Last updated on October 23rd, 2023 at 08:16 pm
Some plants cope wonderfully with cold, while others are extremely sensitive to low temperatures and like it comfortably warm in winter. This article is all about the terms hardy or winter hardy. You will learn which winter hardiness zones there are, which factors influence them and whether hardy plants can survive even very severe frost.
In autumn we slowly adjust to the cold season and begin to prepare the balcony and garden for the winter. Now is also the time to consider which plants can withstand the cold at all. Do you grow hardy plants, frostbite or annual species? To properly assess this, we need to consider several factors and clarify a few things in advance. For example, what is the difference between hardy and winter hardy, and what role does the location play.
Annual and perennial plants in winter
In the plant kingdom, a distinction is made between annual and perennial plants. The question of how much cold a plant can tolerate in winter is only of interest for perennial species. Annual plants die before that. For example, cucurbits such as zucchini or melons begin to wilt in late summer and die in the fall. Even a mild winter would not cause them to resprout the next spring.
Other plants, on the other hand, are actually perennials, but are cultivated as annuals in our latitudes, for example tomatoes. Therefore, these are plants that are not hardy in our country. To overwinter them in good conditions is very laborious.
Many garden plants are also perennial in our country and can survive deep sub-zero temperatures. Others need frost protection, but then they can still spend the winter outside. And then it also makes a difference whether a hardy plant grows in a container or outdoors. Let’s take a look at it one by one.
Winter hardy – what does that actually mean?
The term “hardy” indicates whether a plant can cope with cold and survive the winter outdoors. Depending on the region, however, the meaning of the term can vary greatly, because winter-hardy in Mallorca is of course something completely different than in deepest Siberia. If we now buy a hardy plant in our Central European latitudes, the term refers to the local temperatures. In the trade, however, in addition to hardy fellows, there are also those that are called “hardy”. These plants can not cope with severe frosts. They are just not quite as sensitive to the cold and – whether outdoors or in a container – definitely need protection in the winter!
The winter hardiness zones
To make the whole thing a little clearer, the regions of our earth are therefore divided into so-called winter hardiness zones. There are a total of 11 of them. In regions of zone 1 it can get below -45 degrees in winter. In zone 6, temperatures are around the -20 degree mark, while in zone 11, winter temperatures are above 4 degrees. Of course, these are only average values of how cold it gets on average at the respective locations – fluctuations always occur.
In Europe, however, there are only zones 6 to 8, which can be further differentiated:
- Zone 6a: -23.3 to -20.6 degrees
- Zone 6b: -20.5 to -17.8 degrees
- Zone 7a: -17.7 to -15 degrees
- Zone 7b: -14.9 to -12.3 degrees
- Zone 8a: -12.2 to -9.5 degrees
- Zone 8b: -9,4 to -6,7 degrees
Online you can find maps of Europe where you can see in which winter hardiness zone you live (for example in the European plant forum). A look at the map shows that winters become more severe on average from northwest to southeast. For example, parts of Bavaria are in zone 6a, while most coastal areas are in zone 8b.
So why all the effort with the different zones? Quite simply, as mentioned above, winter can strike with completely different severity in each region. So these zones help us determine whether or not the particular plant in our area will survive the winter reasonably unscathed. For example, if you read that a plant is hardy up to zone 8, it will tolerate a maximum of about -12°C. If you live in a zone 7 location, you will have to protect the plant in winter.
Temperature is not everything
While winter hardiness zones and their respective temperatures are a great guide for us, they are not set in stone. External factors, such as location or weather, have additional influence on the damage that can be caused by frost.
For example, in freezing cold with bright sunshine, plants may dry out because they cannot absorb water through the roots (because it is frozen). However, the sun’s rays cause the plant to lose water. Conversely, prolonged heavy rain in cold weather can cause waterlogging and thus root rot. Mulching provides a remedy in both cases and is thus useful for the cold-resistant plants as well.
The location is also important. If hardy plants stand freely in the area, it will definitely be colder and frostier for them than if they are in a protected niche or near a house wall. In a sheltered location, heat is better retained and strong winds are slowed down. And soil type also affects how frost affects soil and plants. Loose and permeable soil causes fewer problems, but very heavy and impermeable soil can also cause waterlogging. Finally, where you live also plays a role – urban environments are often a little milder than rural areas.
Generally, a little more caution is needed with potted plants than with those grown outdoors. Even if the plant that is in the tub is actually hardy, it can suffer frost damage. In the container, the root ball is less protected, which is why cold and frost can attack the plant from all sides. Thus, even hardy plants need frost protection around the pot.
Finally, we have compiled a few examples of hardy, winter-hardy and annual plants for you. Behind the plants you will find information about the maximum temperatures they can tolerate in winter. If the plants can grow both outdoors and in a container, the information refers to the outdoors. The soil in the container can freeze much earlier, depending on the location.
Please note: Even hardy plants look different in winter than in summer. They lose their leaves and also partially freeze completely above ground, for example, the rhubarb. The underground part of the plant then sprouts again in the spring. There are also different varieties and cultivars, so you should pay close attention to what winter hardiness claims are made for each of your plants.
Hardy plants: Minus degrees? No problem!
Frost is no problem for these plants:
- Berry bushes (blueberries, black and red currants, raspberries, blackberries, gooseberries, goji berries).
- fruit trees like apples, pears, plums, cherries, peaches, apricots and nectarines
- Herbs such as thyme, oregano, sage, true lavender, mint, parsley, chives, lemon balm
The younger and planted later in the year, the more sensitive a plant is. In their very first winter, young fruit bushes especially appreciate a little frost protection. Figs and kiwis, depending on the exact variety and their location, will need protection in the first few years. Peaches, apricots and nectarines are at risk of having their blossoms damaged by late frosts.
Winter hardy plants: Outdoors only with frost protection
Once a hardy plant is growing in a container, it should really be considered hardy only. The roots can be damaged if the soil in the pot freezes. Therefore, even hardy plants then need frost protection. However, they can still be overwintered outside.
- Olive tree: can tolerate even slight sub-zero temperatures.
- Rosemary: can remain outside only in very mild regions.
Not winter-hardy: These plants must be kept in winter quarters
- Rosemary: can only tolerate light frost and must be overwintered indoors in most regions
- Citrus trees such as lemons, limes and kumquats
- Crested lavender
Theoretically perennial, practically annual.
These plants can be overwintered under very good conditions. However, this is costly, so they are cultivated like annuals.
- Tomatoes, peppers, chilies, eggplants
- Basil and marjoram
Annual plants for one season
- True coriander
- Legumes (peas, beans)
- Cucurbits (pumpkins, zucchini, melons, cucumbers)
Some vegetables don’t flower until their second year, when they are not harvested. So you can leave some carrots, kohlrabi or beet and protect the bed from frost. The next year the plants will bloom and you can get your own seeds.