Lowest Temperature Rosemary Can Tolerate (5 Ways To Protect)


When winter weather arrives and frost threatens, it is time to think about protecting rosemary plants. Rosemary is sensitive to cold, but the question is: how sensitive?

So, what is the lowest temperature rosemary plants can tolerate?  A temperature of 10 degrees Fahrenheit (-12 Celsius) or below will kill many rosemary plants. Some varieties (such as Arp) are cold hardy and can survive temperatures as low as -10 degrees Fahrenheit (-23 Celsius). Rosemary seeds germinate best at 60 to 70 degrees Fahrenheit (16 to 21 Celsius).

Remember that rosemary plants come from the Mediterranean region, which has a mild climate.  While you are waiting for those warmer days to arrive, there are some steps you can take to protect your rosemary plants from the cold.

In this article, we’ll talk about the temperatures that various rosemary plants can tolerate. We’ll also look at cold-tolerant rosemary varieties and how to protect your plants from frost.

Let’s get started.

What Is The Lowest Temperature Rosemary Can Tolerate?

Rosemary is an evergreen shrub with fragrant green leaves and white, pink, purple, or blue flowers that is native to the Mediterranean region.  As such, it is sensitive to extreme cold, which it does not experience in its native climate.

Rosemary blue flowers
Rosemary is a perennial evergreen shrub with fragrant green leaves that are often used in cooking. The blue flowers bloom in spring or summer.

If you live in an area where rosemary is hardy (or if you chose a cold-tolerant variety), you can leave the plant outdoors all winter.  However, if the winters get cold enough, you will need to bring your rosemary plant indoors (growing in a container, such as a pot or grow bag, will make this easier).

To be safe, you can wait until a week or two after the last frost date in your area.  You can use this resource from the Old Farmer’s Almanac to find frost dates in your location.

frost
Rosemary can tolerate light frost, but it will not do well at colder temperatures unless it is a cold hardy variety.

For example, if you live in Boston, MA, your last frost date in 2022 is April 8.  Waiting until 2 weeks (14 days) after the last frost date would mean putting your rosemary outside on April 22 * + 14 = 22).

The table below summarizes various USDA Plant Hardiness Zones and temperature ranges (to find your plant’s hardiness zone, check the plant label when you buy it, or look up the variety and its cold hardiness online):

USDA Plant
Hardiness
Zone
Minimum
Temperature
Range
6-10 to 0 F
(-23 to -18 C)
70 to 10 F
(-18 to -12 C)
810 to 20 F
(-12 to 7 C)
920 to 30 F
(-7 to -1 C)
This table shows some USDA
Hardiness Zones and the minimum
temperatures for those zones.

Most rosemary varieties are hardy in zone 9, meaning they may survive temperatures as low as 20 degrees Fahrenheit (-7 degrees Celsius).

Some varieties are hardy to zone 6, 7, or 8 which means they can tolerate even lower temperatures (more on some specific varieties later).

According to the Michigan State University Extension, rosemary prefers daytime temperatures of 60 to 65 degrees Fahrenheit (16 to 18 degrees Celsius) and nighttime temperatures of 40 to 50 degrees Fahrenheit (4 to 10 degrees Celsius) during winter.

To keep rosemary warmer, you can use cloches to protect individual plants, or row covers to protect an entire row (more on this later).

The table below summarizes temperature ranges and hazards for rosemary plants (keep in mind that cold tolerance varies with your variety and its minimum cold hardiness zone).

Temperature
Range
Effect On
Rosemary
Below -10 F
(Below -23 C)
death for most
rosemary varieties
-10 to 0 F
(-23 to -18 C)
death for rosemary
not hardy to zone 6
0 to 10 F
(-18 to -12 C)
death for rosemary
not hardy to zone 7
10 to 20 F
(-12 to -7 C)
death for rosemary
not hardy to zone 8
20 to 30 F
(-7 to -1 C)
most rosemary can
tolerate this range
30 to 40 F
(-1 to 4 C)
cold but tolerable
night temperature
40 to 50 F
(4 to 10 C)
ideal nighttime
winter temperature
50 to 60 F
(10 to 16 C)
cold but tolerable
daytime temperature
60 to 65 F
(16 to 18 C)
ideal daytime
winter temperature
65 to 75 F
(18 to 24 C)
ideal temperature for
seed germination
75 to 80 F
(24 to 27 C)
upper end of ideal
daytime temperature
range
over 80 F
(over 27 C)
above ideal daytime
temperature range
This table summarizes temperature ranges
and the effect on rosemary plants.

