiness Zone Map divides the US and Canada into 11 different zones. These zones are based on the lowest average temperature each area is expected to receive during the winter.  The averages used to develop the current map are based on winter temperatures from 1974 through 1986.  

 

The first step in accurately predicting whether a plant will be hardy for you is to find out what your USDA hardiness zone is.  If you don’t know your zone you can use the Hardiness Zone Finder to learn what zone you live in.  Click this link to see a map that includes Canadian zones.  This finder allows you to type in your zip code and it will tell you what zone you live in.

Once you know which zone you live in, it is relatively easy to figure out whether a plant will be perennial or annual.  Simply compare your zone to the zone or zones listed on the plant tag or website or within a gardening book. 

If your zone is equal to or higher than the zone listed for the plant it will be hardy for you, it is perennial in your area.  An example, I live in zone 5.  If the plant tag says a plant is hardy in zones 5 to 9 the plant will be perennial for me.  It would also be perennial in zone 6, 7, 8, and 9.  If the plant tag says a plant is hardy to zone 4 then the plant will be perennial for me.

 

If your zone is lower than the zone listed on the tag then the plant will not be hardy for you, it is an annual in your area.  An example, if the plant tag says a plant is hardy in zones 6 to 10 the plant will be an annual for me in zone 5.

The same zones that apply to annuals or perennials also apply to shrubs and trees.

Once you know what zone you live in and understand a bit about how hardiness zones work, deciding whether a plant is likely to act as a perennial and survive your winter becomes much easier.

There are some other things to consider as far as hardiness goes.

  1. The hardiness zone map assumes that the plant will be planted in the ground not in a raised pot or window box.  The ground (soil) remains slightly warmer and won’t freeze as solid as a raised pot will.  The pot will frequently thaw out from time to time even in the winter.  It is this thawing and re-freezing that will cause many perennials and even some woody shrubs to be less hardy in a container than the landscape.  The zones listed in catalogs and on tags and websites refers to the zone a plant is hardy to if planted in the ground.  If you are planting in a pot you will need to choose plants that are two zones hardier than the one you live in.  So for me to have a plant hardy in a pot it would need to be hardy to at least zone 3 rather than zone 5.

    You could also dig a hole in your vegetable garden or an out of the way spot in your garden.  Place the pot into this hole and fill in soil until it is level with the lip of the pot.  This will keep your pot as warm as the surrounding ground helping the plant within the pot survive the winter.

  2. Plant breeders are constantly working to select plants that are hardier than the regular species.  It is possible that a newer, named cultivar could be hardier than the regular species.

    That being said, it is also possible that a newer, named cultivar is less hardy than the regular species.  This will sometimes happen when a new flower or leaf color (or pattern) is discovered.  Sometimes these genetic breakthroughs are initially linked with less hardy plants.  So if a plant tag lists a zone that is either hardier or less hardy than the general species it isn’t necessarily a mistake.
  3. It is also sometimes possible to take a plant that is one zone less hardy than your zone and bring it through the winter.  If you live in zone 5, a zone 6 plant could be perennial for you.  There are several reasons this could be true.  First the published zone information might be incorrect.  Second, it is often possible to find spots within your garden or even within your community that stay a few degrees warmer than the rest of the general area.  For instance, areas that are protected from the wind are often warmer.  Areas next to your house, especially south facing exposures, or those near a brick or stone wall can retain more warmth also. 

 

The moisture level, how well your soil drains, how much you mulch, when you mulch, and how much snow cover you have can all affect whether a plant is perennial or not.  Many plants that should be hardy will suffer in soils that don’t drain well.  Thick layers of mulch, properly applied, can increase hardiness as can a thick, insulating layer of snow.  The changing weather conditions each year can allow some plant to ‘perennialize’ for a few years and yet we all know that sooner or later, the real climatic zone will catch up with the hardy annual.

 

What does this all mean to you?  Hardiness zones are essential information for choosing which plants are likely to be perennial, however, by experimenting with placement and plants that are almost perennial, it is possible to expand the selection of plants that are perennial in your garden.

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Anonymous's picture
Anonymous Sun, 01/15/2017 - 2:36pm

Hello,

I live in zone 6 but am right next to the zone 5 line it looks like. It's really a toss up as to what weather we get between the two areas because it's a matter of 1/2 of a county where the line is. Do I plant for zone 6, both, or steer towards zone 5. What would be some good shrubs to plant that will be happy in shaded as well as mostly sunny areas? Also good low maintenance plants that love shaded areas, as I have a half circle area that is overcast by tall Elm trees.

Thanks so much!

