Plant hardiness ratings explained

When we buy a plant, there’s always the temptation to make an impulse purchase. Yet the first question that should pop into a gardener’s head is ‘will it actually thrive in my garden?’


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In a climate as varied as the British Isles, one of the main factors used to work this out is whether it can survive the winter outdoors unaided – what we call ‘hardiness’.

Throughout Gardens Illustrated there are references to a plant’s hardiness described using two separate scales: one from the Royal Horticultural Society (RHS) and the other from the United States Department of Agriculture (USDA). Both are based on accumulated experience of a plant’s ability to endure cold conditions, or in the UK, the ability to withstand changeable periods of freezing and relatively mild, wet weather. Plants adapt to tolerate cold to varying degrees. Some will take a light frost (temperatures just below freezing) for a couple of hours, others cope with long periods of freezing to remarkably low temperatures.

The USDA scale was developed in the 1960s for North America but it is now widely used in other countries around the world. It’s based on minimum average temperatures in a range of zones – as shown in the two illustrations – which increase in steps of 10°F. The US zones have been mapped in remarkable detail and plants can be assigned a rating according to the coldest zone in which they can grow or, as we have done here, a range of zones in which the plant is known to grow. These range from 1 (very hardy) to 13 (least hardy), with each divided into two 5°F subzones ‘a’ and ‘b’. This USDA system has been applied to the UK and Europe and gardeners here can use the mapped zones as a general guide.

The RHS scale, introduced in 2012, is a more descriptive system of hardiness ratings. Like the USDA system, it uses a scale (this time in 5°C steps) from H1 to H7, based on minimum winter temperatures. However, it has two major differences. The first is that it runs in the opposite direction to the USDA system – 1 is very tender, 7 is very hardy. It also offers a description for the garden conditions to help reflect the variable nature of UK winters. These are summarised in the scale below, but you can find more detailed descriptions on the RHS website. The most tender rating, H1, has been subdivided into three categories A, B and C.

Perhaps, though, the more important difference is that the RHS scale is a rating of the plant’s hardiness and hasn’t been translated into mapped zones for the UK.

All rating given in the magazine are accurate to the best of our knowledge. Not every plant has been given a rating by
the RHS, but where none is available our contributors will sometimes rate the plant based on their own trials. USDA scales are taken from the website of the Missouri Botanical Garden ( or based on growers’ experiences in the USA. Both are included as a guide only to help you predict what you should be able to grow in your garden. n

USDA and RHS scale (US and UK systems)

For those keen to grow tender plants, a south-facing aspect is desirable to maximise exposure to the sun’s heat. Likewise, shelter is critical, either to protect plants from cold winds or from cold air flowing down hillsides.

Gardeners will talk about frost pockets or frost hollows. These are where cold air drains off exposed slopes, as cold air is heavier than warm air, and collects in valleys or in sheltered areas where it cannot escape. This leads to cooler overnight temperatures and more frosts.

Where possible, lay out your garden to avoid slowing or trapping cold as it filters down slopes. Hedges, fences and walls can be strategically placed to protect plants or to provide sheltered nooks where you can grow plants that would not flourish in the open. Walls provide an additional benefit because they absorb heat during the day, which they then give off again at night. Cleverly, this keeps the surrounding air several degrees warmer, which can make all the difference to your plants on a frosty day.

Bear in mind, though, that structures such as walls, fences and hedges also cast shadow. This can mean that after an overnight frost, even though the daytime temperature rises above freezing, the ground remains frozen when in permanent shade.

Useful Information

RHS hardiness ratings
USDA hardiness maps


The Gardener’s Guide to Weather and Climate by Michael Allaby (Timber Press, 2015). Weather in the Garden by Jane Taylor (John Murray Publishers, 1996).

Hardiness Zones In Britain – USDA And RHS Hardiness Zones

If you’re a gardener in the United Kingdom, how do you interpret gardening information that relies on USDA plant hardiness zones? How do you compare UK hardiness zones with USDA zones? And what about RHS zones and hardiness zones in Britain? Sorting it out can be a challenge, but understanding zone information is important because it helps you select plants that have the best chance of surviving in your particular climate. The following information should help.