Can Rosemary Survive A Freeze In Winter?

Most rosemary varieties can survive a light frost or freeze in winter.  However, a severe freeze will damage many rosemary plants.

frost on grass
Most types of osemary can survive a light frost, but colder temperatures may be a problem without protection (or a cold hardy variety).

For example, a temperature of 30 degrees Fahrenheit (-1 Celsius) is no problem for most rosemary varieties.  However, a temperature of 5 degrees Fahrenheit  (-15 Celsius) will kill most rosemary plants (except cold hardy varieties).

Which Rosemary Is The Most Cold Hardy? (Cold Tolerant Rosemary Plants)

One way to help protect your rosemary plants from cold is to choose the right varieties in the first place.

rosemary plant
If you choose cold hardy rosemary varieties, they can survive much harsher winter conditions.

There are several rosemary varieties that can survive temperatures down to 0 degrees Fahrenheit or even negative temperatures (-18 C or below).  Some of these varieties include:

  • Alcade Cold Hardy Rosemary (Zone 6 to 9)
  • Arp Rosemary (Zone 6 to 10)
  • Athens Blue Rosemary (Zones 6 to 10)
  • Hill Hardy Rosemary (Zones 6 to 11)
  • Salem Rosemary (Zones 7 to 9)

Let’s take a closer look at each of these rosemary varieties and find out just how cold hardy they are.

Alcade Cold Hardy Rosemary

This rosemary variety grows upright, reaching a height of 30 to 36 inches and a width of 24 to 36 inches.  It has olive green leaves and pale blue flowers that bloom in early summer to fall.

It resists drought and is hardy to USDA Zones 6 to 9, meaning it can tolerate temperatures down to -10 degrees Fahrenheit (-23 degrees Celsius).  In zone 6, it should be planted in spring or early summer at the latest, to give the plant time to get established before winter.

If you live north of zone 6, you can keep Alcade as an indoor plant for the coldest part of the year.

You can learn more about Alcade Cold Hardy Rosemary from High Country Gardens.

Arp Rosemary

This rosemary variety grows upright, reaching a height of up to 36 inches and a width of up to 36 inches.  It has pale blue flowers that bloom in late spring to early summer.

It resists drought and is hardy to USDA Zones 6 to 10, meaning it can tolerate temperatures down to -10 degrees Fahrenheit (-23 degrees Celsius).  In zone 6, it should be planted in spring or early summer at the latest, to give the plant time to get established before winter.

If you live north of zone 6, you can keep Arp as an indoor plant for the coldest part of the year.

You can learn more about Arp Rosemary from High Country Gardens.

Athens Blue Rosemary

This rosemary variety grows upright, reaching a height of 48 to 60 inches and a width of 24 to 36 inches.  It has blue flowers that bloom in spring.

It resists drought and is hardy to USDA Zones 6 to 10, meaning it can tolerate temperatures down to -10 degrees Fahrenheit (-23 degrees Celsius).  In zone 6, it should be planted in spring or early summer at the latest, to give the plant time to get established before winter.

If you live north of zone 6, you can keep Athens Blue as an indoor plant for the coldest part of the year.

You can learn more about Athens Blue Rosemary from Pixies Gardens.

Hill Hardy Rosemary

This rosemary variety grows upright, reaching a height of up to 48 inches and a width of up to 24 inches.  It has light blue flowers that bloom in spring.

It resists drought and is hardy to USDA Zones 6 to 11, meaning it can tolerate temperatures down to -10 degrees Fahrenheit (-23 degrees Celsius).  In zone 6, it should be planted in spring or early summer at the latest, to give the plant time to get established before winter.

If you live north of zone 6, you can keep Hill Hardy as an indoor plant for the coldest part of the year.

