When it come to soil temperature, I normally get asked this question for 1 of 3 reasons:
1. You're trying to help temperature-sensitive houseplants survive winter,
2. You want to propagate even though it's winter,
3. Your indoor plant needs repotting now but it's winter and you're worried about recovery.
Step one is usually a head pad or seedling mat and step two is a soil thermometer. But assuming you've ticked those off your list, here's the temperature range you're aiming for...
What's the ideal soil temperature for houseplants?
For the ideal soil temperature for houseplants for root growth in general, and for recovery after repotting or for propagating, you want to be above 15 degrees Celsius, ideally between 15 and 24 degrees. Or for those who use Fahrenheit: Aim to maintain the soil temperature above 59 degrees Fahrenheit, ideally between 59 and 75.
Below 10 degrees Celsius (that's 50 Fahrenheit), and root growth slows down and can stop altogether depending on the plant. Above 25 and soil can dry out too fast unless humidity is high. Root function begins being impaired again once you get around 35 degrees or higher (in Fahrenheit that's 95 or higher). Keep in mind different plants have different temperature tolerance levels, so this is just a guide for houseplants in general.
What's the soil temperature in a houseplant's natural habitat?
In the Amazon rainforest where a lot of our popular houseplants originate from, the soil temperature averages 24 to 25 degrees Celsius all year round (that's around 75 to 77 Fahrenheit), but humidity is also high, around 70% to 90% humidity all year also. Higher humidity helps soil from drying out too fast at high temperatures.
What temperature does a heat mat maintain?
A heat mat without a controller is usually set to maintain a temperature about 10 degrees above the ambient temperature. The temperature reading on your controller will typically be higher than the soil temperature as heat dissipates, more with good airflow and nothing to keep it contained (like a humidity dome or cabinet over the top). You can also get heat mats that monitor the temperature and you can set the range, like the Inkbird Dual.
PS: If you want to convert Celsius to Fahrenheit on the fly: Double Celsius and add 30 to get Fahrenheit, or subtract 30 then halve it to get Celsius.
Happy growing,
Anna
ABOUT THE AUTHOR: Anna is the owner of plant store Love That Leaf which specialises in products and advice for indoor plants. She is also the resident writer for NZ Gardener magazine on the topic of indoor plants. You'll see her houseplant care articles in-stores and online on Stuff, The Post, NineHoney, Express, Echo and more.