Seedlings are the immature versions of bigger plants. The seedlings of cool season plants (like lettuces) will usually do well from about 40° F (4° C) upwards. They will easily tolerate a light to moderate freeze (25° F – 32° F / -4° C – 0° C), but should not be exposed to temperatures below 25° F (-4° C). The seedlings for warm season crops will not do well at temperatures below 50° F (10° C).
Seedlings are not only the younger versions of the older plants, they are little living plants all of their own and you should always consider the temperatures they can grow at.
Some seedlings can grow well at temperatures lower than those of the bigger plants.
Spinach seedlings, for example, can grow at temperatures even up to 20 degrees lower than the adult plants.
Vegetable seedlings are generally quite hardy and can tolerate quite low temperatures.
In fact, most vegetables need a period of very low temperatures (even below 32° F (0° C) as an important part of their growth cycle.
You can think about the minimum temperature for vegetable seedlings to grow as being somewhere between 35° F – 55° F (2° C – 13° C).
Warm vs. cool season seedlings
However, vegetables are broadly divided into cool and warm season crops.
Each type of crop has an optimum temperature for growth.
Cool season vegetable crops will usually do well from about 40° F (4° C) upwards.
A light to moderate freeze (25° F – 32° F / -4° C – 0° C) will usually not harm the seedlings of cool season crops.
However, any temperature below 25° F (-4° C) is likely to cause damage to even the hardiest cool season seedlings.
Warm season crops usually require a low temperature of between 50° F and 60° F (10° C – 16° C) to grow.
This means that the seedlings for warm season crops will not do well at temperatures below 50° F (10° C).
There are, of course, hardier plants and the seedlings may tolerate lower temperatures, even a light freeze.
As a rule, though, warm season seedlings will not tolerate any form of freeze.
How cold can plants tolerate?
Surely growing plants need sun and warmth.
Well, not always.
There are many plants that tolerate very low temperatures.
On the other hand, though, there are also many plants that don’t do well when it’s very cold.
As a group, plants can tolerate a range of different temperatures, from well below freezing to very warm.
Many types of vegetables will grow and even do well in colder temperatures, usually above about 40 °F (4° C).
However, most plants are far less tolerant when the temperature drops below 32° F (0° C), since water in the plants’ cells freezes and the plants can die.
It’s possible to protect plants against the effects of freezing temperatures by covering them, particularly seedlings.
Just about every variety of plant has its own tolerance of cold.
Will frost kill plants?
Frost is frozen drops of water on plants that form when the temperature drops below freezing in the morning.
When frost forms, the water in the cells can freeze and expand, which can cause the cells to burst. Ice can also form in the tissue, preventing the plants from absorbing water, which will eventually kill the plants.
Some plants are adapted to the cold and frost will not kill them.
Frost forms on plants when the temperature drops below freezing, which is at 32° F (0° C).
As the air settles on the cool surface of a plant, it condenses and forms water droplets.
When the temperature is low enough, the water droplets freeze and frost is formed.
The freezing temperature of the frost is then effectively transferred into the plants’ cells, where the water freezes.
The cells expand as they freeze and eventually burst.
The freezing temperatures associated with frost can also affect the plants by forming ice inside the tissue itself.
This then draws water out of the cells, so they effectively dehydrate.
Without sufficient water, plants cannot photosynthesize, which means they won’t produce the energy the plants need.
At best, the plants will be damaged.
At worst, they will die.
Of course, there are plants that are adapted to the cold and won’t be killed by frost.
How do plants survive freezing?
Plants that are adapted to cold climates can survive freezing because they have proteins that protect their cells against dehydration.
It’s sorta like anti-freeze.
The proteins fill the spaces in between the cells.
It is these proteins that freeze, not the water in the cells.
In this way, the plants are protected from freezing and won’t be killed by frost.
For this reason, most plants that are not adapted to the cold don’t need to be protected from frost.
Sometimes, there can be an extended, very hot summer, followed by a dip in the temperatures.
When the temperature is low enough and approaches freezing, an unseasonal frost can strike.
This is when the plants can be damaged by the frost.
It is unusual that the plants will be killed by this type of frost, but they will be damaged.
The most lethal form of frost is black frost, which turns the leaves and stems of a plant black.
Black frost causes the plant to freeze internally and is always lethal, killing the plants.
Can plants survive freezing?
Given the huge variety of plants on Earth, there are lots of plants that can survive freezing temperatures. These are usually plants adapted to extremely low temperatures & cold climate zones. Some types of plants need the soil in which they grow to freeze, as it is part of their growth cycle.
