Lowest Temperature Basil Plants Can Tolerate (How Cold?)


Are you worried about your basil plants being out in the cold overnight?  If so, you are probably wondering just how much cold they can take, and how you can protect them.

So, what is the lowest temperature basil plants can tolerate?  Basil plants can tolerate temperatures down to 50 degrees Fahrenheit (10 degrees Celsius).  Basil is frost sensitive, so temperatures of 32 degrees Fahrenheit (0 degrees Celsius) or below will damage the plant.  Any temperature below 50 degrees Fahrenheit can harm the plant or turn the leaves black.

Luckily, there are some ways to protect basil plants from cold.  We’ll get into that, but we’ll start with the temperatures that basil can tolerate and how they grow best.

Let’s get going.

What is the Lowest Temperature Basil Plants Can Tolerate?

According to the University of Illinois Extension, basil plants can tolerate a temperature down to 50 degrees Fahrenheit (10 degrees Celsius).  Any lower than that, and the plant will start to suffer.

basil
Basil can tolerate temperatures down to 50 degrees Fahrenheit (10 degrees Celsius), but any colder than that and you will start to have problems.

Any temperature below 50 degrees Fahrenheit will damage basil.  One sign of damage is black leaves on the plant.

Basil is frost sensitive, so temperatures of 32 degrees Fahrenheit (0 degrees Celsius) or below can mean the end for a basil plant.

Most basil varieties are annual, meaning that they only survive for one year.  In general, they grow until fall frost arrives.

However, an early spring frost can put an end to your basil plants if you don’t prepare for it.  Let’s look into the ways you can do that now.

-How to Protect Basil from Cold and Frost

There are many ways to protect basil plants from cold and frost.  One of the best ways to do it is to choose the right varieties, so let’s start there.

Choose Fast Maturing Basil Varieties

If you choose the right basil varieties, you can avoid danger of cold and frost to the plants.  With faster-maturing varieties, you can:

  • plant later in the season to avoid a late spring frost
  • get a harvest before a fall frost threatens your plants
frosted leaf
A fast-maturing basil variety will help you to avoid cold and frost damage to your plants.

Many basil plants can take 75 to 80 days to mature.  However, there are some varieties that only take 60 to 65 days to mature.

By choosing some of these varieties, you can save yourself 10 to 20 days (1.5 to 3 weeks) of growing time.  This makes a big difference in a cold climate with a short growing season.

Here are some of the fast-maturing basil varieties I found at Johnny’s Selected Seeds:

  • Lime Basil – this basil variety adds a nice citrus flavor to your dishes.  The plants are compact at only 16 to 20 inches (40 to 50 centimeters) tall, with 2 inch (5 centimeter) leaves.  Maturing in only 60 days, lime basil is a great choice if you want to harvest fast!  You can find Lime Basil on Johnny’s Selected Seeds.
  • Mrs. Burns’ Lemon Basil – this basil variety is sweet and tangy.  The plants grow to a height of 20 to 24 inches (50 to 60 centimeters) tall, with 2.5 inch (6.3 centimeter) leaves.  The plant has white flowers, and it matures in only 60 days, making it another choice if you want to pick basil leaves sooner.  You can find Mrs. Burns’ Lemon Basil on Johnny’s Selected Seeds.
  • Kapoor Tulsi (Holy Basil) – this basil variety is spicy with coffee and chocolate flavors.  The flowers are purple, and the plant matures in only 60 days, making it yet another great choice for a fast harvest.  You can find Kapoor Tulsi (Holy Basil) on Johnny’s Selected Seeds.
  • Sweet Thai Basil – this basil variety grows as a compact plant, at a height of 16 to 20 inches (40 to 50 centimeters) with 2 inch (5 centimeter) leaves.  The flowers are purple, and they are edible!  This basil matures in only 64 days, giving you a harvest in just over 9 weeks.  You can find Sweet Thai Basil on Johnny’s Selected Seeds.
  • Cinnamon Basil – this basil variety grows very tall, at a height of 26 to 30 inches (65 to 75 centimeters) with 2 inch (5 centimeter) leaves.  The stems are violet, and the flowers are lavender (and edible!).  This basil matures in 65 days, meaning you get a harvest much sooner than other varieties.  You can find Cinnamon Basil on Johnny’s Selected Seeds.

Start Seeds Indoors

It is important to plan ahead when planting basil seeds (or any seeds for that matter).  You can plant seeds directly in the garden, but you will have better results with transplants.

This means starting your seeds indoors, and transplanting them outside after they mature a bit.  The Washington State University Extension suggests starting basil seeds indoors 6 weeks before the last frost date.

To find the last frost date for your area, enter your zip code into this tool from the Old Farmer’s Almanac.

For example, in Avon, Massachusetts (zip code 02322), the last frost date is May 8.  Working backwards 6 weeks (42 days), I get a date of March 27.

So, I would start my basil seeds indoors on March 27.  This would give the plants 4 days in March, 30 days in April, and 8 days in May (42 days or 6 weeks total) to grow before I transplant them outside.

When you plant your basil seeds, bury them only 0.25 inches (0.6 centimeters) deep.  Basil seeds need light for germination, so keep them in an area where they will get enough light (a windowsill or under grow lights).

