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.
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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.
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.
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) |
| 7 | 0 to 10 F (-18 to -12 C) |
| 8 | 10 to 20 F (-12 to 7 C) |
| 9 | 20 to 30 F (-7 to -1 C) |
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).
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 |
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.
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.
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).
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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.
When To Plant Rosemary Seeds
However, if you want to start rosemary from seed, keep the following in mind:
- Rosemary seeds are slow to germinate, taking 14 to 21 days at ideal soil temperatures of 65 to 70 Fahrenheit (18 to 21 Celsius).
- Rosemary seeds that do germinate have a low germination rate (Texas A&M University reports that rosemary seed germination rates can be as low as 15%, or 3 seeds out of 20).
- New plants grown from seed take a long time to grow large enough for harvesting.
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!
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.
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.
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!)
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.
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.
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.
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.
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.
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 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.
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~Jonathon