Stacey Hirvela's picture
Stacey Hirvela Mon, 01/23/2017 - 11:31am

To be on the safe side, plant mostly zone 5 hardy plants. If you have some sheltered areas (and if you typically get reliable snow cover) you can get away with more zone 6 species. Your local garden center will be a great resource on what will and won't work depending on your exact location. You can search for shade tolerant plants on the top of our search pages: https://www.provenwinners.com/plants/search/shrubs. Most every shrub we offer would be considered low maintenance, as they need pruning once a year or not at all; they also have minimal water needs once established. 

Anonymous's picture
Anonymous Mon, 12/26/2016 - 5:47pm

I live in zone 10 and want to plant a deciduous magnolia for zones 4-8.
What should I do to ensure my tree grows successfully in my warmer zone?

Anonymous's picture
Anonymous Tue, 01/03/2017 - 3:56pm

Hi There!
OK so you want to plant a deciduous magnolia in zone 10, but the plant shows that it grows in zones 4-8. The key here is where are you in a zone 10 out west or down south?

I live in zone 9a-b and am growing some of the "Tulip" deciduous Magnolias, I assume this is what you are wanting to grow?

From what I have seen the most important thing is your soil type, if you live on sandy soil it makes it much harder to grow the trees, since they are stressed by higher temperatures as well as the low water holding potential of the sandy soil. If you have clay soil I think you would be OK since it holds water and helps the tree adapt to the warmer zone. If you are out west most of the soils are clay so it works in your favor, but if you are in the south the soils are often sandy.

I would definitely try the tree but I would start very small (1 gallon pot size or smaller), it means you may wait a few years before it blooms heavily, but those years will be spent developing a good root system, and it is a cheaper investment if things do not work out.

The last thing that may be a problem is insects, there are some borers that can decimate a magnolia and seem to be more common in the south. However if you are in the west this will be less of an issue.

Key factor will be regular deep soaking watering to help those roots get a good strong grip in the soil.

Please let me know if you have more questions!!

Anonymous's picture
Anonymous Tue, 06/21/2016 - 10:26am

I live in Illinois and I would love to add these plants to my garden. If planted in ground how can I protect them in the winter without pulling them out and transplanting them in pots. Thank you,

Sarah Geoghegan's picture
Sarah Geoghegan Fri, 06/24/2016 - 9:28am

Kimberly Queens are generally an indoor plant, but they will survive outdoors during the summer. However they will not survive winter temperatures outdoors, even with protection. There are ferns that are perennials though, I would suggest checking with your local garden center to find something similar that is hardy enough to get through an Illinois winter. Hope this helped!

Anonymous's picture
Anonymous Sat, 04/16/2016 - 4:52pm

Hi I ***thought*** the tag on my perennial tagged plant said hardy to ***negative 10 negative 20***. Turns out it says hardy to 10 to 20 degrees ***above*** zero Fahrenheit (zone 8, we live in zone 6)! Of course, I didn't find all this out until AFTER I get the plants in the ground! Should I try to return them for a different plant or should I keep them and cover them in cold weather, or take them in for the winter? I've been struggling with gardening and our stores are so NOT helpful!

Barb Balgoyen's picture
Barb Balgoyen Thu, 04/21/2016 - 7:18am

Good morning,

We are curious about the plants that you purchased, were they annuals, perennials or shrubs? Have you asked the garden center if they would work with you to get "Hardy" plants into your landscaping?

What did you purchase, maybe we can make some alternative suggestions.

We would like to help you through this frustrating situation, we just need more information.

Please email me back at your earliest convenience.

Thank you for your interest in Proven Winners plants.

Barb Balgoyen
Walters Gardens, Inc.
Proud Supplier of Proven Winners Perennials

Anonymous's picture
Anonymous Thu, 08/20/2015 - 9:42am

do you sell seeds. want purple fountain grass. if not what is the earleist I can get the plant. live in Sedona az

Cindy Meyers's picture
Cindy Meyers Thu, 08/20/2015 - 2:58pm

We do not sell seeds. We do have the Purple Fountain Grass available in our online store at: https://www.provenwinners.com/plants/pennisetum/graceful-grasses-purple-fountain-grass-pennisetum-setaceum-rubrum
Or you can use our Find A Retailer page and enter your specific location using your zip code to find retailers near you who may have it available. Here is the link: https://www.provenwinners.com/retailers/locate

Thank you for contacting Proven Winners!

Anonymous's picture
Anonymous Fri, 09/05/2014 - 10:47am

I have seen in the past a difference between temperatures between the canadian and us zones. Now when I look at catalogues and thier stated zones I 'm wary because I don't know if they are canadian or us figures or if both are the same zone.

Kerry Meyer's picture
Kerry Meyer Wed, 09/10/2014 - 3:53pm

The Canadian Zones match the US zones for temperature range - the ranges are universal.  So the zone infiormation is the same for both the US and Canada.  You will need to use a Canadian map to figure out which zone you are in, but the zones listed for each plant are applicable for both the US and Canada.