USDA Plant Hardiness Zones

USDA (U.S. Department of Agriculture) plant hardiness zones, based on minimum ten-year average temperatures, were created in the 1960s and are used by gardeners around the globe. The purpose of the designation is to identify how well plants tolerate the coldest temperatures in each zone.

USDA zones begin at Zone 1 for plants that tolerate severe, sub-freezing temperatures to tropical plants that thrive in Zone 13.

RHS Zones: USDA Zones in Great Britain

RHS (Royal Horticultural Society) hardiness zones begin at H7 (temperatures similar to USDA Zone 5) and are used to designate very hardy plants that tolerate sub-freezing temperatures. On the opposite end of the temperature spectrum is zone H1a (similar to USDA zone 13), which include tropical plants that must be grown indoors or in a heated greenhouse year round.

Does Britain Use USDA Hardiness Zones?

While it’s important to understand RHS hardiness zones, much of the available information relies on USDA zone guidelines. To get the most benefit from the wealth of information on the Internet, it’s a tremendous help to arm yourself with information about USDA zones in Great Britain.

Most of the United Kingdom is located in USDA zone 9, although climates as chilly as zone 8 or as mild as zone 10 aren’t uncommon. As a general rule, the UK is marked primarily by cool (but not frigid) winters and warm (but not scorching) summers. The UK enjoys a fairly long frost-free season that extends from early spring to late autumn.

Keep in mind that UK zones and USDA zones are intended to serve as guidelines only. Local factors and microclimates should always be taken into consideration.

Garden Fundamentals – become a better gardener

Planting zones, also called hardiness zones and gardening zones, help you select plants that will grow in your climate. In this blog I will answer the following questions:

  • What are planting zones?
  • How do you determine your own hardiness zone?
  • Are the USDA zones the only planting zones? What about other countries?
  • How do you select better plants by knowing your planting zone.

Berkheya purpurea by Robert Pavlis

What are Planting Zones, Hardiness Zones and Gardening Zones?

Planting zones, hardiness zones and gardening zones are all names for the same thing. USDA zones is another name for the same thing.

Consider the above picture, Berkheya purpurea, from my garden. Before I grew the plant, I didn’t know if it could survive our winters. How much cold can Berkheya purpurea take in winter? Planting zones provided a way for me to know this without buying the plant.

I know my planting zone is 5, or more precisely 5b. With this information I can look up the plant on the internet to find it’s hardiness zone. If the plant’s hardiness zone is the same as mine, or a smaller number, then it should grow here.

Plants generally have a range of hardiness zones. So a plant might have a zone range of 4 to 8 (usually written as 4-8). This means that the plant should grow in any planting zone between 4 and 8, inclusive. The smaller the number, the colder the climate – just like a thermometer. So if you live in zone 3, a 4-8 plant will probably not make it through the winder.

If you live in a warm planting zone like 10, then this plant will also not do well because of the heat. It does not want to grow in a zone warmer than 8.

Finding Your Planting Zone

There are two ways to find your planting zone. You can ask an experienced gardener in your area. They will almost certainly know their planting zone and if you live nearby – yours will be the same.

You can also look on the internet for a planting zone map for your area. Some magazines and books also have this information but older written material may be out of date.

USDA Planting Zones

The USDA has created a number of plant zone maps over the years, with the latest one being released in 2012. It’s official name is the USDA Plant Hardiness Zone Map. The map is based on the average annual minimum winter temperature, and is divided into 10-degree zones (F).

Each zone is further divided in half to have an ‘a’ and a ‘b’, with ‘a’ being 5 degrees colder than ‘b’. To be honest, very few sources of plant information bother with the ‘a’ and ‘b’. They will just report the number.

If you live in the US, you can get your planting zone here:

Canadian Planting Zones

Canada has taken a slightly different approach. In addition to the minimum winter temperature, they also take into account other variables like: length of frost-free period, summer rainfall, maximum temperatures, snow cover, January rainfall and maximum wind speed. In theory this makes the maps more accurate than the USDA zone maps.

The latest map is from 2000 and it can be found here: Unfortunately, these maps use stupid colors so it is hard to tell one zone from another.

