Save Your Garden by Understanding Frost Terminology

It's early spring, and you've taken loads of time choosing the plants you want to grow this year, you've invested in your seeds, you've carefully tended the seedlings in your house, and now it's time to take them to the great outdoors to finish their life (and ultimately, improve yours!)

BUT, if you're not knowledgable on the subject of FROSTS and all it entails, you could very likely experience great loss and watch all your hard work, hard-earned money, and precious seedlings wash down the drain.

How can you avoid loss in the garden during this early spring season when frosts are still occurring and the temperatures are fairly unstable? Well, it comes down to understanding the different types of frosts (light, moderate, and heavy) and the frost tolerance level of your plants (frost hardy, frost tolerant, frost sensitive, and warm-loving).

When it comes to planting your early spring garden, as with any garden, there are certain measures and precautions you should take in order to help your plants survive and thrive.

But the BIGGEST PRECAUTION you should take, and sometimes the biggest learning curve, is Understanding  "Frost" Terminology (this also includes knowing your specific "frost dates").

So, let's dig into it, shall we?

Term #1 - What is a "Frost" or "Freeze"

If you've been gardening for any time at all, and even if you haven't, you likely know that some plants can withstand (and even thrive) in colder/cooler weather, while other plants wilt at the very mention of "cold"!

You've also probably heard the following terms: frost tolerant, frost hardy, cold hardy, frost resistant, frost sensitive, light frost, hard frost, and hard freeze. 

But what exactly do these terms mean? What's the difference between "frost tolerant" and "frost hardy"? And what is a "light frost" versus a "hard freeze"?

Trust me when I say, there IS a difference and if you don't FULLY UNDERSTAND what temperature ranges define each term AND which plants fall into which "frost" category, you could lose your garden just like I did. ⬇️

So, let's define the terms and save you from losing YOUR garden...

#1 Why are there differences in temperature tolerance between plants?

It comes down to the cellular makeup of the plant. Plants with higher levels of starches (or sugars) in their leaves will freeze at a lower temperature (just as sugar water freezes at a lower temperature than plain water).

So, the higher the starch content, the lower temperatures that particular plant can handle.

The other concept you should understand is:

#2 WHY do plants experience frost damage in the first place?

Frost occurs when outdoor temperatures hover around the freezing point, or 32°F/0°C (although frost can actually happen up to around 36°F). It occurs when moisture, or water on the plants, in the air or within the plant itself, freezes.

Because plant cells contain water, the cells freeze at these lower temperatures (keeping in mind the adjustment for the sugar/starch levels). And what happens when water freezes? It expands, which causes the cell wall to burst, damaging the structure (if not outright killing) your plant.

So, with that knowledge, which plants can handle those lower temperatures?

Well, before we go there, you also need to understand the temperature ranges that define the two types of cold hardiness and, therefore, the level of cold tolerance these plants will have.

Term #2 - Cold Hardy Plants

The first level of cold hardiness is COLD HARDY, or in some cases, FROST HARDY.

A mature, cold hardy plant can easily handle light to moderate frosts (29-32°F) without being damaged. And they can generally tolerate a hard frost quite well, which is defined as temperatures at or just below freezing (32°F/0°C).

Some cold hardy plants can even handle a HARD FREEZE, which is defined as a period of at least four consecutive hours with air temperatures reaching sub-freezing temperatures that are below 28°F (–2°C). But survival at these temperatures doesn't usually happen without some measure of precaution, to include hardening off or a protective cover.

Mature, cold hardy plants that have been exposed to these colder temperatures off and on will very likely survive a hard freeze. However, tender baby seedlings will not fare as well and will likely need to be hardened off 4 to 6 weeks BEFORE your last frost date.

By hardening them off, you slowly acclimate your plants to the colder temperatures and natural elements that they likely haven't experienced in your house.

To harden off cold hardy plants, start setting them outside when daytime temperatures are above 40°F. If evening temperatures stay above 40°F, you can leave them outside overnight. BUT if temperatures are forecasted to dip BELOW 36°F, bring them back indoors for the night and start again in the morning.

Depending on the forecast, you can typically transplant your cold hardy plants to the garden 2 to 4 weeks before your last frost date, AFTER they've been hardened off for at least a week or two.

BUT, if temps creep near that 36°F mark, either hold off transplanting them or use a thick frost cloth to protect them (again, IF they've been hardened off), keeping the frost cloth mere inches above your seedlings (versus feet above).

* Image courtesy of Charles Dowding

I CANNOT STRESS THIS POINT ENOUGH... Even though a seed packet or a website (or wherever you find your plant information) tells you a plant is "cold hardy" and can handle freezing temperatures, don't risk losing your precious seedlings to a frost because you didn't harden them off OR cover them!

The exception to this rule would be if you've been growing your seedlings in an outdoor, unheated greenhouse where seedlings are more regularly exposed to lower temperatures and sunshine. They will fare much better then those grown inside a temperate house.

I do want to mention that the acclaimed "no dig" garden pro, Charles Dowding, recommends transplanting smaller seedlings into the garden (versus more mature seedlings) and he DOES NOT harden them off. However, he grows his seedlings in an unheated greenhouse, so they are more acclimated to lower temperatures. 

The choice is obviously yours, but I suggest (from experience) erring on the side of caution.

Cold Hardy Plant List

• Arugula
• Asparagus
• Beets
• Broccoli
• Brussels Sprouts
• Cabbage
• Carrots
• Collards
• Endive
• Garlic
• Kale
• Kohlrabi
• Leeks
• Lettuce (most varieties)
• Mustard Greens
• Onions (Seeds, sets, bunching)
• Parsnips
• Potatoes
• Rhubarb
• Rutabaga
• Spinach
• Turnips

Term #3 - Frost Tolerant Plants

Also known as, "frost resistant" plants, these plants can handle "light frosts", but not for long.