You can learn more about Hill Hardy Rosemary from the Growers’ Exchange.

Salem Rosemary

This hybrid rosemary variety grows upright, reaching a height of up to 36 inches and a width of 16 to 24 inches.

It resists drought and is hardy to USDA Zones 7 to 9, meaning it can tolerate temperatures down to 0 degrees Fahrenheit (-18 degrees Celsius).  In zone 7, it should be planted in spring or early summer at the latest, to give the plant time to get established before winter.

If you live north of zone 7, you can keep Salem as an indoor plant for the coldest part of the year.

You can learn more about Salem Rosemary from the Plant Addicts.

Rosemary Seed Germination Temperature

According to the University of Massachusetts Amherst Extension, rosemary seeds will germinate best in soil at 60 degrees Fahrenheit (16 degrees Celsius).  Just to be safe, you should start rosemary seeds indoors to keep them warm enough for germination.

(You can learn more about how to keep seeds warm here).

Rosemary Planting Schedule

To avoid cold temperatures and frost, it is important to plan ahead.  That way, you can plant rosemary seeds, transplant outdoors, or bring containers indoors at the right times.

Rosemary
Plant rosemary at the right time and you will be rewarded with brilliant blue flowers and fragrant leaves!

When To Plant Rosemary Seeds

According to the University of Illinois Extension, rosemary is best propagated from stem cuttings, rather than from seed.

However, if you want to start rosemary from seed, keep the following in mind:

rosemary seeds
Rosemary seeds take a long time to germinate, have a low germination rate, and the plants take a long time to grow.

If you are not deterred, then Johnny’s Selected Seeds suggests planting rosemary seeds in trays 10 to 12 weeks before the last frost date in your area.

For example, in Boston (last frost date of April 8), you would work backwards 10 to 12 weeks (70 to 84 days).  This would mean starting rosemary seeds between January 14th and January 28th (3 to 17 days in January + 28 days in February + 31 days in March + 8 days in April = 70 to 84 days total).

Cover the seeds lightly with very little soil, since rosemary seeds need light to germinate!

soil
Rosemary seeds need light to grow, so cover them with only a little soil.

The soil for your rosemary should be damp, but not soaking wet.

After the seedlings emerge, wait until they have 4 true leaves to transplant into individual pots.

When To Transplant Rosemary

Johnny’s Selected Seeds suggests transplanting rosemary outdoors after the last frost date in your area.

rosemary transplants
Transplant rosemary outdoors after the last frost date.

The rows should be 18 to 36 inches apart.  For individual plants:

  • Space plants 8 to 10 inches apart within a row for annual plantings.
  • Space plants 24 to 36 inches apart within a row for perennial plantings.

When To Bring Rosemary Indoors

Most rosemary can tolerate temperatures slightly below freezing (32 Fahrenheit or 0 Celsius).  So, if you bring in your rosemary around the first fall frost date in your area, you should be fine.

clay pots
Grow rosemary in pots that will be easy to move indoors for winter.

If you want to leave your rosemary outdoors as long as possible, just keep an eye on the weather forecast.  Find out the cold hardiness zone of your rosemary variety, along with the coldest temperature it can tolerate.

For example, Salem rosemary is cold hardy to zone 7 (down to 0 degrees Fahrenheit).  So, keep an eye out for temperatures that get close to 0 Fahrenheit, and bring your plants indoors before then.

How To Protect Rosemary From Cold & Frost

There are many ways to protect your rosemary plants from cold and frost. Let’s start with cold frames and go from there.

Use A Cold Frame For Rosemary Seedlings

Sometimes, you would like to transplant your rosemary seedlings outside a little earlier in the season than recommended.  (Of course, there are other times when your spouse yells at you for having too many plants in the house!)

cold frame
Use a cold frame to help keep newly transplanted rosemary warm in spring.

In any case, a cold frame keeps your plants warm after you move them outdoors. A cold frame is a short wooden structure with a glass (or plastic) top, kind of like a miniature greenhouse.

Some cold frames open automatically when it gets too hot inside.  They close again when it cools down, thus “self-regulating” their internal temperature to some extent.