An easier way to find your Canadian hardiness zone is to look your town on this list. The headings are bit confusing. The first column is the old zone number, the second the new and current zone number.

Be careful with older Canadian information. Prior to the 2000 update, the Canadian system used a different number scheme. In my case I used to be a 6, and now I am a 5. They did this to harmonize the numbers with the USDA maps.

Note added Dec 23, 2015: I compared the US maps to the Canadian zone map of 2,000, and found that Halifax, Quebec City, Edmonton and Guelph had the same zone number on both maps. Winnipeg and Richmond BC were not the same. It is clear that not all parts of Canada are harmonized with the US system – you need to check this for yourself.

Planting Zones for Europe

The most used planting zones for Europe follow along the same lines as the USDA Plant Hardiness Zones. A map for Europe can be found at:

Planting Zones for the UK

The UK uses the same zones as Europe – sort of. Planting zones in the UK range from 7 to 9. If you read any plant information from the UK, they really only have two types of plants; hardy and not hardy. Since almost everything grows in zones 7 and 8, most plants are hardy.

Trying to get hardiness information on a particular plant from UK sources is, for the most part, a waste of time.

The RHS also has their own system of H#. If you want more information about this system have a look here: RHS hardiness system.

Planting Zones Globally

Most other countries have adopted the USDA Plant Hardiness Zones, and some countries have developed their own system. It really makes sense to stick to the US system or at least keep their numbering system, because that is the information you will find in most reference material, including the internet.

Here are maps for some other regions/countries:

Planting zone for Africa:

Planting zone for Australia:

Planting zone for China:

Planting zone for Japan: A pdf for Japan

Planting zone for New Zealand:

Planting zone for Ukraine:

Let me know in the comments below if there are maps for other countries.

The Limits of Planting Zones

Planting zone information can be extremely valuable when selecting plants. Most plants sold in North America have the USDA planting zone right on the label. Many websites that discuss plants also list the planting zones for a particular plant. All good stuff – but there are limitations.

Accuracy of plant zones

Who determines the hardiness zone for a particular plant? Newly developed plants don’t have a hardiness zone since few people have grown it. any reported values are a guess that gets fine tuned over time as more and more people grow the plant.

Sometimes the zones are determined by testing plants in various regions. A few years ago I was asked to trial a new cultivar called Liriope ‘Super Blue’. The growers who were introducing the plant wanted to know how well it would grow in zone 5b. They gave me 12 plants and asked that I grow them in various locations at Aspen Grove Gardens – my private garden. The story and results are fully documented on my GardenMyths blog in a post called Hardiness of Liriope ‘Super Blue’.

Liriope ‘Super Blue’ by Robert Pavlis

If you go to 5 websites and check on the plant hardiness numbers you are likely to get 2 or 3 different answers. Some sites are very conservative. Fine Gardening seems to always understate the hardiness of plants. Other sites overstate the hardiness – are they trying to sell more plants?

The plant in the first picture in this post is Berkheya purpurea. It is rated as a ‘6b’ plant. From experience I know 6b plants don’t survive our 5 b winters. I also know that many plants native to South Africa are hardier than reported. I grew this one from seed, and it has survived 4 winters so far, two of which were extremely cold. Maybe I have been lucky, or maybe this is not a zone 6b plant. It looks more like a 5b plant and other growers are now reporting similar findings.

I usually check several websites for zone information. If they all match, I am fairly confident in the numbers. If they don’t match, I usually go to Dave’s Garden. I find the data on this site to be the most reliable. The easiest way to find the information if you are not a member of Dave’s Garden, is to add the word “Dave’s” to your Google search.

Local Micro Climates

I don’t believe that you have different small micro climates (ie low temperatures in winter) around a normal small sized lot – although many sources claim you do. However, your zone might be different than a garden up the road if one of the two sites is located at a higher elevation or is more exposed to winds. Published zone maps don’t take this into consideration.

I live just outside of a city of over 100,000 people. Inside the city, 4 km away, the low winter temperatures are a couple of degrees warmer in winter than my place. Some of my shrubs die back to the ground every winter, while in town they don’t.