According to Clemson University, basil seeds germinate best at a soil temperature of 70 degrees Fahrenheit (21 degrees Celsius).  At this temperature and with proper soil moisture, basil seeds will germinate in 5 to 10 days.

When the basil plants each have 2 sets of true leaves (not counting the first leaves that emerge from the seed), repot them.

basil and parsley in pots
If you don’t want to grow basil from seed, you can buy established plants in pots from Bonnie and other suppliers.

This will give each plant more space to grow.  It will also prevent the roots of nearby seedlings from getting tangled up with each other.

You can either grow the basil in pots, or transplant them right into the soil outdoors.  Just be sure to watch the weather forecast before you transplant them!

Watch the Weather Forecasts

Before you transplant your basil plants outdoors, make sure the weather will cooperate.  Usually, you should not transplant outdoors until after all danger of frost has passed (last frost date or later).

Basil grows best when daytime temperatures are in the 70s Fahrenheit, and when nighttime temperatures are over 50 degrees Fahrenheit.

It is a good idea to “harden off” the plants to get them used to the outdoors gradually.  If you want to learn more about how to harden off plants, check out my article all about it here.

If you leave the plants in pots for the first couple of weeks outdoors, it makes it easier to harden them off.  You can also bring the pots back indoors for a night or two if temperatures threaten to go below 50 degrees Fahrenheit (10 degrees Celsius).

Select the Best Location

When planting basil outside, be sure to choose the best location possible.  Basil prefers full sunlight, meaning 8 or more hours of sun exposure per day.

In addition to helping the plant grow better, more sunlight will also help to warm up the soil.  This protects basil against cooler nighttime temperatures.

sunlight through forest
Choose a spot with lots of sunlight for your basil. This will keep the soil warmer during the season.

Basil prefers soil that drains well.  If your soil is clay, you may need to add compost to improve drainage.  You can learn more about how to make soil drain better in my article here.

When you transplant basil into the garden, leave 4 to 8 inches (10 to 20 centimeters) between plants.  This reduces competition between plants and prevents the spread of disease.

Mulch after Planting

After you plant your basil in the right location, it’s time to do some mulching.  According to the North Carolina State University Extension, mulching is good for basil plants because it:

  • Retains soil moisture by preventing evaporation due to sun and dry air
  • Slows weed growth by smothering existing weeds and preventing new ones
  • Prevents disease by keeping soil off of plant foliage

Mulch also helps soil to warm up faster in the spring, especially if you use black plastic mulch.  However, you can use lots of materials as mulch, including:

  • Wood chips
  • Grass clippings
  • Leaves
mulch
You can use wood ships, grass clippings, or leaves as mulch to insulate the soil and keep basil plants warmer.

Just be sure not to use too much mulch at once.  A thin layer, about 2 to 4 inches (5 to 10 centimeters) thick, will usually do it.

Keep in mind that it is possible to hurt your plants with too much mulch.  To learn more, you can read my article about over mulching plants here.

Use a Cloche, Cold Frame, or Greenhouse

If your basil is already planted, there are still ways to protect it from cold and frost.  One of the best ways is to use some kind of shelter to protect them.

Cloche

A cloche is a cover that protects plants from cold, wind, and pests.  A cloche is often made of plastic, although historically they were bell-shaped and made of glass.

You can make your own cloche from a clear plastic bottle by cutting out the bottom.  Then, put the bottle over your basil plant to warm up the air and soil inside.

water bottles
Cut off the bottom of a clear plastic bottle and you have yourself a cloche!

You can even use the cap on top of the bottle as a vent to cool the inside of the cloche on a hot day.  You can learn more about cloches (and how to buy or make them) in my article here.

You can also wrap a blanket or towel around the outside of a cloche to provide additional cold protection.

The downside of a cloche is that it can only keep one plant warm.  You will need a cloche for each plant you want to protect.

Cold Frame

A cold frame is a short structure that keeps multiple plants warm at once.  It is close to the ground, and you can open it during hot weather and close it at night to keep plants comfortable.

A cold frame operates on the same principle as a greenhouse.  However, a cold frame is much smaller than a greenhouse.

A cold frame won’t hold taller plants or trees like a greenhouse would.  However, basil would fit perfectly in a cold frame if you want to provide them with extra cold protection.

Greenhouse

A greenhouse is a structure that keeps many plants warm at once.  It is tall enough to walk into, and takes advantage of the greenhouse effect (sunlight heats up air and soil, heat energy gets trapped under the plastic or glass) to keep the inside warm.

greenhouse
A greenhouse will keep basil and other plants warmer throughout the growing season.

You can buy a greenhouse kit, build your own from scratch, or have someone build it for you.  A greenhouse can keep lots of plants warm, but keep in mind that it can get expensive to build one.

Conclusion

Now you know the lowest temperature basil plants can tolerate: 50 degrees Fahrenheit (10 degrees Celsius) is about as cold as you want them to get.

You also know how to protect your basil plants from frost and how to prepare them for cold nights.

I hope you found this article helpful.  If so, please share it with someone who can use the information.

If you want to read some of my most popular posts, check out the “Best of GreenUpSide” page here.  Enjoy!

~Jonathon

jonathon.david.madore

Hi, I'm Jon. Let's solve your gardening problems, spend more time growing, and get the best harvest every year!

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