Seedling Heat Mats are used by quite a few people and many web sites suggest that they are required for germination. But other people never use them; so are they really necessary? What, if any advantage do they offer? Can you grow better plants if you use a seedling heating mat to germinate seeds?
In this post I will look at the truth about seedling heat mats.
What Are Seedling Heat Mats?
A seedling heating mat is a sheet of plastic that has heating elements embedded inside. When the mat is plugged in, it heats up, which in turn heats anything that is sitting above it.
Lower priced mats are designed so that they only heat up to a fixed temperature, while higher end products are equipped with a temperature regulator so that you can control the temperature.
The mats are normally used early in the year to provide extra heat, either in greenhouses or under light stands. Homeowners use then in cold basements to start seeds in late winter.
I did a product review for the Redi-Heat Heavy-Duty Propagation Mat By Phytotronics, here: https://youtu.be/OKPQiomsH8k
Check out this FREE 10 part Vegetable Growing Master Class.
Amount of Heating
Most of the products on the market claim to raise the temperature of the soil by 10-20 F (5-11C) over ambient (room temperature). I could not find any data to confirm the actual temperatures they reach.
Is Wet Soil Cooler?
One of the claims made by many gardening sites is that wet soil is cooler than the air around the pot. It then follows that even if the room temperature was ideal for seed germination, the soil would be too cool and therefore you should use a heating mat.
There are also claims that wet soil is cooler than dry soil. The explanation for this is that the water, which evaporates from wet soil, has a cooling effect.
What is the truth?
The first thing I tested was to compare the temperature of wet and dry soil, in my basement growing area. Several measurements were taken over a couple of days, using ProMix, a common seed starting soiless mix. They showed that both had the same temperature (+/- 0.2C).
Next I compared the temperature of soil and air. Same setup with numerous measurements over a couple of days. Air temps were measured at the same height above the table, and a few inches from the pots. Consistently, the soil was warmer than the air temperature, by about 0.5C.
The temperature of wet soil, dry soil and the ambient air around the pots are essentially the same, which eliminates one of the reasons for using a heating mat.
Not Every Seed is a Vegetable
A lot of web sites which discuss heating mats deal with vegetable seeds, even if they are not explicitly mentioned. Many growers start these seeds in late winter and due to short growing seasons they want quick germination. People forget that most types of seed are not vegetables and germination speed is not always important.
Research looking at North American native seeds found that they had high germination rates, and germinated quickly at 71F (22C).
I have germinated over 1,000 different species and for most, temperatures above a cold basement are not required. In fact, I am continually amazed at how many species germinate in a refrigerator.
Winter sowing is a common method to start annuals and perennials – they certainly don’t have a warm cozy heating mat, and they germinate just fine.
Any statement that implies “most seeds need a heating mat” is incorrect.
Starting Vegetable Seed
Speeding up the germination of vegetables, in climates with a short growing period, may be of some advantage.
Consider starting tomato seeds. If a heating mat germinates them 3 days faster, is that an advantage?
Some claim it is an advantage, but why? Why not just start the process 3 days earlier? I have been germinating tomato seeds for 40 years and I have never felt the need to speed things up. I use my baggy method, and place them on my desk until they germinate.
The next question to ask is, “will a heating mat speed up germination”?
Ideal Germination Temperatures
Lets look at some specific vegetables and their ideal germination temperature. J. F. Harrington, Dept of Vegetable Crops, University of CA at Davis has prepared a good list for us.
My basement is mostly unheated and is around 68F (20C). At this temperature tomatoes take 8 days to germinate.
If I used a heat mat and increased the temperature by 18F degrees to 86F (30C) they would germinate in 6 days, a difference of two days.
The most common suggested range for germinating tomato seeds is between 75F and 80F (24-27C), and a research paper suggests 75-85F (24-29C) for optimum germination. Stokes seeds suggests 75°F/24°C for Ultra-sweet tomatoes.
Peppers, the other vegetable that is commonly quoted as needing higher temperatures, will germinate in 12 days at the lower temperature and 8 days at the higher one, a difference of 4 days.
The difference in germination rate for the two temperatures mentioned above is minimal for most vegetables.
There are some, mostly the melon family, that do benefit from higher temperatures; cucumbers, muskmelon and watermelon.
The real benefit of a heating mat depends very much on the temperature of your growing space. At 68F (20C) there is limited benefit for most crops. As the ambient temperature drops, the benefit goes up.