Snow cover also plays a big role. I have trouble growing some plants because we have limited snow cover most winters. North of here, it is certainly colder, but they have reliable snow cover. So their air is colder, but the perennial, hiding under the snow is warmer. Their effective zone is actually warmer due to snow cover.

Dry Can Be More Important Than Cold

Some plants can take very low temperatures; most alpines for example. But some can’t take wet conditions in winter. If you have warm spells in winter, and winter rains, it can spell death to a plant, even though it should be hardy in your area.

Experience Fine Tunes Your Planting Zone

Planting zones are not perfect, but they are a great help in selecting plants – make use of them. Use them as a guide, and over time, experience will tell you if you are in a slightly warmer or colder zone. You will probably have to kill some plants to really find out.

1) Photo Source: all pictures by Robert Pavlis

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If you’re a beginner gardener or even a seasoned one, there are certain no fuss, no fail garden plants that gardeners turn to in their outdoor space. These flowers and foliage plants bring color, interest and texture to the garden, and they can take dry, drought-like conditions.

When you turn to these low-maintenance plants, your garden will reward you with endless beauty.

8 Hardy Plants You Can’t Live Without in the Garden

1. Tickseed

Drought-resistant, perennial tickseed, also known as coreopsis, is a perennial that grows well just about anywhere. It brings pretty blooms and wispy foliage.

Just give tickseed full sun to part shade and watch it thrive.

2. Sedum

Colorful perennial sedum succulents spread easily in containers and in the garden. They’re easy to grow in sun and shade and you can take down a stalk or clipping and replant it elsewhere in the garden and it will thrive.

Sedum comes in many varieties and provides a blend of color and textures. It grows tall or can be low-growing. For vertical planting or groundcover, try a sedum paver. It works well on slopes, along sidewalks and even in the hellstrip, or the space between the sidewalk and the street. Check out this DIY birdhouse project that uses sedum.

3. Coneflower

Coneflower, known also as echinacea, is a perennial that brightens up your garden with blooms that remain steadfast even during the hottest times of the season. Coneflower is among those summer favorites that provide drought-tolerant and low-maintenance blooms and grows well in the ground or in containers.

Coneflower comes in a variety of colorful shades, including bubble gum pink, fuchsia, sunny yellow, ruby red, cotton white and spicy orange.

Just plant where they will get six or more hours of sun and mulch to keep them moist during dry periods. Coneflower forms clumps so divide them every few years for free plants to place elsewhere in your garden or containers.

4. Mint

As fresh herbs go, this garden plant is an easy one to grow. You can clear cut, step on and pick mint until the plant is bare and this herb will still grow back as a perennial.

Some gardeners don’t like its spreading habits so it’s possible to contain them in planters or stick them in tough spots around your landscape.

Another solid reason to like mint? It smells fragrant when you brush past it and you can pick it for use in beverages, marinades, ice cream or as a garnish. Yum!

5. Begonia

Give these little annual flowers some shade and they’ll bring you big color without fail. Versatile begonias work well in flowerpots, window boxes and hanging baskets or plant them as a border plant in your garden.

6. Hosta

Shade-tolerant perennial hosta thrive in a variety of garden settings. With their flashy foliage, hosta can play star in the garden or take a supporting role with their color and texture framing lively blooms from annuals and bulbs.

Although hosta send up stalks of lavender and white flowers each summer, the number one reason to plant hosta is for the magnificent foliage. With colors ranging from shamrock green to chartreuse to teal blue, hosta offer contrast to pink, red, orange and yellow from spring through fall.

7. Nandina

Called heavenly bamboo, perennial nandina grows in sun, in shade and pests don’t bother it, either. It’s a fast-growing shrub that spreads and is hard to kill once it’s planted. The best way to grow nandina is in containers and keep it under the cover of a porch or outdoor room.

Important to note: trim off the red berries on nandina since they can be toxic to birds.

8. Spirea

Pollinator-friendly spirea teems with flowers and foliage that can take a beating by the weather or gardener neglect. This perennial shrub grows in full sun or part shade and requires little pruning or maintenance.

Spirea is an adaptable shrub that creates layer of blooms wherever you decide to plant it.

