We have all the technologies we need to make our homes cosy and warm during hard winter and cool during bursting summers. However, what about your indoor herbs? What about those herbs outside? Some herbs are very sensitive to temperature changes and small amounts of either heat or cold can kill them.
Hence, what is the ideal temperature for indoor herbs to thrive? The ideal temperature for most indoor herbs, as advised by experts, is between 65 to 70°F (18-21° C) day and 55 to 60°F (13-16° C) night. The ideal temperature for the most common herbs is shown below
Ideal Temperature Range
Why is it so important to get the temperature right for your herbs? And how can you make sure you create an environment where they can thrive? Let’s dive in
Temperature of the soil is vital for both optimum germination and growth of indoor herbs. Remember, the table above shows the ideal temperature in which the plant will produce the higher harvest or the larger amount of leaves.
A study conducted in 2014 confirmed the effect of temperature on herb growth. It found that increasing the temperature from 18˚C up to 27˚C increased the fresh weight in basil by 106%, sage by 95% and rosemary by 126%. An increase from 18˚C to 24˚C increased the weight in lemon balm by 78%, cilantro by 41%, oregano by 40% and thyme by 58%.
Temperature tolerance is the term used to describe a plant’s reaction to temperature extremes. Herbs with a high-temperature tolerance can withstand a wide variety of temperatures, whereas, low tolerance signals that the herbs are more sensitive to temperature fluctuations.
For example, Basil will grow best when the soil temperature is at least 70°F (21°C). However, even at just 50°F (10°C) the Basil plant can be damaged by the cold and it’s leaves may blacken. In warmer climates, Rosemary is a perennial herb that can last for years. However, in climates cooler than the ideal temperature, it behaves as an annual, being very unlikely to survive the winter unless cared for indoors.
What temperature is too cold for basil?
A temperature that is constantly (more than a couple of nights) below 10C (or 50F) will start damaging a basil plant.
To create a standard understanding of plant tolerance, countries prepare Plant Hardiness Zone Maps as discussed. These maps outline zones of the country based on the lowest temperatures in winter. The zones are then used on seed packets and in gardening information to provide a guide as to the suitability of certain herbs for the zone you are located in. While a useful simplification, these zone maps are only a very broad guide and of course do not take into consideration any other factors other than low temperature that can impact upon the herb growth.
If all of this is sounding far too complicated, then most seed packets will have all the information you need to care for your herbs including optimum soil temperature for germination and growth.
You can also roughly categorise herbs into warm climate and cool climate herbs.
|Warm Climate Herbs (Zone 8 and above)||Cold Climate Herbs (Zone 5)|
What are the strongest herbs?
Mint and Rosemary and Oregano are two of the hardiest herbs (and so my favourite). While Mint does run rampant, it does require water so is not drought resistant. For hot and dry regions then, the winner of the hardiest herb vote is Rosemary, with Oregano.
From my personal experience in the UK (not that cold) basil is one of the worst. It is very cold sensitive, so avoid it for any outdoor application.
What temperature is too cold for rosemary?
Rosemary is a hardy herb that can hence tolerate a light freeze with no issue. Temperature below -5C (23F) can start affecting the plant that will then go dormant. Hence, such herb can be left outside with no issue if the winter is not extremely cold.
Can herbs stay outside in winter?
Not all herbs can stay outside in winter, especially the no hardy ones. Herbs likes basil, cilantro, dill cannot tolerate cold temperatures (below 10C, 50F). On the other hand, the mind and rosemary can easily survive very cold weather by slowing their growth. Their stems will look bare, the leaves will fall. However, the roots are still alive and the plant will likely come back from spring.
Can basil survive 100 degrees Fahrenheit?
Basil is a plant original from the warm India continent, where high temperature and humidity level are relatively common. A basil plant can withstand such temperature for a couple of days.
Important, in this case, is to provide water more regularly (check the soil) and avoid direct sunlight. On such a day it is easy for your herb to get sunburn. However, for more than a couple of days a basil plant might struggle and die at such a high temperature.
Out of the Temperature Range? What is Going To Happen
If the soil temperature goes above the recommended range, the germination success rate will definitely be compromised. Instead of merely stimulating the germination of the seeds, the heat will cook the seeds in the soil! In addition, for established herbs, temperatures that are too high dry out the soil.
If the soil temperature is below the optimum range then the seeds will remain dormant. More extreme cold may result in the seeds being frozen in the soil, damaged by the cold and therefore no longer able to germinate. If you are talking about a grown plant, cold can easily turn foliage of less hardy plants to black colour and kill the herb stems. When the soil is too cold, the roots are unable to transport water and nutrients leading to wilting of the foliage, susceptibility to disease, and plant death.
How To Keep the Temperature In Check?
One of the benefits of growing herbs indoors is that you have much greater control over the environment. So what can you do if the soil or air is too hot or too cold compared to the optimum range?
What can you do if the soil of your potted plants is too hot?
You can use four methods to cool down the soil:
- Organic mulch. Wet soil is cooler than air, so applying an organic mulch to the plants is one way of retaining moisture and therefore keeping the soil relatively cool.
- Location. See if you can find a cooler location for the herbs. As a general rule, downstairs is colder than upstairs, and north facing rooms are also cooler than other parts of the house.
- Ventilation. Increase natural ventilation in the herb location either by opening windows, using fans or portable air conditioners.
- Lights. Dim or turn off any lights in the location where the herbs are growing. Avoid (as also illegal in many countries) the use of incandescent bulb. They produce way too much heat (and waste tons of money).
What can you do if the soil of your potted plant is too cold?
Here some 4 methods to bring some warmth to the soil:
- Plastic mulch. In contrast to organic mulch, plastic mulch increases the soil temperature and therefore can be used to warm your herbs. However, it is criticized for not being environmentally responsible and does not help build the soil.
- Location. Finding a warmer location is also an option. If you have upstairs spaces these are likely to be warmer, and south-facing rooms could also provide more heat.
- Heat mats. Used under the plants, heat mats provide a stable source of heat and are an effective way of ensuring consistent temperatures for seedlings and herbs with low-temperature tolerance.
- Cloches. These are glass domes that can be used to cover your herbs and act as a portable greenhouse, trapping the warmth inside.
Do herbs survive in a fridge? cut herbs usually survive a refrigerator for a couple of days no problem. Freezing should be avoided as, if done inappropriately, can turn herbs into mush.
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