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How to Use Heat Mats for Starting Seeds

January 01, 2023 11 min read 0 Comments

How to Use Heat Mats for Starting Seeds

Everything you need to know about using a seedling heat mat for seed starting in less than 10 minutes. Heat mats can be a vital part of your production plan when used properly. The number one thing to consider is that not all crops are created equal. Starting seeds means taking a look at the individual needs of each crop and sometimes each individual variety. 

Germination Requirements For Starting Seeds (soil, light, water, temperature)

Consider soil preference, temperature preference and light preference for all of the crops you will be starting from seed.

In this article we will be dealing exclusively with the temperature preference of the soil. Using the correct germination temperature for your seed starting process has a number of benefits including; increased seed germination rates, quicker germination times and healthier plant starts overall.

Market Garden Mixed Crops vs. Monocrops

If you plan to start seeds in one to four flats total for the year or for the season this likely means you have a mixed market garden or large scale home production garden. If you're trying to do all that all in just a few cell trays that contain mixed varieties keep in mind that you may want to find the best average temperature to set your heat mat on. You may also decide to use other methods to take care of those soil temperature needs such as starting some trays inside and others in an unheated space like a shed or garage.

Now if you're starting seeds year-round, monocropping a significant number of rows of plants or you just have a couple of different varieties of seeds, you might have a couple of different soil temperature setups that you need to accomplish this. It helps to just know that this is what the bigger farms have as a standard practice; Multiple heating mat set ups for multiple varieties.

Do You Need a Heating Mat to Start Your Seeds?

One of the main questions we get is, "Do I even need a heat mat to start seeds?" If you are just starting a few seedlings indoors to save a little money on your spring garden plants then you may not need one.  However, considering the soil preference, temperature preference and light preference can be different for every variety of seeds the answer is probably yes, if you are growing a wide variety of crops from seed.  

The germination requirements for lettuce seeds are very different from those for chili peppers or tomatoes. While your lettuce seeds may germinate just fine in an unheated room or shed where the temperatures range from 40-65℉ in the Spring, other gardening staples will not. Tomatoes for example will rarely germinate below 75 degrees. To start these properly you will need to warm the soil using seedling heat mats.

You will need a heat mat if...

If you are planning to start seeds that have special requirements or even just in a space that is outside their preferred temperature, having a seedling heat mat can definitely be the key to a successful start to every growing season. 

If you are going to germinate heat loving crops like zinnias or chili peppers, using a heat mat can give you a significant jump on the growing season. These crops frequently take 7-14 days to germinate if left to their own devices. Grown on a heating mat, these seeds can germinate quite well in just a couple days.

If you need to start a large quantity of seeds and ensure uniform germination and growth, using a heat mat for propagation is one of the best ways to get an even start.

If your seeds have very specific temperature needs for proper growth of roots or development of specific traits. For example zinnias grown outside of their preferred conditions may revert to single type blooms instead of the larger multi-layered petal style.

Looking Up Plant Varieties’ Needs on Your Seed Supplier’s  Website

Most reputable seed suppliers will provide you with all of the information you need to get your seeds started right. If you have a favorite seed supplier take the time to look through their website. You may have to scroll down past the general description to find the ideal soil temperatures for seed germination. 

Generally listed under Growing Information or Attributes within a seed’s description you will find a list of information about the growing preferences of the plant. This will include days to germination, preferred temperature, and  proper seed spacing as well as differences between indoor and outdoor starting recommendations.  

To find the best temperature to set your mat at you should make a list of the preference range of each variety you will be starting. Once you have all of this information at your fingertips you can move forward with examining your ideal ranges.  

The Bell Curve of Planting Temperatures

It can be very helpful to think about plotting your varieties along a bell curve. Some plants like lettuces will germinate in the lower ranges of 55℉- 65℉. Most plants will happily sprout in the 65℉-75℉ range. A few species of plants including peppers, tomatoes and some flowers prefer ranges from 75℉ to as high as 90℉. 

Seed Packet Information

Some companies include ideal ranges in the information listed on the seed packet but most have only general information such as days to harvest and plant spacing. For more specific information you will need to refer to the website or research your variety online with a reputable source. 