For example, this vent opener from Univent will respond to temperature changes. It opens a vent in the cold frame automatically to keep plants from overheating.

Once your rosemary seedlings are mature and ready for transplant, you can put them right into a cold frame.  To see if your cold frame is warm enough for rosemary transplants, put a thermometer inside and monitor it every day.

A max/min thermometer (such as this one from A.M. Leonard) will help you to get an idea of the coldest and warmest temperatures in the cold frame over any time period.

Keep an eye on the weather forecast and be ready for frost or unseasonably cold weather!

Use Black Plastic To Warm Up The Soil

If the days are cool but sunny, you can use sunlight to your advantage and warm the soil.  Get a length of black plastic and lay it down over the soil where you want to plant your rosemary.

The black plastic is dark, so it absorbs most of the sunlight that hits it.  This in turn warms up the air and soil beneath the plastic.

daylight
Black plastic will absorb sunlight and trap heat in the air and soil underneath.

Eventually, the greenhouse effect will prevent the heat from leaving and keep things warm for transplants.  When the soil is warm enough, cut planting holes in the plastic (a trowel or scissors work well for this), and put your rosemary transplants into the holes.

Remember that plastic is a water barrier, so you should water the soil before putting the black plastic in place.

Use Cloches For Young Rosemary Plants

A cloche is a simple and cost-effective method to keep young rosemary plants warm on cold days.  A cloche is just a cover used to protect plants from cold and wind (as well as pests).

Originally, a cloche was a bell-shaped glass cover to put over plants.  Now, a cloche is usually made of plastic, making it cheaper, lighter, and less likely to shatter.

glass cloche
This is what a cloche used to look like!

You can also use a wire cloche to protect plants from rabbits and other animal pests.  However, a wire cloche by itself won’t keep the plants warm – for that, you will need to drape plastic or row cover material over it.

If you want to make your own cloche, simply take empty clear gallon containers of water or milk.  Then, cut out the bottom, and put the container over your plants.

water jugs
You can use water or milk containers with the bottoms cut out as cloches.

On hot days, take off the cap on top of the gallon jug to let your plants breathe.  After all, the last thing you want is to kill them with heat after protecting them from the cold!

The one drawback of using a cloche is that your rosemary plant will eventually outgrow it.  However, a cloche is a great method for keeping young plants warm if they are transplanted outside early in the season, or if cold weather comes late.

Use A Greenhouse For Taller Rosemary Plants

You might need to protect your established rosemary plants from cold as they get bigger over the years.  In that case, putting them directly in your greenhouse is a good choice.

You can buy a pre-fabricated greenhouse and put it together yourself, hire someone to help you build one, or make one from scratch yourself.

greenhouse conservatory
A greenhouse is a great option to keep rosemary warm (along with your other plants).

Before you transplant rosemary, put a thermometer inside the greenhouse and check the temperature to make sure it is warm enough.

After moving rosemary plants in the greenhouse, keep the door closed at night to trap heat inside.  Otherwise, you will damage or kill your rosemary plants on a single cold night!

Use Row Covers For Taller Rosemary Plants

If your rosemary plants are too tall for cloches and you don’t have a greenhouse, then row covers could be just the thing you need.

One way to support row covers is to bend flexible plastic rods into half-hoops, with both ends stuck into the ground on opposite sides of the planting row.

row cover
Use bent rods, curved into hoops, to support a row cover.

When you have several of these curved rods lined up over your row of rosemary plants, drape a piece of garden fabric (row cover) over the hoops.

You might need to put bricks, rocks, or pieces of wood on the edges to keep the fabric in place.  You might also need to take off the row cover on hot days, so watch the weather.

row cover
Row cover is made from a light, breathable fabric that is water permeable but keeps plants warm.

Row covers will also protect your rosemary plants from pests – just an added bonus of this method of cold protection! Remember that row covers block some sunlight – the thicker the cover, the better the cold protection, but the less sunlight that can get through.

Conclusion

Now you know how much cold your rosemary plants can tolerate before they die from cold.  You also know how to protect your rosemary plants from cold weather and extend the growing season.

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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