If you’ve spent any time around gardeners, you’ve heard terms like “frost tolerant” or “warm weather crops.” This is the idiosyncratic and vague, but purposeful, language of experienced gardeners.
This post seeks to demystify those terms and also teach you which specific plants fall into which category.
What we’ve found in our subarctic gardening experience is that depth of knowledge about this subject has become one of the most important things we have learned.
The four major categories that we will discuss are cold hardy, frost tolerant, frost sensitive and warm loving plants. The subject of plant tolerance is one of the most vital things you can understand to make you a better cold climate gardener!
Why Are Some Plants More Frost Tolerant Than Others?
This is a really good question to start with! Why are there differences in temperature tolerance between plants?
Frost occurs when outdoor temperatures hover around the freezing point, or 32F/0C. It occurs when moisture (water) on the plants, in the ambient air or within the plant itself, freezes.
Frost, or freezing, can cause cellular damage to plants that don’t have sufficient protections from it. Frost can occur above actual freezing temperatures.
The generally accepted reason for some plants being more frost tolerant than others is due to having higher levels of sugars built up in their leaves. Sugar water will freeze at a lower temperature, when compared to plain water. These higher levels of sugar allow certain plants to avoid freezing and thus, they also avoid cellular damage caused by freezing.
There are two types of cold hardiness that are recognized by most gardeners. These two levels essentially describe the level of cold tolerance that the plants have.
Similarly, some plants won’t tolerate frost at all. For these, there are also two levels of tolerance. These two levels again reflect the general tolerance towards cooler temperatures of those plants.
Why Is Plant Temperature Tolerance Important To Know?
As we mentioned above, we have come to the point where we think this topic is one of the most vital to the subarctic gardener.
When you garden in warmer zones, your knowledge of what temperatures a plant can tolerate isn’t often tested.
When growing in the subarctic, you have a very short season bookended by “spring” and “fall” that are measured in weeks, not months.
- Understanding temperature tolerance helps you better understand what gardening techniques might be best for certain types of plants.
- Knowledge of the subject can help inform a phased hardening off process that can accommodate a larger garden and smaller indoor grow room.
- When your indoor seed garden is overflowing, it can tell you what plants can safely be outside and which ones can not.
- Knowledge of plant tolerance helps the gardener develop a strategy for planting a fairly large garden over several weeks.
- Knowing plant tolerance allows the gardener to strategically harvest your garden. What’s important to harvest first? What can you wait on?
- With understanding of what your plants can tolerate, the northern gardener can achieve maximum benefit of free outdoor growing energy, directly saving money on indoor plant lighting when possible.
We heavily use our knowledge of this topic every single year. Every garden we grow and harvest is deeply rooted in these concepts.
What Are Cold Hardy Garden Plants?
Cold hardy plants are the most capable of them all when it comes to cold tolerance. A mature, cold hardy plant can withstand actual freezing temperatures (32F/0C) for a period of time and even temperatures below freezing.
Cold hardy plants can easily handle light to moderate frosts without being harmed. Another way of saying this is these plants can tolerate a hard frost.
When To Harden Off Cold Hardy Vegetables
Plants which are cold hardy can often be put outside before any others.
In general, for new gardeners, a minimum temperature of about 40 degrees Fahrenheit is a good general guideline. Once outdoor low temperatures are consistently above this temperature, you can consider leaving these plants outside.
This time frame is typically going to be 4 to 6 weeks before last frost, or roughly mid to late April in the Interior of Alaska.
Of course, you need to be sure to ensure these plants are hardened off well before doing so.
You can also push these limits even further, if needed. We’ve found cold hardy seedlings to be rather resilient, adapting to temperatures even in the mid-30’s and below.
We do advise bringing your cold hardy vegetables back indoors (or into a greenhouse) in the event that a temperature dip towards freezing temperatures is expected. Young plants are still susceptible to freezing temperatures.
When To Plant Cold Hardy Vegetables
We typically plant cold hardy plants in our garden anywhere from two weeks before last frost all the way up to last frost.
For cold hardy root vegetables, these can be direct sowed up to two weeks before the last frost date.
In general, we look at the 10 day weather outlook when we get to two weeks before last frost. If there’s no predicted snowfall or unseasonable temperature dips expected, it’s usually safe to plant cold hardy plants.
Cold hardy plants have an excellent chance at surviving a frost if the weather patterns change.
When To Harvest Cold Hardy Vegetables
Cold hardy vegetables can also be the last plants you harvest in the fall, at least if they haven’t flowered or bolted yet. We are often able to harvest these plants well past first frost and up to the point where the cold really starts to set in.
This is advantageous as we are often quite busy with preservation during harvest season. These varieties are the ones you can put off just a little bit longer, if needed. Don’t wait too long, though, as they aren’t invincible!
Notes About Cold Hardy Vegetables
A few cold hardy plants can also be planted in late summer, allowing for a second harvest. These are typically very fast growers, such as spinach, lettuce and mustard greens. If you want to do this, think about planting these about 15-30 days before the first frost for best results. You can often harvest after the first few frosts and get some extra fresh late season veggies.
