The Complete Guide to Using Seedling Heat Mats (plus common mistakes to avoid)

I used to think heat mats were only for serious outdoor gardeners. I was picturing greenhouses full of rows and rows of seedling trays. But as my indoor jungle reached 100+ plants (I stopped counting after that), I soon caught the propagating bug, and the humble heat mat quickly became one of my favourite plant tools. Propagating was SO satisfying and rewarding. Watching cuttings root then grow into a whole new plant. There was no way I wanted that to stop just because of winter.

I also discovered the value of heat mats for being able to repot my indoor plants any time of year. Now I could repot when the plant needs it rather than being dictated by the seasons. I just pop the plant on my heat mat to help roots recover and reduce transplant stress. These days I'll pull out my heat mats any chance I get, for everything from Tomatoes and Peppers to String of Turtles and Philodendrons. 

How long do you leave heat mats on for?

When using a seedling heat mat indoors for starting warm-weather crops like tomatoes, you want to keep your heat mat on 24/7 until germination to keep an even soil temperature, then turn it off a couple of days after germination. Turning heat mats off during germination, such as overnight when lights are turned off, causes uneven temperature swings that can interrupt and stunt development and delay sprouting.

For propagating tropical indoor plants, you can also leave your heat mat on 24/7 in winter until cuttings are well rooted and ready to pot up (then I put the newly potted plants on the heat mat also), or since they're indoors, if the air temperature at your place doesn't drop too much overnight, you can turn your heat mat off overnight if you prefer. 

However when using your heat mat in an unheated environment (such as outdoors in a greenhouse or garage), keep it during germination then on for longer after germination until the area stays at least above 65 degrees Fahrenheit day and night (around 15 degrees Celsius), ideally into the 70's / over 20 degrees. Again, check the temperature preferences of the plants you're growing (this is just a general guide).

For cool-weather crops, like lettuce, kale, cabbage, peas and broccoli, it's best to either not use a heat mat for germination (unless the ambient temperature is too cold for them), or turn it off asap after germination, otherwise when things heat up they tend to bolt and quickly become leggy. A heat mat with a thermostat (set to turn off when it gets too warm), is helpful for avoiding things heating up too much.

Do you have to use a thermostat or temperature controller for heat mats?

No you don't have to have a thermostat or temperature controller, but there are times you might want one. Most people find standard heat mats without thermostats do a great job for most plants and the best selling heat pads don't have a temperature controller.  

However, if you're growing (or baking / brewing), something that requires set temperatures and sulks if it fluctuates outside their happy range, like temperature-sensitive plants, sourdough, or water and milk kefirs, then you have two main options: Get a heat mat with a built-in controller, or buy a controller unit separately to upgrade your standard heat mat.

Remember when monitoring your heat mat, it's the soil temperature (not the air temperature), you want to keep an eye on. Easy to do using a soil temperature meter like the simple ReoTemp Soil Thermometer from Amazon. Some heat mats also come with a soil temperature probe that you put into the soil.

What are the best heat mats to get for plants?

The two most popular standard seedling heat mats without a controller are the Lerway Heat Mat and Inkbird Plus (or for those not based in New Zealand, have a look at the Vivosun Heat Mat available on Amazon). The best seller with a built-in controller is the Inkbird Plus Dual (in NZ), or BN-Link Heat Mat with Thermostat (on Amazon). Or you can upgrade your standard heat mat and buy a separate heat mat controller unit, like the Inkbird Temperature Controller (link for those in NZ), which is also available here on Amazon for those overseas.

Do you need a heat mat to start seeds?

Soil temperature plays a huge part in successful sowing and planting of crops. If you're wanting to start seeds and seedlings indoors any time of year (especially in winter), ready to plant outdoors, the general answer is yes, most plants benefit from being started on a heat mat. Most, but not all.

It comes down to the plant's preferences. Most prefer soil temperatures between 65 to 75 Fahrenheit (18 to 24 Celsius). If the air temperature is already 70 to 75 degrees F (around 20 to 24 C), then you probably don't need a heat mat. Keep in mind you also want to avoid big swings in soil temperature for germinating and growing seedlings, so even if your greenhouse or garage gets lovely and warm during the day, you may need to turn a heat mat on in the evenings to keep seeds and seedlings toasty overnight. Heat mats really do work wonders.