The easy answer to this question is: Yes, plants can survive freezing.
But, it depends on the type and variety of plant.
There is as wide a range of temperatures at which plants can survive.
Of course, this reflects the huge number of types of plants there are.
Some plants are chill-resistant plants and can survive very low temperatures, but not freezing, while other plants, that are cold-hardy, can survive periods of freezing.
At the other extreme, are plants that grow in hot and humid, or warm and dry, areas and will not survive low temperatures.
These will definitely not survive temperatures of 32° F (0° C).
Most flowering summer annual plants can’t tolerate very low temperatures and are almost certain to die if they are exposed to frost or other freezing temperatures.
What plants survive freezing?
Then, there are other plants that can survive the coldest temperatures, even below 32° F (0° C).
Particularly notable here are coniferous trees, which we describe as being evergreen.
The sharp, needle-like leaves of these trees prevent water loss and the bark contains air pockets, rather than cells filled with water.
This means the stems of the trees can’t freeze easily.
Another plant that is adapted to the cold is the Rhododendron, which has waxy leaves, so they are protected from some of the effects of the extremely low temperatures.
Plants that grow from bulbs are also able to survive freezing.
In fact, extremely cold temperatures are part of the life cycle of bulbs.
Without freezing, the plants won’t produce a good show of blooms in the spring.
What temperature should you cover plants at night?
Plants should be covered at night if the temperature drops below what they can tolerate.
A good approach is to cover plants when the temperatures go below 32° F (0° C). It is usually advisable to cover seedlings if the temperatures drop below 32° F (0° C).
Different plants can tolerate different temperatures, so there is not one standard measure according to which you will know to cover your plants at night.
However, just about all plants, even those that are cold-hardy, do not tolerate winter freezes well.
A good overall strategy is to cover your plants when temperatures go below 32° F (0° C), or the freeze may harm them.
A very effective way to cover plants is to use white copoly.
You should find out as much as you can about the cold-hardiness of your plants, so you know when they may need protecting.
Some plants need the freezing, dormant period to grow well.
These include some vegetables, potatoes and the Japanese quince.
Most perennial plants aren’t damaged by very low temperatures, as they adjust to the changes in temperatures.
These types of plants won’t need to be covered at night.
There are, however, perennial plants that are sensitive to frost, such as Hostas.
You will need to cover these plants when the temperatures drop below 35° F (2° C).
Most leafy, green vegetables (such as cabbage, kale, spinach and bok choy) are cool season crops and won’t need to be covered when the temperatures drop towards freezing.
It is usually advisable to cover the seedlings, though, if the temperatures drop below 32° F (0° C).
How to protect plants from frost
The easiest way to protect plants from frost is to cover them with a frost cover or a blanket. The whole plant must be covered. Plants in pots can be moved to a sheltered area. Another way to protect plants from frost is to surround the stem with mulch.
To protect your plants from frost, it may be your instinct to cover them.
This isn’t always necessarily the best thing to do, and it’s certainly not the only way to protect them.
One method you can use is simply to move the plants indoors, or at least to a more sheltered spot, when the temperatures drop very low.
Of course, you can only do this with plants that are growing in pots that can be moved.
Plants growing outside are not protected so easily, but there are things you can do to stop the frost damaging them.
To cover your plants, don’t use plastic sheets, such as paint drop sheets or tarps.
These don’t ‘breathe’, so no moisture can escape, which means it can then freeze under the covering.
The plant may also receive too much moisture.
If you cover your plants to protect them from frost, rather use a material that allows air through.
You can get horticultural fleece, or a similar material, from your garden shop, or nursery.
Cheap ways to protect plants from frost
If you want to avoid that expense, you can also use old blankets or towels, or even newspaper to cover the plants.
Don’t put the cover on too tight, or it will effectively harm the plant.
Form the fleece into a loose, tent-like shape over the plant and weigh down the edges.
You can remove the covers during the day, if the temperatures rise sufficiently.
Another way to protect your plants against frost is to cover the ground around them with mulch.
This will keep the ground insulated and the water in the soil, while also protecting against frost.
Is 40 degrees too cold for plants?
Depending on the plant, 40 degrees can be too cold.
Or it could be just right.
The average temperatures found in tropical areas range between 77° F and 82° F (25°C – 28° C), which means that 40 degrees is too cold for them.
A lot of vegetables will grow below 40 degrees, but there are numbers of blooming plants for which 40 degrees is too low.
Fruit trees are notably hardy plants that routinely tolerate extended periods where temperatures go below 40° F (4° C).
