Have you noticed that a lot of the advice about when to plant herbs and vegetables is inconsistent across the internet? Do you find that one site says to plant tomatoes on April 20th in zone 6 while another says to plant them at the end of May? If so, you may be asking yourself whether garden hardiness zone data is reliable.

Is this article we take a look at why that is and how to get the most accurate gardening data for your area.

Why Garden Hardiness Zones Can Be Deceiving

Garden Hardiness Zones are often used as a guide for determining which plants will thrive in a given location. However, if you’ve gone strictly by the book – following the USDA Hardiness zone map as a gardening guideline – you’ve probably been frustrated as a result of killing some plants you thought were going to survive.

Why is that?

Well, it’s for several reasons, that all kinda boil down to this fact: Mother Nature is unpredictable.

What’s more, when you mix earth’s erratic weather with complex geography, you get a very complex weather system. In the end it’s just hard to formulate a simple chart to account for a complex system.

Humans by nature want things to fall into tidy categories that are easily explainable. It’s why we love lists and ranking things in order. It’s also why the hardiness zone map is so appealing to us as gardeners, especially beginning gardeners who yearn for guidance.

The truth is – and this may be disappointing to some gardeners – there is no system that is going to function as a perfect guideline for you. The only thing that comes close is experience, and even that will falter from time to time.

The world is a living being, after-all, and like us, it’s ever-changing. Mother nature tends to make plans that we often cannot for-see.

What Determines the Hardiness Zones according to the US

The hardiness zones are based strictly on average minimum temperatures, which were tracked over a 30 year period from 1976 – 2005. This means that they don’t take into account other important factors like humidity, rainfall, elevation, and many others which we’ll get into in a bit.

One piece of data that is often included with hardiness zones is the first and last frost date of the year. But there are microclimates that cause the first and last frost date to fluctuate within a given hardiness zone. For instance, a low-lying area in zone 6a might experience frost a week or two later than a hilltop in the same zone.

Although you’ll often find frost dates lumped in with hardiness zone data, first and last frost dates of the season are NOT what determine hardiness zones. Frost data is collected across 15,000 weather stations across the country under the National Centers for Environmental Information. The hardiness zone study, on the other hand, was done by the USDA in a study spanning from 2005-2012, updated recently by PRISM, a research group within Oregon State University. Two totally different organizations & data sets.

Mixing Hardiness Zone and Frost Date Data

The first and last frost dates often don’t correlate with the hardiness zones, which can result in bad advice when they are lumped together.

For instance, Holy Cross and Barrow are two Alaskan cities in hardiness zone 2b. Plugging them into this frost date calculator, which pools data from The NCEI, we find that the first and last frost dates in Holy Cross are September 14th and May 29th, respectively. In Barrow, however, the first frost is August 8th, the last frost date is July 28th. That’s a huge difference in the gardening season between two cities in the same hardiness zone.

Drastic changes in geography within a hardiness zone (cliffs), proximity to the ocean, and wind chill are examples of conditions that cause one garden to experience frost earlier or later than another garden within the same hardiness zone.

Climate Change and its Effect On Hardiness Zones

It’s important to also consider that the USDA hardiness map is not static. It changes also over time as the climate changes. For instance, some places that were once too cold for certain plants are now within the ideal range due to climate change.

What all of this means is that the hardiness zones can give you a false sense of security when it comes to your plants. Just because a plant is said to survive in your zone, doesn’t mean that it will actually thrive there.

Are the first and last frost dates associated with hardiness zones accurate?

As we saw in the Alaskan example above, they are often inaccurate, meant to serve as a guideline rather than be relied on strictly. The USDA actually never mentions anything about first and last frost dates on their website. Again, they use the average minimum temperatures to delineate plant hardiness zones.

The most often cited data on first and last frost dates come from The NCEI, and even that data should be used with caution when used to determine your area’s gardening season.

Probabilities of Frost on Certain Dates

Frost dates were recorded over a 30 year period at the 15,000 NCEI weather stations across the country. They then determined the probability of frost on any given date using that data, from a 90% chance of frost down to 10%.

When that data is cited on websites like, The Old Farmer’s Almanac, the top Google hit for frost dates at the time of this publication, they often use the date at which the 50% threshold is crossed. Meaning, at what date in the spring has the chance of frost reduced to 50%.

The problem is, a 50% chance of frost are not good odds when it comes to deciding when to plant tomatoes, which are notoriously frost intolerant. If you plant tomatoes on that date, half the time they will die that evening. It’s best to wait until the frost likelihood drops significantly below 50%. In fact, the 10% threshold is probably closer to the date you want to be planting in the spring and harvesting in the fall.

An experienced gardener balances these probabilities with the length of the growing season for any given plant vs its frost tolerance. Some plants can tolerate light frost, which occurs at around 28°F and 32°F. If you live in an area where the time between frosts is short, then you’ll be limited to plants that have a fast fruiting/flowering cycle, or plants that can tolerate medium to light frost.

You can always cover your garden beds or put plants in a greenhouse to extent the growing season as well.

How to find plants for your garden

All of this has been leading up to one important question being, What plants should I plant in my garden?

You’ve probably already searched online for the answer to this question and found there are a number of plant finder tools, such as the one on Gardenia’s website.

But for the same reason that the hardiness zones are aren’t perfect, so too are garden assistant tools like this. It’s just too hard to account for the subtle fluctuations in the environment that affect plants hardiness. Examples of factors that can vary within each hardiness zone are:

• Length of the growing season (first and last frost date)
• Amount of direct sunlight
• Soil density & PH
• Wind
• Altitude
• Natural Irrigation

There are others…many others. It’s simply too many factors to account for.

Experimentation in the Garden

The best way to find out what grows best in your yard is to experiment. Use the tools as a guideline and just go for it. Better than the tools – to ask your neighbor who has a plush garden what their thoughts are. Go to the local nursery and ask an expert. The USDA even offers the same advice on their website:

“No hardiness zone map can take the place of the detailed knowledge that gardeners pick up about their own gardens through hands-on experience.”

– USDA Plant Hardiness Website

If you’re an inexperienced gardener or just inexperienced in the zone you are planting in, you may want err on the side of caution and choose plants that are a zone or two lower than what is listed for your area. This will give them a better chance of survival, even if the weather takes an unexpected turn.

Again, just plant something in the ground you think has a good chance of survival. See how it does. It’s ok if some plants fail. Trial and error is the best teacher for a gardener. Start out mostly with plants that thrive in the zone below yours. Try planting one that might be a little risky.

For instance Rosemary, a cold hardy herb, is typically hardy in zones 7-10. You live in zone 7a (the colder half of zone 7). In your first year, don’t go out and buy 5 rosemary plants. Buy one and see how it does. You can even put it in a pot. That way if you see it start to die as winter sets in you can bring it inside.

If it does well, not only do you know that rosemary does well in your yard, there’s a good chance other zone 7 hardiness plants will do well too.

Conclusion

The USDA plant hardiness map, the first and last freeze date estimates, and plant finding tools found online are excellent guidelines to help you get an idea of what will do well in your garden.

However, the best advice on what to plant in your garden will likely come from a local master gardener or plant specialist at your local nursery.

Due to microclimates that can exist, even in your yard, truly the best thing you can do to find out what will do best in your garden is to experiment. That trial and error process is part of what makes gardening interesting. Don’t be afraid to get out there, get your hands dirty, and have fun!