Tomatoes for example rarely germinate below 75 F / 23 C, whereas lettuce prefers it on the cool side, around 50 to 72 F / 10 to 22 C and don't sprout easily when the soil temperature's over 72 F / 22 C. 

Do you have to use a humidity dome over your seeds on a heat mat?

No, you don't have to, but you will probably want to once you know the reasons why. Without something to keep the heat in, your heat mat will be less efficient at warming the soil and the soil temperature will stay less consistent as the air temperature and air humidity fluctuates during the day. Otherwise a lot of that lovely heat will just disappear. Heat mats also dry out soil and evaporate water much faster than you might realise. A humidity dome also helps avoid your seeds or seedlings completely drying out (and potentially dying if you don't catch it in time).

Ideally you want a vented humidity dome, like the Elho Mini Greenhouse Seedling Tray Kit with Dome (in NZ), or Infinity Humidity Dome Germination Kit (from Amazon). Or to just quite regularly remove the dome at least daily to get some airflow in there. That's because high humidity plus heat with little to no airflow can lead to the growth of nasties like mildew and rot. 

Can a heat mat get too hot?

Heat mats without a thermostat controller tend to maintain a temperature around 10 to 15 degrees above the ambient room temperature, but other conditions can make them feel hot or run hot. Heat mats can feel hotter than you'd expect to the touch, but when you test the soil, it's fine. Two big factors are the surface you've got the heat mat on (carpet is a no-go for example), and the airflow situation (or lack of it), around the mat or the plants.

If your heat mat feels like it's getting too hot, first test the soil temperature. It's the only way to be sure. You can also put heat-absorbing material between your heat mat and your plants, such as a piece of stone (I use leftover ceramic bathroom tiles for mine). You can also increase the airflow and remove anything keeping heat in that's on top of your heat mat or trapping heat in, such as a humidity dome. Avoid wood or anything flammable on top of your heat mat. To check the temperature, use a soil thermometer (like the simple ReoTemp Soil Thermometer from Amazon). Your hand or how warm the bottom of the seed tray or pots are isn't a reliable guide.  

Why does my heat mat feel cold?

If you've only just turned your heat mat on, it can take about half an hour on average for it to reach temperature. But even then, don't expect your heat mat to feel hot to the touch. Your hand temperature is around 32 to 34 C (men tend to run warmer, women cooler), that's from high 80's to low 90's F, so your heat mat will tend to feel warm but not hot to the touch if you lay your hand flat on the mat.

Also, keep in mind it's gently warming the soil of the plants directly on the heat mat, and beyond that, excess heat will usually be lost quickly, so it may not feel all that warm if you just hover your hand over the mat. 

In a large space or unheated area that's cooler, a heat mat may struggle to maintain a warm enough temperature without a little help. Changing what the heat mat is sitting on or enclosing the area both help, as does covering the seeds / seedlings / cuttings with a dome or covering to keep the heat in. Heating the ambient air temperature helps also. If you're unsure if your heat mat's doing its job, get yourself a soil temperature meter (like the ReoTemp Soil Thermometer available on Amazon).

How long do seeds take to germinate?

At the right temperature, most seeds germinate in 3 weeks or less. Some are super speedy and will sprout in less than a week. In ideal conditions, here's a general guide to germination times for different plant seeds. If they're taking longer than this, conditions may need to change:

Days: Basil, Mint.
Around 1 week: Marigolds, Beans.
Around 2 weeks: Chives, Catnip, Rosemary, Parsley, Beets, Lettuce, Turnips, Chillies, Pumpkins, Sage.
2 to 4 weeks: Tomatoes, Peppers, Mango, Lavender.
1 to 2 months: Avocado, most flower seeds.

Are all heat mats waterproof?

No, not all are waterproof. Some are just water-resistant. So the occasional spill or misting of water shouldn't cause a problem, but should still be wiped away. Even if your heat mat is waterproof, you still don't want to leave your heat mat wet as water may travel down the cord to the plug, or get into the area where the cord goes into the mat.

