What to do with frozen plants

It's easy to prepare for the changing seasons. I think we all have more or less an idea of when to take out the coats and blankets, and move the plants indoors among other preparations; but one of the consequences of global warming that we are experiencing is sudden changes in temperature. In the middle of spring, when we are already putting away the sweaters and getting ready for the warmth of summer, we can receive a sudden frost that affects the plants. Or worse, you have plants that are theoretically prepared to withstand the winter in your region, but it got colder than usual.

Whether it is because of global warming, because you forgot to save them for the cold season or because the temperatures dropped much lower than expected and damaged the outdoor plants, those plants affected by the cold can be saved; it is a matter of assessing the damage and learning the first aid to be able to recover them. For this reason we share these tips so that you know what to do with frost-affected plants and everything you need to know to protect them.

what effects does frost have on plants?

Technically speaking, frost occurs when there is a drop in the ambient temperature and the temperature at ground level falls below 0°C, i.e. below the freezing point of water. It is precisely this detail that starts to wreak havoc, as it causes the ambient water vapour and water to freeze.

As temperatures begin to drop and the hardiness threshold of the plant is reached, biological and metabolic functions are disrupted. This happens because ice crystals form in the protoplasm of the cell, in simple terms: the liquid part of the cells freezes, causing them to rupture and destroy; this is called intracellular frost.

But this is not the only damage that plants can suffer, as ice can also form inside the plant but outside the cell. This is known as extracellular frost and is the most damaging to the plant, because when it thaws it ends up dehydrating the plant. But not all frosts are the same. In fact, depending on the environmental conditions, we can predict how much damage will be caused to the plants. That said, we could differentiate frosts as follows:

White frosts

They are characterised by low temperatures, but high humidity in the environment. This causes frost to form on the plants, giving them a whitish appearance. Although they look terrible, they are not so serious because the frost protects the internal structures of the plant and prevents intracellular freezing.

Black frost

Silent and lethal, they are very damaging to plants. humidity levels are low, the air is very dry so no frost or ice forms. The cold directly attacks the inner tissue of the plant, leaving it black.

Advection frosts

These are caused by the irruption of large masses of cold air, these are those cold waves that arrive suddenly and destroy plants, especially because they are accompanied by a lot of wind.

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Radiation frost

They are caused by the cooling of the ground, which gives up its heat to the atmosphere during the night and is accompanied by cold air. They usually occur in spring and are terrible because of the lack of humidity and wind.

Evaporation frosts

When the water covering the plants evaporates, the internal temperature of the plants drops exaggeratedly. It usually occurs at dawn, when the dew evaporates. This frost affects flowers, new leaves and fruits that are just growing.

Just as not all frosts are the same, not all plants are equally sensitive. Each species, depending on its place of origin, has a temperature limit that it can withstand. We call this characteristic hardiness .

The hardiness of plants

When we talk about hardiness we refer to the ability of a certain plant species to survive conditions that make it difficult to grow, i.e. it is a number that indicates how much of a warrior that particular plant is. Normally, hardiness indicates resistance to cold, heat, wind, drought, among other variables but is almost always limited to reporting a temperature range for the plant.

Hardiness is indicated by a number, which shows a range of the hardiness zone and should serve as an indicator of whether a certain plant will be able to grow successfully in a certain region. The truth is that this number is a very relative value, not only because each country or region has developed its own scale, but also because it does not take into account other environmental factors. It seems practical, but the truth is that this number cannot be taken seriously, but only as a guide.

The problem with relying too much on hardiness indicators, either the number or the temperature references, is that not all frost conditions are the same. For example, if an alpine plant that survives happily in extreme temperatures is subjected to the same temperatures but in a region with higher humidity, the plant will die irretrievably.

Other important factors are related to the type of substrate, as a clay soil retains too much moisture in winter and can choke and kill a plant, regardless of whether it can withstand low temperatures, because of poor drainage. This would be solved with a looser, better-drained substrate. Other factors such as micro-climates formed by the location will also affect the survival of the plant.

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My advice is not to take this information at face value. You can use it as a guide when choosing plants, but don't blindly trust it when it comes to the lowest temperatures they can withstand, as this can result in a few plant deaths that could have been avoided with a little care. But if the worst has already happened, don't despair, we can apply first aid.

