If you’ve spent any time around gardeners, you’ve heard terms like “frost tolerant” or “warm weather crops.” This post seeks to demystify those terms and also teach you which specific plants fall into which category.
The four major categories that we will discuss are cold hardy, frost tolerant, frost sensitive and warm loving plants. For the cold climate grower, these concepts are one of the most important things to understand.
Why Are Some Plants More Frost Tolerant Than Others?
This is a really good question to start with!
Frost occurs when outdoor temperatures hover around the freezing point and moisture on the plants or in the ambient air freezes. Frost can cause cellular damage to plants that don’t have sufficient protections from it.
The generally accepted reason for some plants to have more frost tolerance than others is due to having higher levels of sugars built up in their leaves. Sugar water will freeze at a lower temperature than plain water will. This allows the more frost tolerant plants to avoid actually freezing and thus 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 vegetables have. Similarly, for plants that won’t tolerate frost at all there are also two levels of tolerance. Those which are just sensitive to frost and those that really require warmer temperatures at all times.
What Are Cold Hardy Garden Plants?
Cold hardy plants are the most capable of them all when it comes to cold tolerance. A mature plant can withstand actual freezing temperatures (32F/0C) for a period of time and even temperatures below freezing. Cold hardy plants can also handle light to moderate frost without being harmed. Another way of saying this is these plants can tolerate a hard frost.
When To Plant Cold Hardy Vegetables
Plants which are cold hardy can often be planted outside before any others. Once outdoor low temperatures are consistently at or above 32F, you can consider leaving these plants outside. This is typically going to be 3-4 weeks before last frost, or roughly early May in the Interior of Alaska. Of course, you need to be sure to ensure these plants are hardened off well before doing so.
We do still advise being able to bring your cold hardy vegetables back indoors (or into a greenhouse) in the event that an temperature dip below freezing is expected. Young plants are still susceptible to freezing temperatures. We typically plant cold hardy plants in our garden anywhere from two weeks before last frost all the way up to last frost.
For the root vegetables, these can be direct sowed up to two weeks before the last frost date.
When To Harvest Cold Hardy Vegetables
Cold hardy vegetables can also be the last plants you can 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 of these 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 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. While we don’t advise keeping them in the ground all winter, 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. The best measurement is when the soil is workable and absent of ice. They will germinate in the 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 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 Plant Frost Tolerant Vegetables
Frost tolerant plants can be placed outside once temperatures are consistently above freezing, but frosts may still be prevalent. This is often a week or two before the last frost, or about mid-May in the Interior of Alaska.
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 the last frost time, but often prioritize them more than truly cold hardy varieties.
Notes About Frost Tolerant Vegetables
While these plants are frost tolerant, be careful about exposure to freezing temperatures when the plants are young. Whether you are setting the plants outside or putting them in the ground, be somewhat sensitive that these plants are not extremely cold hardy.
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. 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.
A List Of Frost Tolerant Vegetables:
- Bok Choy
- Chinese Cabbage
- Peas (Garden variety)
What Are Frost Sensitive Plants?
These varieties feature very little frost tolerance and could be expected to be severely injured or even will die once frost hits them. They do often tolerate somewhat cool temperatures, at least down to the low 40’s or high 30’s where frosts aren’t also present.
When To Plant Frost Sensitive Vegetables
With these varieties, we typically wait until at least our last frost date plant them outside. Sometimes we will wait until a week or so after last frost, just to be safe. While we are hardening them off, we look for days that will be in the 40’s or above. These are also varieties that we will try to protect with frost cloth in the event a late frost comes up in the early season.
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 some degree of warm temperature soil to germinate. The seeds of these plants will not germinate well, or at all, in cold or even 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 usually only found in the hardiest of varieties.
A List Of 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?
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.
When To Plant Warm Loving Vegetables
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 maintain above 50 degrees. As for planting, we often will wait until at least last frost to put them in our garden.
With peppers specifically, we wait until almost a week or two after last frost to plant them in our garden. It’s very important to maintain flexibility with these plants and allow them to be brought inside if conditions warrant.
When To Harvest Warm Loving Vegetables
As for harvesting, as with frost sensitive plants, these are always the first to get harvested. We try to aim for before or right after at the first frost to harvest them. While most of these 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
As you might imagine, 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.
These varieties 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.
At least here in the subarctic, we find that we have to finesse our peppers into full maturity. While we often have some peppers that are fully ripe at or before harvest time, many are not. We’ve found it to be a good practice to return peppers into our indoor grow room for full maturation. This process usually happens within days upon bringing them inside, radically improving our harvest rates.
A List Of Warm Loving Vegetables:
- Summer Squash
- Winter Squash
- Sweet Potato
- All Melons (Marginal subarctic crop)
Final Notes On Hardiness & Frost Tolerance
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.
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 early season and 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 the last frost date.