Carrots will actually improve in flavor if you allow them to withstand some levels of frost. It’s always a good idea to harvest these late in the season after you’ve seen a few frosts or you are in the final stages of your garden clean up.
It should be noted that with the root vegetables in this category, you can keep the plants in the ground well past your last frost date. We don’t advise keeping them in the ground all winter, but some people do so and harvest when they are needed. We advise a full harvest and storage in appropriate conditions or the use of preservation techniques.
When it comes to most lettuce-like greens on this list, you can also direct sow these in your garden up to two weeks before last frost. Some years will support even greater season extension.
Most lettuce and leafy greens will germinate in cool soils and will sprout when conditions are right for them. These direct sowed plants will also be more vigorous than indoor grown and transplanted varieties.
A List Of Common Cold Hardy Vegetables
- Brussels Sprouts
- Lettuce (most varieties)
- Mustard Greens
- Onions (Seeds, sets, bunching)
What Are Frost Tolerant Plants?
The next level of cold hardiness is simply frost tolerance. These plants are a little bit more sensitive than truly cold hardy plants. There is enough of a cold tolerance difference to classify them just a little differently in our book.
Frost tolerant plants are those that will, as described, tolerate fairly light frosts. The biggest difference between these plants and cold hardy varieties is that these don’t tolerate freezing temperatures quite as well. Some varieties can withstand short periods of freezing temperatures without trouble.
When To Harden Off Frost Tolerant Vegetables
Most of the time, we treat our cold hardy and frost tolerant plants very similarly.
Once temperatures consistently get into the 40’s, we are eager to start hardening off our frost tolerant plants.
In most years, this is 4 to 6 weeks before last frost. They can often start living outside about 3 to 4 weeks before last frost, at least in most seasons.
In general, we try to avoid having our frost sensitive plants outside when temperatures dip into the mid-to-low 30’s.
When To Plant Frost Tolerant Vegetables
Frost tolerant plants can often be treated very similarly to cold hardy plants. Meaning, they can be placed outside when temperatures are consistently above freezing, and preferably somewhat above it.
As with cold hardy plants, we recommend bringing young plants inside (or in a greenhouse) in the event of a dip into freezing temperatures.
We typically plant our frost tolerant plants in our garden a week before last frost and up to last frost.
When To Harvest Frost Tolerant Vegetables
As with cold hardy plants, these are often plants that you can hold off on harvesting until after the first frost, if needed. We try to harvest most of these at or around our first frost time, but often prioritize them more than truly cold hardy varieties.
These plants are susceptible to damage from hard frosts. So, to maximize your harvest quality, it’s a good idea to harvest them before such damage can occur.
Notes About Frost Tolerant Vegetables
While these plants are frost tolerant, be careful about exposure to freezing temperatures when the plants are young. Younger plants must still be treated with care.
With radishes, you can get some late season harvests in. Planting radishes about 3 to 4 weeks before first frost and even up to first frost can allow for some nice, quick garden snacks quite late in the season. Radishes are typically very fast growers, reaching maturity in 3-4 weeks.
Artichokes do surprisingly well within the Interior of Alaska. Artichokes are typically difficult to grow, but our northern climate supports them well. Our cool temperatures lend well to the plant’s requirement to spend a couple of weeks below 50 degrees for a good fruit set. Artichokes in sub-irrigated containers have performed quite well for us in interior Alaska.
A List Of Common Frost Tolerant Vegetables
- Bok Choy
- Chinese Cabbage
What Are Frost Sensitive Plants?
These varieties feature very little frost tolerance and could be expected to be severely injured or can die once frost hits them.
These plants do often tolerate somewhat cool temperatures, at least down to the low 40’s where frost is unlikely to occur.
When To Harden Off Frost Sensitive Vegetables
Frost tolerant plants can be brought outside on days where temperatures are expected to be in the mid-40’s and above. We definitely want to avoid any risk of freezing, so treat frost sensitive plants with more care.
While we are hardening frost sensitive plants off, we generally look for days that will be in the mid-40’s or above.
Frost sensitive plants can often tolerate temperatures into the low 40’s and sometimes even high 30’s.
When To Plant Frost Sensitive Vegetables
With frost sensitive plants, we typically wait until at least our last frost date to plant them outside. If the weather isn’t behaving, sometimes we will wait until a week or so after last frost, just to be safe.
Remember, last frost dates are an average of typical last frost dates. It’s entirely possible to get frosts after your last frost date!
For this reason, we are particularly sensitive to looking at our 10-day weather outlook around last frost for any unseasonable temperature dips. This entirely informs our decision on whether to actually plant frost sensitive plants.
When To Harvest Frost Sensitive Vegetables
When we go about our harvest strategy, these are always at the top of our list to get out of the garden as soon as possible.
While you can sometimes wait for the first frost to kill off the plant, we are aiming for harvest usually right around first frost.
We won’t often try to put frost cloth on them late in the season, mostly because growth and maturation of fruit is almost at a standstill in cool temperatures.