See our other lists for an EASY, low-maintenance garden:

  • Spend Less Time Watering with These 7 Tricks
  • Toughen Up Your Landscape with 5 Drought Tolerant Shrubs
  • Garden Smarter When You Plant These 5 Low-Maintenance Perennials

About The Hardy Plant Society of Oregon

The mission of the Hardy Plant Society of Oregon is to nurture the gardening community. Our vision is to make the world a better place through horticulture.

The Hardy Plant Society of Oregon (HPSO) is a membership organization for gardeners at all levels. Founded in 1984 to serve Oregon and Southwest Washington, the Society now has over 2800 members throughout the Pacific Northwest and beyond.


Organized to foster an understanding of herbaceous perennial plants (or “hardy perennials”), HPSO now promotes education about a wide range of plants, garden design, and gardening techniques. We sponsor workshops, lectures, study weekends, classes, publications, book sales, plant and garden art sales, trips and tours. We organize member gatherings and encourage member networking, support worthwhile community gardening projects through grants, sponsor plant and seed sales or exchanges, and encourage the preservation of special gardens of botanical, horticultural, or historic interest.


HPSO is a tax-exempt, 501c(3), non-profit, charitable and educational organization, incorporated under the laws of the State of Oregon and governed by a Board of Directors elected by the Society’s members. HPSO maintains an office in Northwest Portland. Part-time staff oversee the office, and many volunteer members manage or support the Society’s activities, including our publications, our Open Gardens program, our plant sales, and our speaker programs.


HPSO started in the 1970’s, when Marvin Black, a Seattle horticulturist, on a gardener’s exploration of England, discovered Great Britain’s Hardy Plant Society. Marvin was impressed with the organization’s possibilities and turned to his gardening friends for support in organizing the first American Hardy Plant Society. In June of 1980, this group sponsored the first Pacific Northwest Hardy Plant Society Study Weekend at Edmonds Community College.

Eventually, a group of Oregonians, tired of traveling to Seattle for garden events, met with Marvin to plan the beginnings of the Hardy Plant Society of Oregon. Started by a small group of friends in 1984, HPSO grew to nearly 2,000 members in 1995 and surpassed 2,400 members in 2014.

The Hardy Plant Society of Oregon has subsequently inspired the development of several other gardening groups, not just in the Pacific Northwest, but here and there throughout the USA. There are now Hardy Plant groups all across the country.


Join one of the West Coast’s largest gardening communities and meet other gardeners who share your passion.



Word family (noun) hardship hardness hardiness (adjective) hard hardened hardy (verb) harden (adverb) hard hardly From Longman Dictionary of Contemporary EnglishGardening, Plantshardyhar‧dy /ˈhɑːdi $ ˈhɑːrdi/ adjective 1 STRONG PERSONstrong and healthy and able to bear difficult living conditions hardy mountain goats2 DLGHBPa hardy plant is able to live through the winter —hardiness noun Examples from the Corpushardy• Red deer are hardy, adaptable animals.• A hardy and easy plant to grow in the aquarium.• Very hardy and easy to grow.• The people who lived in the hills were a hardy and hard- working race.• Charolais cattle do not like rain or too much cold. They are not hardy animals.• A very hardy Aponogeton, and one of the best to grow in the aquarium.• Small specimens do fairly well in tanks, but they are not among the most hardy aquarium fishes.• To survive the cold, Winnipeggers are not just hardy but imaginative.• Usually Guppies are hardy fish and adapt to most water conditions, but they do prefer alkaline water.• A few hardy joggers were out running in the cold.• It is an excellent, hardy plant, which is also easy to grow.Origin hardy (1200-1300) Old French hardi, from hardir “to make hard, make brave”