Plants that are found growing naturally in a certain area will be adapted to the temperatures in that area.
This means that plants growing in the tropics are used to warmer, humid conditions.
Plants that are found in colder areas will be adapted to the cooler temperatures.
For example, quite a few varieties of vegetables are tolerant of temperatures below 40° F (4° C).
These include a lot of green vegetables, such as kale, lettuce, green onions and celery.
Carrots and potatoes are also tolerant of colder temperatures.
All of these types of vegetables can grow in temperatures as cold as 40° F (4° C).
There are also flowering plants that will tolerate colder temperatures and even produce flowers during the cool months.
The cold is actually part of their life cycle: they hibernate during the winter months, then when the temperatures start to rise, they begin to ‘wake up’ and will flower in spring.
Is 40 degrees too cold for tropical plants?
The average temperatures found in tropical areas range between 77° F and 82° F (25°C – 28° C), which means that these plants won’t do well at lower temperatures.
Some varieties of warmer climate plants (such as Hibiscus, Bird of Paradise and some varieties of Monstera) can tolerate temperatures down to 50° F (10° C), but any temperature below will damage them.
This means that 40° F (4° C) is definitely too cold for tropical plants.
Is 50 degrees too cold for plants?
The cold-tolerance of a plant depends on the type of plant. For tropical plants that grow in warmer climates, 50 ° F (10° C) is too cold; their growth is stunted and they may die.
Flowers also differ: 50 ° F (10° C) is usually too cold for flowers such as Marigolds, Hibiscus, Zinnias, Geranium and Petunias.
Plants that occur naturally in warmer climates, such as the tropics, are not always tolerant of lower temperatures.
Most of the popular house plants are tropical, which means they grow naturally in warm, humid environments.
These plants prefer to grow in higher temperatures, between 65° F – 75° F (18° C – 23° C), which means that 50 ° F (10° C) is too cold for these plants.
This is why plants such as Monstera, Pothos, Palms and Philodendrons are grown indoors, even in colder climates.
There are some indoor plants that can tolerate temperatures as low as 50 ° F (10° C).
These include Maidenhair Ferns, Chinese Evergreens, Clivias, Jade Plant and the Fiddle Leaf Fig.
Plants that grow outside have to tolerate specific temperatures, or they will die.
There are many varieties of trees for which 50 ° F (10° C) is not too cold.
The Live Oak, for example, can even tolerate much lower temperatures, while the simple lemon tree can even survive if the temperature drops to about 30° F (-1° C).
Flowering shrubs and plants in your garden will generally prefer warmer temperatures and grow well in the sun.
This means that 50 ° F (10° C) is usually too cold for flowers such as Marigolds, Hibiscus, Zinnias, Geranium and Petunias.
How does cold temperature affect plants?
When plants experience temperatures that are too cold for them, the water in their cells freezes and the plant can no longer absorb water.
With a lot of plants, this occurs when the temperature drops below freezing (32° F / 0° C).
With some plants, cold temperatures tend to decrease the rate at which the enzymes act in the plants and they can no longer digest the nutrients and minerals they need to grow.
Plants that are intolerant of low temperatures can become damaged if they are exposed to very cold conditions.
How a hard freeze affects plants
The water in the plants’ cells freezes and the plant can no longer absorb water.
The plant effectively suffocates and can die.
This can also occur if the temperature drops too quickly.
The plants can’t adjust to the freezing temperatures and are most likely to die.
You will find that some plants may be damaged and can survive freezing temperatures.
When the temperature drops below freezing (32° F / 0° C), plants’ water in the cells in the leaves and stems can freeze.
This causes the plant to appear to be brown and even a bit soggy.
The damage can affect the whole plant, but the fruit and flowers are most vulnerable, as they grow quite quickly and don’t have time to get used to the colder temperatures.
Plants that grow naturally in hot and humid areas will be more severely affected by very cold temperatures.
This applies to most popular houseplants, which originate in the tropics.
Cold temperatures tend to decrease the rate at which the enzymes act in the plants.
This then means they can no longer digest the nutrients and minerals they need to grow.
Ultimately, the plants’ growth is stunted, or they may die.
Another effect of very cold temperatures on plants can be that the cellular membranes become less porous.
The plants then take up fewer necessary nutrients, which will also affect their growth.
You will notice the damage to your tropical houseplants when they discolor, turn white or develop yellow spots.
Then, there are plants that seem to be unaffected by extremely low temperatures.
These include evergreen trees, leafy vegetables and flowers such as the crocus, snowdrop and primrose.