Kerry Meyer
Proven Winners

Anonymous's picture
Anonymous Mon, 05/05/2014 - 2:51pm

I WANT TO ORDER ON LINE. HAVING TROUBLE DOING SO. FRANK PHELPS FKPPA@SBCGLOBAL.NET

Kelly Geoghegan's picture
Kelly Geoghegan Mon, 05/05/2014 - 4:59pm

Call us at 815-895-8130 we are open from 9am-5:30pm Monday through Friday! Call us with any question!

Hope this helps! Kelly Geoghegan PW

Anonymous's picture
Anonymous Mon, 03/31/2014 - 3:13pm

Hi, I live in Coquitlam BC, just outside of Vancouver by about 25 kilometers, but I when we get snow, say 2 feet this winter, Vancouver didn't have any. So what zone would I be please?
Thank you,
Barb

Kelly Geoghegan's picture
Kelly Geoghegan Tue, 04/08/2014 - 5:13pm

This is the website we use for the Canadian zones. I would say your around a 7, maybe 6b-7a!

http://atlas.agr.gc.ca/agmaf/index_eng.html#context=phz-zrp_en.xml&extent=-3820622.0026645,-574774.2920971,4348384.0026645,3744546.2920971&layers=place37M,place25M,place15M,place5M,place1M,place500K,place250K;rivers25M,rivers15M,rivers5M,rivers1M,rivers500K,lakes37M,lakes25M,lakes15M,lakes5M,lakes1M,lakes500K,Roads25M,Roads15M,Roads5M,Roads1M,Roads500K,ferry500K,bndy5-37M,bndy1M,BndyLn1-5M;PlantHardinessZone2000;

Hope this helps! Kelly Geoghegan PW

Anonymous's picture
Anonymous Wed, 01/29/2014 - 11:07am

In the main article you say the zones are for cold weather and if your zone is higher than the plant your fine. But in response to another's comment you say it can't be higher because it would be too hot. I live in a zone 9/10 and am confused to what zones I should be looking for

Kerry Meyer's picture
Kerry Meyer Tue, 02/04/2014 - 9:58am

This article is specifically about USDA cold hardiness zones.  In general, a plant is considered a perennial if it lives through the cold of your winter, which is what most people are worried about.  In warm winter climates, the problem is a bit different.  Some northern perennials end up being annuals in warm winter climates because they cannot handle not having a cold period.  So if the USDA hardiness zones are listed as 4 to 8, then the plant would only be expected to be perennial in those zones.

Anonymous's picture
Anonymous Mon, 09/24/2012 - 10:55am

If I plat a tree zoned 5-7 in my zone 8-9 will it survive? How to I care for it in the heat of the summer?

Anonymous's picture
Anonymous Tue, 11/01/2016 - 2:25pm

Both your question and the answer from Laura are too general to be useful. "A tree"? Which tree? Tree species and varieties are different, and respond differently to different stresses. It's very possible that some trees labeled 5-7 would do fine in 8, or would do ok, or would struggle but still survive. Also, zone 8 on the west coast is different from zone 8 on the west coast, because the USDA map only considers temperature. There are other factors that partly determine hardiness, such as winter rainfall, summer humidity, etc. And there are microclimates: next to a south-facing stone wall is different from open north exposure.
Also--and the article doesn't mention this, but it's true--the hardiness of plants is not written on stone tablets handed down from on high. There are differences of opinion among experts, and certainly among nurseries, as to how hardy certain plant are. Sometimes there are scientific trials, but these don't occur fo all plants, are often region-specific, and become outdated as new varieties are introduced. As a person who once made a living by researching such information for pot tags, I can assure you that, for many plants, three different nurseries will give you three different hardiness zones. It's the same with plant size and color, by the way. To be sure, you need to do some research on your own, and the internet makes that easy. I always look first at sites that don't have a vested interest in selling plants, such as botanical garden, plant society, and university websites.

Laura Geoghegan's picture
Laura Geoghegan Tue, 01/07/2014 - 10:21am

It's to hot in zone 8-9 for the tree to survive.  Your zone needs to be on the tag for it to work in your area.

Laura Geoghegan PW

Wilma's picture
Wilma Mon, 05/28/2012 - 3:59pm

Your information is great. I have learned more reading this than all the garden books I have purchased in the past. From now on I will visit this web site first before I purchase any other flowers, shrubs, or misc. garden needs. My books will visit the yard sale so I can purchase more flowers.

Anonymous's picture
Anonymous Thu, 04/12/2012 - 1:36am

Wow! Great info! So now what to say to the people at the nursey? Well my plant could actually be a perennial as long as ...... No need to spend so much money on the same plants year after year. Thank you!

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