Examining Temperature Ranges

Now that you have an idea of what the ideal ranges are for all of the varieties you will be starting you can find the Goldilocks zone for your seedling heat mat setting.  The Goldilocks zone is just what it sounds like, not too hot and not too cold for everything that needs a little boost of warmth to get started.

In an indoor setup where you're already at 70 degrees many of the plants that prefer to be on the lower end of the bell curve will germinate well, possibly only needing the heat mat when overnight temperatures drop. However with peppers and tomatoes you will want to just bump that heat up around 10℉ to get you more in the high end of the range of that bell curve. 

Grow Room Ambient Temperatures and Handling Fluctuations

If you are starting all of your seedlings indoors you will likely have control over the temperature of the room and will be able to control the temperature of your propagation trays with the mat. However if you are starting your seeds in an unheated space like a shed or garage overnight temperatures may drop below what the mat can compensate for. The mats can really only raise the soil temperature a maximum of 20℉ above the temperature of the space that they are in. 

If you will still be within ideal ranges your seeds should sprout just fine. If you know that it will fluctuate beyond that range you may want to bring the trays into a more protected space overnight. You can also add humidity domes to your seed germination set up to help retain a little extra heat. A dome will also protect your seedlings from drying out. The heating mat will cause more water to evaporate from your growing trays and can lead to growth problems if you forget to water.

Using a Heating Mat to Start Flower Seeds

Many flower varieties are even more finicky than vegetables when it comes to optimum germination temperatures. Lisianthus, for example, requires a very specific temperature range of 70℉-75℉ during the 10-15 day germination phase. This must then be reduced and maintained at 60-70℉ for an additional 45-50 days for proper development.

If you will be starting multiple varieties of flowers for a cutting garden or farm, you may find that having two distinct set ups gives you the best starts. Just as we recommend above with vegetables, research the ideal germination temps for each variety and group them by Goldilocks’ zone.  Keep in mind that many types of flowers will need the temperatures changed higher or lower after germination depending on your desired transplant date. 

Heat Loving Flowers

Zinnias, sunflowers, celosia and many other high performing, heat tolerant cut flowers prefer to germinate at higher temperatures. 75℉ is a great general temperature for germinating many summer flowers. Some, like zinnias, will do best when started out even hotter in the 80-85 degree range. 

Cool Germinating Flowers

Scabiosa, cosmos, yarrow and many other flowers that bloom in spring and summer prefer cooler temperatures for germination. Most of these flowers prefer to be in the 65-70 degree range until they have sprouted. 

Be sure to properly harden off all seedlings started indoors before transplanting out. You can review our advice for hardening off seedlings at the end of our Seed Starting 101 Guide.  


What's the Right Seedling Heat Mat for Your Scale of Growing?

Depending on what type of grower you are you may have very different needs when it comes to heat mats and thermostats. While the home gardener may only need one small mat, the large scale farm or homestead may need multiple mats and multiple temperature range options. 

Seed Starting for the Home Garden

When you choose a heat mat the number one thing to start out with is identifying your scale. Here at Bootstrap Farmer we have a variety of sizes available. 

With all of these hobby/home gardener heat mats, if you just plug them into the wall are they always going to be on and give you about 10 degrees over the ambient temperature. So if you're in your house and it's 70 degrees and you're trying to plant lettuce it's probably going to be on the hotter side of that spectrum and you may not even need it. However, if you're in your house and your household thermostat is set 70 ℉, 10 degrees over ambient temperature does put you right nicely in that Goldilocks’ zone for the varieties that prefer 80 to 90 ℉.

heat mat controller

Commercial Farms and Market Gardens

For our bigger growers we have commercial heat mats in both a master and an add-on. Both of these are going to be a 20” x 48” size. The master mat has a plug that you can plug into a thermostat which we'll talk about here in the next section. On the opposite side it has a pigtail to plug into the add-on mat which can then be further pigtailed depending on how many cell trays you're going to germinate. 

How to Use the Thermostat Controller for Seedling Heat Mats 

We really encourage the use of thermostats for heat mats. Thermostats are the only way you can control whether the heat mat is going to turn on or off based on the climate in your growing space and keep a steady temperature. Without the use of one you're just going to let your mat run that 10-15 degrees above ambient temperature.  Keep in mind 10 degrees is not necessarily at the soil when using a mat on its own. 