A "light frost" is defined as temperatures down to (but not below) 32°F. A "moderate frost" is considered to be between 29°F and 32°F and would most likely be too cold for 'frost tolerant' plants.

The gist of this group of plants is, stay above 36°F and you'll be fine. Although I would still recommend covering them with a frost cloth to play it safe. 

There is such a things as a "microclimates" that can cause the temperatures around your plants to differ just enough than those temperatures in the surrounding (and forecasted) areas.

In the same fashion, harden off your frost tolerant plants just as you would your Cold Hardy Plants, making sure to bring them in the minute temperatures dip below 40°F!

Frost Tolerant Plant List

• Artichoke
• Bok Choy
• Cauliflower
• Celeriac
• Celery
• Chard
• Chinese Cabbage
• Peas
• Radish
• Radicchio

Term #4 - Frost Sensitive Plants

Frost Sensitive Plants are those that fall between the categories of Frost Tolerant and Warm Loving.

They can handle "cooler" temperatures down to the 40s but can by no means handle a frost and most definitely will die with a freeze.

These plants should be hardened off 2 to 3 weeks before your last frost date and SHOULD NOT be planted before your last frost date, unless recommended on the seed packet. For example, some snow peas can handle light to moderate frosts.

Make sure you are keeping an eye on the forecast BEFORE you plant them in the garden and even AFTER you've planted them, in case there is an unexpected freeze. 

If an unexpected freeze happens, read "how to prepare for a late frost" in this article to know how to protect your plants.

These plants would do really well placed or planted in a greenhouse or hoophouse, if you have one. They will appreciate the extra warmth in most cases.

Frost sensitive plants will also experience stunted growth when temperatures get into the mid-40’s or so. While they can often tolerate cooler temperatures, the benefits to planting them out too early is generally minimal.

Frost Sensitive Plant List

• Snap Pea
• Sweet Corn
• Tomatoes
• Beans
• Some herbs (basil, lemon verbena, stevia, lemongrass, calendula, borage, dill, cilantro) - although I've had pretty good success with dill and cilantro in cooler weather.
• With few exceptions, most flowers are frost sensitive. Exceptions include perennial flowers that are naturally cold hardy.

Term #5 - Warm-Loving Plants

These plants absolutely do not like temperatures below 50°F and SHOULD NOT BE PLANTED before your estimated last frost date AND could even wait to be planted well AFTER the last frost date.

If exposed to temperatures below 50°F, these plants will experience significant damage, stunted growth, and possibly death.

Warm-loving plants perform better when they are, well, warm! They will grow faster, stronger, and will produce healthier fruit. Anything you can do to keep the soil warm for these plants will equate to healthier plants and better production.

Techniques such as "soil warming" and "ambient air warming" are created by using raised beds, greenhouses, and high tunnels or hoop houses.

Harden off warm-loving plants 2 to 3 weeks prior to last frost or about 4 weeks prior to last frost if you can place them in a temperature-controlled greenhouse.

When hardening off warm-loving plants, place them outside when temps are in the mid-50’s and above. Don’t leave them outside at night unless the temperatures are expected to remain above 50 degrees.

DON'T BE IN A HURRY TO HARDEN OFF YOUR WARM-LOVING PLANTS!!! The benefit to getting your plants out earlier does not outweigh the damage they might experience if the temperatures dip too low.

So, without further ado, these are the plants that you must take the most care of when frost dangers are still present:

Warm-Loving Plant List

• Cucumbers
• Eggplants
• Peppers
• Okra
• Pumpkins
• Summer Squash
• Winter Squash
• Sweet Potato
• All Melons

Final Notes & Thoughts

Unfortunately (or fortunately - however you want to look at it), there is a bit of research, time, experience and sometimes, "failure" associated with the nuances of this subject.

And what I mean by that is, what works in one region of the world may be completely different for another region, even if they're in the same hardiness zone! So, it may come down to you asking neighbors who have gardened in your area what has worked for them and what hasn't.

Additionally, what may be true for one variety of plant (as far as "frost tolerance" goes) may not be true for another variety. 

For, example, there are cold-hardy lettuces and there are frost-sensitive lettuces. And even within the "cold-hardy" lettuce group, there are varieties that can handle the coldest of cold weather better than others. Green Harvest, North Pole, Tango, and Winter Marvel are said to handle temps as low as 5°F! 

FUN FACT! Crops with curled/wavy/textured leaves generally have more frost tolerance than those with smooth or flat leaves. This is partially due to the frost being less able to penetrate the through the textured leaves, but also due to the mini, warmer microclimate created within the leaves. This feature helps the plant to survive colder temperatures better than others. 

Amazing, huh?!

So, you really need to do your research if you're trying to push your limits on planting out BEFORE your last frost date (find your last frost date here).

Time and experience will teach you a lot, but unfortunately, sometimes as gardeners we learn from making mistakes, just like I did this year with my cool season garden. 

I didn't wait long enough and I should've spent more time hardening off my plants, EVEN THOUGH everything I read said the variety of plants I was planting would tolerate a moderate to heavy frost.

So, guess what? Moving forward, I will harden off my cool season plants, just like I do my warm season plants. Lesson learned!

MY PARTING WORDS OF WISDOM? Do your research. Learn from experience. Learn from others. Don't give up! 

We all make mistakes, so learn from them and keep going! There will ALWAYS be another garden season to grow!

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