It's best to pop a tray or saucer underneath the plants, so there's something to catch excess water and keep your heat mat dry. It's always best to locate plugs above your heat mat if you can, or at least have the cord loop down between the mat and the plug to create a 'drip loop' for safety. Like most appliances, heat mats should also be used with a surge protector, especially if you plan to leave it on for long periods of time or unattended.

What's the ideal temperature for indoor plant cuttings to root?

For propagating most tropical houseplants, you want to aim for a soil or water temperature around mid-20's Celsius or high 70's Fahrenheit to encourage cuttings to root. During cooler months, soil can be as much as 10 to 20 degrees colder than the air temperature around it. Bottom heat (from a heat mat), produces better results when propagating than warming the entire room.

Water temperature will also naturally be lower than air temperature, as evaporation takes heat from the water. When humidity's lower - which it typically is in winter as artificial heating zaps moisture out of the air - that also increases the temperature difference between water and air. Heat pads also increase evaporation. In winter I pop both my water and soil propagations on a heat pad.  

What's the ideal soil temperature for different crops?

As a general rule, once you know your plant's preferred temperature for germination, use the middle to upper end of the range for germination, and for seedlings use the full range but avoid going under the minimum. 

10 – 15°C artichoke, broad-beans, carrot, kale, parsley, parsnip, peas, radish, spinach
10 – 20°C beetroot, brassica, celery, chive, garlic, lettuce, leek, onion
15 – 20°C beans
15 – 25°C corn, cucumber, pumpkin, zucchini
18 – 25°C sweet peppers / capsicum / bell peppers, tomatoes
20 – 30°C chillies, eggplant, kumara, melon

Note that these are the ideal soil temperature, not air temperature, and soil on average can be 10 to 20 degrees cooler than the air. I highly recommend getting a soil meter (like the ReoTemp Soil Thermometer available on Amazon) as it really is the only way to be sure you're achieving the right temperature for your plants.

Tips for germinating flower seeds on a heat mat

Flowers tend to require a different approach to germinating crops and herbs. Flower seeds are generally fussier so for them a heat mat with a temperature controller is a better choice such as the Inkbird Plus Dual (in NZ), or BN-Link Heat Mat with Thermostat (on Amazon). Or you can upgrade your standard heat mat and buy a separate heat mat controller unit, like the Inkbird Temperature Controller (for those in NZ), which is also available here on Amazon.

Lisianthus for example prefer a range of 70 to 75 F (21 to 23 C), for the 10 to 15 days they take to germinate, then a drop to maintain a cooler range between 60 and 70 F (15 to 21 C) for a further 45 to 50 days. Zinnias like it even warmer, around 80 to 85 F (26 to 29C) to get started. Even cool germinating flower seeds still prefer comparatively warmer temperatures than cool germinating vegetables. Cosmos for example prefer a range of 65 to 70 F (18 to 21 C). Long story short? Get a heat mat where you can set the temperature. 

What surfaces can a heat mat be put on?

The surface a heat pad is put on changes the temperature by impacting how much warmth is lost or retained. Placing on colder surfaces or materials like concrete will reduce the temperature, and placing on an insulated surface will help maintain a higher temperature.

Don't put heat mats on heat-emitting surfaces (like a heated floor) or on carpet or rugs. Non-flammable surfaces like a stone counter-top or ceramic floor tiles are great options. If you're using a heat mat on open wire shelving, I put cardboard or ceramic tiles under mine to direct the heat upwards.

Also run your hand over the surface first to check for any sharp or rough areas that might damage the heat mat. Never double-up or overlap heat mats and avoiding folding or creasing your heat mat.

How much do seedling heat mats cost to run?

Using one of the standard Inkbird Plus heat mats as a guide, based on running a heat mat 8 hours a day, 7 days a week for 3 months over winter; a heat mat would cost around $4.50 in total for 3 months, or less than $2 a month. That's based on $0.32 cents per kilowatt-hour (kWh) based on the average 2023 NZ electricity rates published by Canstar Blue.

Beware under-watering

Before you now go get yourself a heat mat, remember that the extra warmth heat mats provide also increases the rate at which soil dries out. If you don't pop a humidity dome or similar over your seedlings (more about humidity dome mistakes above), the soil will dry out faster than you might realise. Same for water propagating, as water will evaporate much more rapidly on a heat mat. If you do use a dome, you may not need to water until after germination (when you would typically remove the humidity dome).