Winter Hardiness and the Science Behind Freezing Temperatures

When it comes to plants and winter, it’s helpful to know what happens as far as plant physiology and how it can effect your plants - especially hardy annuals and biennials.

There are a few factors that determine how cold some varieties of flowers can endure.

First of all, not all hardy flowers have the same level of cold hardiness. Some, such as Icelandic poppies and dianthus and orlaya can endure temperatures down to -5 F. Others, such as scabiosa can endure temperatures down to only 15 F, while nigella endures temperatures only a bit below freezing at around 23 F (and of course there are exceptions to these temperatures, but more on that in a minute)

Let’s see what happens as we head into fall. As temperatures get close to 32 F (freezing) we notice that plants start to change their physiology. Tender annuals stop growing for the most part (and may even experience some light frost damage in the mid-thirties) and hardy annuals start getting ready for cold weather by slowing down as well.

Many plants in essence start hibernating for the cold. Not in the same sense as animals, but their growth just sort of slows to a crawl as the days get shorter and colder.

When temperatures hit 32 F, water starts changing its properties, forming crystals of ice at that stage. These crystals have the potential to puncture cells - think a needle going into a balloon - and can cause damage. Frostbite or frost burn in animals is one such example, but plants can also experience frost in a similar way.

Tender annuals like dahlias and basil will immediately turn black when getting into freezing temperatures, their stems and leaves and flowers basically collapsing inwards as each cell is punctured and releases liquid.

Hardy annuals like Icelandic poppies and orlaya on the other hand are much more hardy when it comes to the cold. Although they can be completely covered in frost and endure temperatures below freezing, they are adapted to the cold in a way that the dahlias and basil are not, and it allows their cells to pass through freezing temperatures unscathed.

Plants that are able to survive frosts and freezing and cold temperatures have adaptations that allow them to do so. If you’ve grown winter vegetables, you may know that carrots and kale get much sweeter after a frost - this is due to the plants converting their starches into sugars, which keeps the water in their cells from freezing (think putting salt on ice - the water may be very cold, even below freezing temperature but it doesn’t form those damaging ice crystals, keeping the plant’s cells intact and keeping them from turning into mush).

It’s interesting that the damage comes not necessarily from the cold itself, but by the desiccating (water-withdrawing) nature of freezing. The frost damage you see in the form of blackened or browned leaves that are left behind after a freeze almost look like they were burned or dried out - and that’s because they froze, not because they dried out. Or in the case of the dahlias and basil, the ice crystals that formed just shredded the entire plant.

In many cases, the hardy annuals and biennials we are trying to grow are relatively hardy - with the majority being hardy well below freezing. In a high tunnel or low tunnel or cold frame or covered garden bed, these flowers can survive a winter even without supplemental heat so long as you keep them from the damaging effects of freezing temperatures.

Wind or exposure does play a factor as well as far as cold temperatures. Wind is a very big factor here in terms of reducing temperatures far below the actual air temperature, making conditions very frigid - after all, as they say in the Midwest it’s not the cold but the wind that makes things unpleasant. Wind quickly will not only drop temperatures even lower but will also dry out and dedicate plants and foliage, which increases the damage that the cold can do.

Some flowers such as sweet peas are actually pretty cold hardy, but with wind or freezing rain the foliage can be damaged and can set them back or even kill them if it gets to be too cold.

If you have good microclimates set up, you can actually really help to increase your chances of your flowers overwintering well. Buildings of course can help to decrease the amount of wind exposure (which is why courtyards and walled gardens have much higher success overwintering than open fields and spaces) but even hedgerows, ornamental grasses and fences can help to shelter your plantings. And if you live in an urban area, you can also count on stored heat from concrete and buildings to keep your garden a little bit warmer during the winter as well (especially if you get a good amount of sun).

Snowfall can increase the change of survival, acting as a form of insulation if you get enough of it. In fact, places that get a thick layer of snow that lasts throughout the winter can often overwinter flowers that other places with less snow (but warmer temperatures) cannot.

Rainfall on the other hand, can effect cold as well by causing freezing rain that can damage plants in addition to lower temperatures. Freezing temperatures with rain can freeze plants solid, and if not well protected the ice that forms can turn the foliage into mush due to the damage from the ice that forms.

There’s also the fact that very wet and soggy winters and wet and soggy soil decreases the chance of your hardy annuals and biennials of surviving because they will have damage from the substantial and chronic moisture in the soil (sometimes creating anaerobic environments that are terrible for root health).

Temperature changes can causes issues as well. For example, oscillating temperatures - freezing and warming up and freezing and warming up can cause problems as well by causing plants to heave out of the soil.

Alternately, temperatures that are warm for an extended period of time can cause plants to discard their adaptations to cold and put out lush and tender growth - but if you get a polar vortex situation of cold temperatures for an extended period of time, your plants can get shocked and die off as well.

In short, the more stable you can keep the temperatures around your plants, the better off you will be. Even if they are colder temperatures, so long as you can keep them in the realm of surviving your flowers will overwinter.

There’s also the fact that smaller plants or seedlings are much more adaptable to changes in temperatures. Many times seedlings that have just one or two sets of true leaves will be able to handle temperatures that are far lower and conditions that are far more harsh than fully grown plants - I’m not sure of the physiology behind it, but I think it has to do with the fact that the smaller surface area to volume ratio of the seedlings’ structures and leaves means that they are much more adaptable to whatever Mother Nature can throw at them (older larger plants will turn to mush even if they are the same species and have grown in the same conditions and temperatures).

And remember that you are just wanting your plants to overwinter, not necessarily be growing actively throughout the winter. Survival is technically what we are looking for, not necessarily flowering at this point of time - that will come later in the spring as the days get longer and warmer.