Hardy Annual

An annual is a plant that only lives for a single growing season. Thus, they are not generally thought of as being hardy. However, hardy annuals do exist, and they can provide many benefits to the outdoor gardener. A hardy annual is nothing more than a plant that will only live for a single growing season, but that can withstand wintertime temperatures and possibly frost.
Hardy annuals begin life as a seed, germinate and grow into an adult plant, and then produce seeds before dying at the end of the season. However, because they are tolerant to the cold, they can be planted well before other plants can, which would be damaged or killed. For instance, snapdragons can be planted in the fall or early spring, giving gardeners a head start without having to worry that the plants will sustain damage from cold temperatures.
You will also find that hardy annuals can make do in poorer soil and harsher growing conditions than many other plants. This is often because they were originally adapted to semi-arid growing conditions, or evolved to deal with soil that lacked many nutrients. However, while they will grow in poor soil, it is recommended that they receive a mild fertilizer (7-7-7) to help support growth and the development of strong stems and vibrant flowers.
Finally, understand that hardy annuals and half-hardy annuals are not the same thing. Half-hardy seeds cannot be planted before the last frost, but can be sown before cold temperatures are gone for the year.

We all love a good stew. After letting it simmer all day in the crock pot, it becomes a delicious and filling meal that can feed a family for days.

But should we call it a hearty meal or a hardy meal? Separately, should you plant hardy plants in your garden or hearty ones?

These two words are easy to mix up, especially since their meanings can seem very similar if not used carefully. Read on to discover the best contexts for these confusing adjectives.

What is the Difference Between Hearty and Hardy?

In this article, I will compare hearty vs hardy. I will give a few example sentences for each word, and I will also discuss a useful memory tool that will help you decide which of these words to choose.

Is it hardy or hearty? Let’s find out.

When to Use Hearty

What does hearty mean? Hearty is an adjective that has many different senses.

Here are a few examples,

  • “Make me a hearty stew!” cried Marilyn, after she returned from her hunting trip.
  • The parishioner offered a hearty greeting to everyone who walked through the doors of the church.
  • “Huzzah!” was the pirates’ hearty cry after they had captured another naval ship.
  • This warming, savory, hearty baked rice casserole was originally meant to be an Indian-style biriyani, but my larder was stocked with Gallic ingredients: mushrooms, thyme, garlic, parsley. I switched gears, heading in a French direction. –The New York Times

Expressions Using Hearty

In most cases, the word you are searching for it hearty. This is the correct choice for the following expressions,

  • A hearty appetite.
  • A hearty meal.
  • A hearty handshake.
  • A hearty laugh.
  • A hearty applause.
  • Party hearty.

When to Use Hardy

What does hardy mean? Hardy is another adjective and is defined as strong, robust, and capable of enduring difficult conditions. It has some overlap with the second sense of hearty (see above).

For example,

  • I will plant some hardy geraniums in my flower garden, because they can survive my poor gardening skills.
  • The rookie boxer fights with passion, but his veteran opponent is hardy and has taken many blows in his long career.
  • The Irish come from hardy stock, and they will not go down without a fight.
  • It is also a hardy virus. Dr. Moe said studies she has done have detected the virus on surfaces three to four weeks after it was put there though it is unclear if it could cause an infection that much later. –The Wall Street Journal

Expressions Using Hardy

There are fewer common expressions that use hardy compared with those that use hearty.

Here are a few phrases that call for hardy,

  • Hale and hardy.
  • Hardy stock.

Most other instances call for the word hearty.

Trick to Remember the Difference

You are not the first writer to be unsure whether hardy or hearty is the correct word. Luckily, there is an easy way to remember the difference.

Hardy contains the adjective hard, which can also mean strong or robust in many contexts. The similarity between these words should help you remember when to choose the word hardy.

Hearty is more complicated. It has many meanings, one of which overlaps with hardy, and is the source of most of the confusion. In most cases, for sturdy or strong, hardy is a better choice, while hearty is more appropriate to describe a hot stew or a welcoming greeting.

To remember hearty, think of things that benefit the heart: a welcoming greeting, a nourishing strew, pleasant laughter or applause.


Is it heart or hardy? Hearty and hardy are both adjectives.

  • Hardy means robust or sturdy, like a plant that is hard to kill or a champion martial artist.
  • Hearty has many meanings, including wholesome and filling when used with regard to food, and enthusiastic and welcoming when used regarding laughter or a greeting.

Hearty also has a meaning that overlaps with hardy, but in these circumstances, hardy is usually the better choice. Since hardy contains the word hard, which is its rough synonym in many contexts, you shouldn’t have much difficulty choosing that word.

Remember, you can always reread this post for a quick refresher if you get confused.

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