Notes About Frost Sensitive Vegetables
A defining characteristic of these plants is that frost sensitive plants always require warm temperature soils to germinate. The seeds of these plants will not germinate well, or at all, in cool soils.
These plants are strong candidates to be placed or planted in a greenhouse, if you have one. They will appreciate the extra warmth in most cases.
Additionally, with tomatoes, some varieties are exceptionally tolerant of cooler temperatures. While they don’t like freezing temperatures, we’ve seen a number of varieties get well into the 30’s before showing signs of cold stress. It’s important to note that this characteristic isn’t universal among tomatoes and is only found in the hardiest of tomato varieties.
A List Of Common Frost Sensitive Vegetables
- Snap Pea
- Sweet Corn
- Most herbs (Parsley is frost tolerant)
- With few exceptions, most flowers are frost sensitive. Exceptions include perennial flowers that are naturally cold hardy.
What Are Warm Loving Plants?
Some varieties of vegetables simply prefer warmer temperatures. We sometimes refer to these as “warm climate crops” as well.
Warm loving plants are usually sensitive to temperatures below about 50 degrees. Temperatures below this can injure the plant, severely reduce growth rates or sometimes even harm productivity.
Warm loving plants must be treated with the utmost care in the subarctic. Our early and late season weather conditions can be particularly ill-suited to these plants!
When To Harden Off Warm Loving Vegetables
We are never in a hurry to harden off our warm loving plants!
In most seasons, these are the last of our plants to get hardened off. We might start thinking about bringing them outside 1-2 weeks before last frost. Sometimes before, if weather conditions permit.
When we are in the process of hardening off these plants, we try to aim for days in the mid-50’s and above. We don’t leave them outside at night unless the temperatures are expected to remain above 50 degrees.
Seriously, don’t mess around with these plants. We’re telling you, cold temperatures can permanently damage them.
When To Plant Warm Loving Vegetables
As for planting warm loving vegetables, we often will wait until at least last frost to put them in our garden.
It is often beneficial to wait 1 to 2 weeks after last frost, in case potentially devastating cool temperatures settle in. With peppers specifically, we usually wait two weeks after last frost to plant them in our garden.
If you have a greenhouse, it’s often safe to put them into the greenhouse 1-2 weeks before last frost. Usually this will provide enough protection from the fairly light frosts you might see during this time of year.
It’s very important to maintain flexibility with these plants and allow them to be brought inside if conditions warrant.
It’s a good idea to have frost cloth (sometimes called remay cloth) on hand, in the event of an unseasonable temperature dip or frost. We keep enough frost cloth on hand to cover all of our warm loving plants.
When To Harvest Warm Loving Vegetables
Similar to frost sensitive plants, these are always the first to get harvested.
We try to aim our harvest for before the first frost to harvest our warm loving plants. While most of these plant types will maintain their fruit reasonably well, even after a frost, don’t wait too long as the fruit will quickly be damaged.
As with frost sensitive plants, we don’t bother putting frost cloth on these in late season as growth has virtually stopped once cooler temperatures appear.
Notes About Warm Loving Vegetables
When it comes to warm loving plants, there aren’t many “fast growers” that can get you vegetables fast. Most all warm weather plants require at least 2-3 months past the transplant date to reach full maturity.
Warm loving vegetables will often perform at their best when brought into a greenhouse. When they are warm, they will grow as fast as they can and mature their fruit and vegetables more quickly. Additionally, anything you can do to keep the soil warm for these plants will be rewarded in better production.
Squash are often recognized as warm loving plants. That’s true, and they will do better when they have great warmth. Some varieties are more temperature sensitive, whereas others could be classified as frost tolerant. All squash will produce better when offered warmth.
A List Of Common Warm Loving Vegetables
- Summer Squash
- Winter Squash
- Sweet Potato
- All Melons (Marginal subarctic crop)
Final Notes On Hardiness & Frost Tolerance
One thing that is important is that hardiness against the cold can vary due to a number of complex factors that go into this.
The most relevant aspect to cold tolerance is often the exact variety, or cultivar, that you are growing.
You might find one type of lettuce that can barely withstand a frost and others that will tolerate a hard frost. These are nuances you just have to learn.
Additionally, crops with curled/wavy/textured leaves will usually feature more frost tolerance than those with smooth or flat leaves. This partially relates to the ability for frost to penetrate the plant, but also the curls will create very small micro climates that help the plant survive colder temperatures better.
In closing, we’ll remind you that young plants don’t have nearly as much frost tolerance as their full grown varieties. It’s always important to keep an eye on the weather in the shoulder seasons! Don’t push things too fast.
We know that it’s exciting when the weather starts to warm up! If you’re a new gardener, play it safe and just plant everything around or after the last frost date.
That’s All We Wrote!
We have an ever growing list of insightful and helpful subarctic gardening articles, waiting out there for you!
- Gardening Basics
- Advanced Cold Climate Gardening Techniques
- Growing From Seed
- Family Scale Food Production
- Subarctic Growing Techniques, Methods & More…