Seedling Heat Mats – Are They Needed?

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Robert Pavlis

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.

Redi-Heat Heavy-Duty Propagation Mat By Phytotronics
Seedling heat mat, Redi-Heat Heavy-Duty Propagation Mat By Phytotronics

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.

Soil Science for Gardeners book by Robert Pavlis

I did a product review for the Redi-Heat Heavy-Duty Propagation Mat By Phytotronics, here: https://youtu.be/OKPQiomsH8k

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.

YouTube video

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

YouTube video

 

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

29 thoughts on “Seedling Heat Mats – Are They Needed?”

  1. One of the main reasons to use a heat mat is protection from damping off, which happens when any number of pathogens that thrive in cool, damp soil start to grow. Seedling affected by damping off will look like they’ve been pinched at the stem and they will die. Keeping the soil warm prevents damp off and it’s much safer than the fungicides used to prevent this problem.

    Reply
    • That is only true in very cold conditions. Heating mats should be removed once seedlings germinate in most cases.

      Reply

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