Realistic Germination Temperatures
Dr. Jerry Parsons, Texas Agricultural Extension Service, discusses the idea of realistic temperatures. He makes the point that, watermelon for example, have an ideal germination temperature of 95F, but if you wait for that temperature, and seed outside, it will be too hot and the plant won’t grow. A more realistic temperature for germination is 72F, which is about the same as my basement without a heating mat.
Other realistic temps include; pumpkin (75), cucumber (64), beans (72), pepper (64), and tomato (55). These are in the range of most peoples growing areas.
Do Seedling Heat Mats Grow Better Plants?
I found no information that suggests heating mats grow healthier plants.
How Good Are Seedling Heat Mats?
The claim is that heating mats increase the temperature by 10-20F (5-11C). None of the manufacturers explain why there is a range, and most have exactly the same range – odd!
Do they normally only heat 10F? Do you need to cover the whole area with a dome to get the 20F increase?
I tried to find some actual data for seedling heat mats but did not find any. If you have a mat and can take some readings, compare the temperature in a pot of soil to the air temperature away from the mat and add the data to the comments.
I tested one mat, the Redi-Heat Heavy-Duty Propagation Mat By Phytotronics, which claims to reach 49C. Without the use of a thermostat, which they do recommend, the highest I got was 30C, which is plenty for starting seeds.
If heating mats only raise the temperature by 10F (5C) then their benefit is much reduced.
Heat Mat Thermostats
Many people suggest that you should use a thermostat to more precisely control the temperature. Some products include them, others don’t.
Do you need them?
It sounds like a good idea. Who would not want better temperature control? The reality is that seeds germinate over quite a wide range of temperatures. Provided you are not at the extreme ends of their range, they will germinate.
The average home temperature is in the good range for most seeds without a seedling heat mat. An unheated basement is cooler and if a mat is used, it is unlikely to reach the high end of the germination range negating the value of a thermostat.
The reality is that a thermostat is not necessary because seeds are not that fussy.
Germination Myths
I found several other myths while reading through gardening information that promotes heating mats.
Species, which require a lot of warmth may need more warmth, tomatoes and peppers are good examples.
People confuse plants that need warmth while growing with the need for warmth during germination. Tomatoes like to grow warm, but they don’t need a higher temperature for germination.
“Any vegetable seeds can germinate only, when the soil temperatures remain in their preferred range. ”
The preferred range is the range that products a high rate of germination in a relatively short period of time. Vegetable seeds certainly germinate outside of this range.
“Place seeds on top of the refrigerator to keep them warm”.
This was good advice 20 years ago, when the tops of fridges were warmer, but they are now designed differently and the tops are no warmer than the rest of the room.
Post Germination
Seedling heat mats should really be called germination heat mats. Once the seed has germinated, they should be removed from the mat. Almost all seedlings grow better at temperatures below the optimum germination temperature.
Do You Need a Seedling Heat Mat?
Most home gardeners do not need one.
They can be beneficial if you are trying to germinate seeds in a colder room. Increasing the temperature, within limits, will reduce the time needed for seeds to germinate and may increase the number that germinate.
Use them only for germinating seeds, or rooting cuttings. Don’t use them to grow seedlings.
Some Tomato Germination Videos
We’ve had success germinating those seeds that require a higher germination temperature (eg hairy melon, winged bean, mango) by placing the seedling tray inside a compost bin that I try to keep at about 25 – 30 degrees C until they’re up. I do this in early spring – once the seedlings are up they can tolerate temperatures considerably lower than that, so the tray only needs to stay in the compost bin for about a week or two.
The warm temperate zone that we’re in obviously has a longer growing season than Canada, but I need this strategy to get a long enough growing season for these sub-tropical plants.
I find the strategy also works for me with some more mundane crops – eg tomatoes can be planted earlier and be ready to eat by late spring before all the warm weather pests arrive in numbers.
And yes I have tried simultaneously planting some more seeds directly in the garden to confirm zero germination rate. I also regularly measure the soil temperature at that time of the year.
Our compost bin is the kind with many small vent holes in. I’ve not tried using the unventilated models.
Your basement has a temperature of 20°C. You must have a very large heating bill, I cannot afford to heat my basement to this temperature as it often is near freezing So to overcome this I have a Grow tent which I keep at about 18°C using heat mats and supplementary heating in the form of an oil heater I am managing to grow lettuce at a huge saving on my heating bill , So your point is only valid if it applies to a fully heated home
1) The article makes numerous points.