While in use the thermostat has a probe that goes directly into the soil right where the seed is going to be. Although the mat will frequently heat up above ambient room temperature it's keeping that temperature in the soil steady at your preset ideal. Which means it's having to go through a 1020 bottom watering tray, the dead air space in between the tray and the cell inserts. 

The water is a factor in temperature fluctuations as well. As you add water it's going to be a different temperature and conduct heat differently.  With all of this in mind it really helps regulate the temperature as best you can at the soil where the seed is actually located.

Looking at the bottom of the thermostat it has three parts on the bottom; the power supply plug that goes into the wall outlet, the probe wire and an outlet for the heat mat.  The plug is just going to go into an outlet to get power to the temperature control unit. The probe wire has a suction cup on it and whether you're doing soilless media indoors or soil as a growing media inside or outside you can place the sensor directly into that seed bed. The little suction cup sticks to the 1020 tray and helps keep the sensor from falling out. Remember the probe goes into the soil not into the water or the bottom of the tray.

heat mat

Changing the Settings on Your Thermostat

To operate your thermostat select first if you prefer Celsius or Fahrenheit. To get Fahrenheit you just press and hold the up button for three seconds and it'll switch it over to ℉. If you want Celsius you press and hold the down button for three seconds to put it in ℃ mode. 

To change the temperature you press and hold the set button for three seconds and then you can press either the up arrow or the down arrow to increase or decrease the soil temperature. When you have reached the desired temperature, you just press set and it'll hold that temperature. 

Once it is set you're going to see that heating light turn off and on throughout the day. If it's off that means that the soil temperature has reached the ideal temp and so power to the heating elements inside of the heat mat are turned off. If it's on that means the soil temperature has dropped below your ideal setting and it's going to kick on to maintain that soil temperature. 

Now do keep in mind if you forget about things for a little bit and your plants are inside a greenhouse on a super hot day this is not going to cool down the soil. Seedling heat mats only warm the soil. So if you go above those germination temperatures in the growing space that may affect seed germination or the health of your plant.

Connecting Several Add-on Heat Mats

To hook up add-on mats to the master mat you have these 21 inch long pigtails on one side of the master and on either side of the add-on mat. Some folks are going to go from the wall to the thermostat, put the probe in the first one, and then they're going to plug in their mat. After which they're going to pigtail another mat and then another. With those cords being 21 inches long you can do vertical shelves as long as the height doesn't exceed the cord length. You can also spread them out horizontally.

The three prong connectors on each mat go into three little holes inside the female side of the pigtail. You really have to line those up as they're pretty small. They will go in super easy but you can't just jam it in there you have to line them up properly first. 

heat mat

Understanding Electrical Requirements for Heat Mats

Commercial thermostats and heat mats are rated for 1500 watts which means you can plug in up to 10 mats for 15 amp general household circuits if there are no other draws on the circuit. You may want to double check your circuit to make sure it's at least 15 amps before you plug it in.  We do not recommend going over 10 mats in a chain. 

Keep in mind that in many cases with household electrical circuits there may be something running on it that you are unaware of. This becomes doubly true in grow rooms. Having multiple items on timers and automated controls can easily overload a circuit if your heat mat, your dehumidifier and your water pump all happen to click on at the same time.

So let's say you had a 30 amp circuit in your house, you don't want to overload that thing so eight mats are a comfortable level. You can do 10 but if you're going to do over 10 just get two thermostats and divide those mats up evenly. This way you can also have two distinct temperature zones.  

Getting the ideal seedling heat mats for your growing space.

The best heat mat for your set up is going to be the one that fits within your budget and accomplishes what you need it to do.

  • For those doing propagation of delicate flowers, having the optimal warmth provided through out the process by a commercial grade heat mat with thermostat is worth the investment. You can avoid ending up with lisianthus that never flowers or zinnias that come out looking like gerber daisies.

  • For the home gardener trying to get a jump on spring planting, a simple heat mat that gives your plants germinating out in the garage a little extra warmth can be just what you need.

  • For market gardeners and those trying to sprout large numbers of cell trays at a time, adding a daisy chain of large seedling heat mats can help you get to market with produce weeks ahead of your competition.


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