How to recover a plant after a frost

Take a deep breath and assess the situation. I know that the first impulse is to take scissors and prune everything, but sometimes it is better to leave the leaves dry because they will protect the plant from the cold and future frosts. If it is an outdoor plant, it is better not to cut it back; if it is an indoor plant and you are taking it to a more sheltered place you can think about it. Let's see how to recover plants after a frost according to their type.

Annual plants

These are plants which, as their name suggests, have an annual life cycle, i.e. they are destined to die in medium and hard frosts. All you have to do is remove them and say goodbye to them. Some annuals can behave as perennials as long as they are kept in a warm climate, so if you didn't remember to save them before the cold season, you will have to say goodbye.

Perennial plants

Some herbaceous or perennial perennials, i.e. plants with thin green stems, can lose their tops (leaves and stems) in the cold but may resprout again in spring. Of course, this depends on the hardiness of the plant and its tolerance to low temperatures, so it is necessary to consult these limits for each species. Assuming that they have a certain hardiness, the important thing is to keep the bulb and/or root of the plant protected. This can be done by mulching or mulching before the winter season, or even covering the plant with a thermal blanket. This is especially useful with outdoor or very large plants, which you cannot keep indoors. If you feel that the frost-damaged plant looks too ugly, you can cut it back to size. This will encourage it to grow again in spring.

Woody plants

Shrubs, trees and woody-stemmed plants that are not native to continental climates but come from tropical or semi-tropical locations can die if the frost is too severe. It all depends on how hardy the plant is and how much damage it has suffered. If you can move it indoors, you can prune the affected parts and apply first aid for its recovery. In case you cannot move it, protect it with a thermal blanket and pray that it will last the season. In any case, the plant will be in a resting period and you can wait until spring to prune it.

First aid

Let's say your plant has been left outside and due to some crazy weather conditions a frost has hit it. Let's apply first aid to get your plant back. First of all, you should shelter it indoors and place it in the ideal spot for it, where there is light but not direct sunlight. Your plant will be dehydrated, but don't start watering it. It is essential that it is in a place with a temperature that allows it to warm up its roots, between 15 and 18° C. Wait a couple of hours and water it with lukewarm water, until you see the water coming out of the drainage holes.

Make sure the environment has an adequate humidity level. You can put it near a humidifier, with other plants and even spray it a little the next day. Remember to avoid pruning the affected parts, unless you see that they are mouldy. Be patient, as only time will heal its wounds. Observe your plant daily. If you see that the trunk remains firm and upright, it will recover, but if it becomes soft and the plant starts to droop, it is possible that it has been badly affected. In any case, give it time and space to recover. Don't overwater, wait for the soil to dry out before watering and forget about repotting for at least a year.

Prevention is the key

Before the winter lashes your plants mercilessly, you can take a few simple steps to protect them from the cold temperatures. This is particularly important for those outdoor plants that have to withstand frost or those that are too big and heavy to move. Even if they are hardy plants that tolerate low temperatures well, a little love and care will make them even stronger and more beautiful in the spring. Pay attention, because the care is very simple:

- Reduce watering. Low temperatures maintain humidity levels for longer, but this is not the only reason to reduce the frequency and quantity of watering, as the water can freeze inside the substrate, affecting the roots. For this reason you should also check drainage, as this will prevent the plant from retaining excess moisture.

- Keep your plants warm. Just as you wear a coat to go outside when it is cold, you can improvise shelters for your outdoor plants. Use bubble wrap (yes, the kind used to pack delicate objects) to protect and insulate, and you will be giving it a second use. You can also use plastic, as long as it is transparent to allow light to pass through, although it is advisable to put it up only during the night, supported by stakes or similar so that its weight does not hurt leaves and branches.

- There is strength in unity. Group your plants together so that you can cover them more easily, it will also be easier to keep an eye on them and they will maintain their humidity levels better, as well as offering greater resistance to winds.

- Protect the substrate. Use mulch such as pine bark, dry leaves or straw around outdoor plants, covering the base and the exposed substrate. This will insulate the substrate, preventing heat loss and freezing of the roots, as well as allowing them to continue to absorb water and nourish themselves normally.

- Protect them from the wind. Place the pots against a wall or protected by a surface or taller trees that cut off the wind. This will prevent the pots from tipping over, breaking branches or blowing leaves in freezing winds. You can also place stones or some weight to make them a little heavier and more stable.