2) They are valid even in a cold basement – “They can be beneficial if you are trying to germinate seeds in a colder room”.
It’s interesting that, although you very often discuss ornamentals in your articles, in this article you dismiss heat mats as largely unnecessary, but based solely on their utility in growing edible plants. Any comment on whether they are useful (or necessary) for starting, say, impatiens? It only seems fair to address that use, given the scope of your other discussions.
Interesting question. The value of a heating mat is to speed up the germination process. That has some value for vegetables when grown in a short season climate. It may also have the same value for annuals.
I see no real benefit for perennials, trees, shrubs etc, because knocking a few days off their growth will make no difference.
I was considering a heat mat to germinate some lithops seedlings but since these will be grown indoors under a light and my temps vary between 68-71 in California in the fall, I will forgo the heat mat, be patient and look forward to them sprouting. Thanks!
Grew some from seed years ago – it is a slow process.
…although, having said that, the last couple years I’ve had good success with putting some difficult seeds on wet paper towels and germinating them in jars floating in a cooler full of warm water, then just planting the sprouts.
I’m rather frugal and while my home tends to be on the cool side in Winter, the area directly near my radiators obviously tends to experience little blasts of warmth from time to time. This is where I put my little containers of seeds to germinate.
I’ll have them all laid out on an ironing board above the radiator when there are a lot of them. Once they’re germinated, I move them to where they will be growing on. They really don’t need to be there for long at all. Just a few days.
Hi Robert, I’d like to know a few more details of the experiment in your basement –
1. Perhaps the reason the dry and wet soil were the same temperature is that the air in your basement was close to 100% relative humidity?
2. What month(s) of the year did you do the experiment?
3. I’m surprised that your unheated basement is around 20 degrees C in Canada (US Zone 5) unless the experiment was done in summer. Ideally this experiment should be conducted in early spring, which is when many gardeners would consider ambient temperature too cold.
4. Perhaps your basement is not completely unheated – ie heat comes thru the basement ceiling from the house above?
I should also mention that those gardeners like me who live in warmer climates usually don’t have basements. We would be considering a heat mat in late winter/early spring and using it in a shed, garage, etc. where the temperature could be quite cool over-night and also the humidity could be quite variable.
Experiment was done in October, humidity is probably 60 -70%. The basement is partially heated but not in the area where I did the tests. Besides we don’t heat very much in October. It stays about the same temperature all winter long.
I don’t think humidity will make much difference unless it was very high, but I’ll try again in winter.
Agree – would be a better test in February but there were business reasons to get the video done now.
I taught horticulture in a school here in Ohio. We had 5,000 sq. ft. of greenhouses. I used heat mats to germinate seed and to root cuttings. I would not want to be without the heat mats. One of the main reasons is the temperature in the greenhouse and especially at night when I would drop the temp. about 10 degrees. I also used clear domes on my flats for seed. The mats had thermostats, which is very important. You have to be careful about high soil temperatures. Some seeds can go into thermal dormancy if the soil is too warm.
I’m not sure a mat would be needed for home use, probably not.
For rooting cuttings in the greenhouse the mats were very important. I used a mist system to keep the leaves from drying out, it was run by a computer system. The bottom heat was important to get quick rooting, I must say I was very successful at rooting herbaceous cuttings .
Commercial greenhouses use growth chambers for propagation so its a different process.
I would never use mats to grow plants on, there’s too much of a problem with stretching and soft growth and disease. Air circulation is very important to help control botrytis.
Just a note. I never used rooting hormones to root cuttings. There’s no need they root just fine without. I would get in unrooted cuttings (saves a lot of money on cuttings and high shipping costs). These cuttings are ones that you are not allowed to take cuttings from because of plant patents. I would usually get near 100% rooting.
It really depends on what you are trying to root. Try most trees and you will likely fail even with rooting hormone.
Great post, as usual. Also loved your pruning course! You mentioned that heat mats might be useful for cuttings. Could you do a post on that? Thanks!
I do have plans to do a lot about cuttings – maybe this winter.
To be honest I don’t use heat mats for cuttings. Most are done outside with normal temperatures.
Thanks for your interesting research. I’m a gardener who I think would have a lot of use of a heat mat. I live in a house which is never heated (latitude 34S) but in late winter / early spring the temperature is mainly between 55F and 61F. Plus I like growing veggies like hairy melon, turkey berry and winged bean. Without using a heat mat there will be a long delay before I can get these started. A friend uses his heat mat to get these seeds started and that results in planting out more than a month earlier than I can achieve.