How to Use Heat Mats for Starting Seeds

Heat mats are a vital part of an efficient production plan when used properly. The number one thing to consider is that not all crops are created equal. Starting seeds in the ideal environment 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 soil temperature preference for your crops. Using the correct germination temperature for your seed-starting process has several 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 the season, you likely have a mixed market garden or large-scale home production garden. If you're trying to do all that in just a few cell trays that contain assorted varieties, remember 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 need a couple of different soil temperature setups to accomplish this. It helps to know that this is what the bigger farms have as a standard practice: Multiple heating mat setups 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?" You may not need one if you are just starting a few seedlings indoors to save a little money on your spring garden plants.  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, 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 to start your seeds if...

• If you are planning to start seeds with special requirements or even just in a space outside their preferred temperature, having a seedling heat mat can 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 a few 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 particular temperature needs for proper growth of roots or development of specific traits. For example, zinnias grown outside 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 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 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 plant's growing preferences. This information will include days to germination, preferred temperature, proper seed spacing, and differences between indoor and outdoor starting recommendations.  

To find the best temperature to set your mat at, list the preference range of each variety you will be starting. Once you have all of this information at your fingertips, you can identify your ideal ranges.  

The Bell Curve of Planting Temperatures

Thinking about plotting your varieties along a bell curve can be very helpful. 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 know the ideal ranges for all 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 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 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 room's temperature and can 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 only raise the soil temperature a maximum of 20℉ above the temperature of the space they are in. 

If you are still within ideal ranges, your seeds should sprout just fine. If you know 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 setup 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 regarding optimum germination temperatures. For example, Lisianthus requires a 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 start multiple varieties of flowers for a cutting garden or farm, you may find that having two distinct setups gives you the best starts. 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 temperatures to change 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 an excellent general temperature for germinating many summer flowers. Some, like zinnias, will do best when started 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 harden off all seedlings properly 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 controllers for different temperature range options. 

Seed Starting for the Home Garden

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

• A 10” x 20” mat will fit one 1020 flat that can hold a 50-cell, 72-cell, 128-cell or a 200-cell either all of one variety or mixed varieties with the same temperature needs.
• A 20” x 20” mat will fit two 1020 flats with cell trays with the same germination requirements.  
• A 20” x 48” which can fit up to four 1020 trays with cell inserts. 

With all of these hobby/home gardener heat mats, if you just plug them into the wall, they will always 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. However, if you're in your house and your household thermostat is set to 70 ℉, 10 degrees over ambient temperature puts you in that Goldilocks’ zone for the varieties that prefer 80 to 90 ℉.

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 will be a 20” by 48” size. The master mat has a plug that you can plug into a thermostat, which we'll discuss 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 are germinating. 

How to Use the Thermostat Controller for Seedling Heat Mats 

We encourage the use of thermostats for heat mats. Thermostats are the only way you can control whether the heat mat will turn on or off based on the climate in your growing space and keep a steady temperature. Without one, you're just going to let your mat run that 10-15 degrees above ambient temperature.  

While in use, the thermostat has a probe that goes directly into the soil, right where the seed will be. Although the mat will frequently heat up above ambient room temperature, it keeps the temperature in the soil steady at your preset ideal, going through a 1020 bottom watering tray, the cell tray, and the soil. 

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.

The bottom of the thermostat has three parts: the power supply plug that goes into the wall outlet, the probe wire, and an outlet for the heat mat.  The plug will just go into an outlet to power 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 the water or the bottom of the tray.

We highly recommend that you make a second attachment point to keep the probe from falling out of the soil. You can do this by taping the wire to the table or using a cable tie and hook to keep the wire in place. If the soil probe falls out, the heat mat can turn itself all the way on and stay that way. This can potentially "cook" your seedlings.  

Changing the Settings on Your Thermostat

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

To change the temperature, hold the set button for three seconds, and then 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 will see that the heating light turns off and on throughout the day. If it's off, that means that the soil has reached the ideal temperature, so the power to the heating elements inside of the heat mat is turned off. If it's on that means the soil temperature has dropped below your ideal setting and will kick on to maintain that soil temperature. 

Now, do keep in mind that 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 your plant's health.

Connecting Several Add-on Heat Mats

To hook 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 that, they're going to pigtail another mat and then another. With those cords being 21 inches long, you can do vertical shelves if 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 have to line those up as they're pretty small. They will go in super easy, but you can't just jam them in there. You have to line them up properly first. 

Understanding Electrical Requirements for Heat Mats

Commercial thermostats and heat mats are rated for 1500 watts, which means you can plug in up to 2 sets of 4 chained mats on a 15 amp general household circuit if there are no other draws on the circuit. You may want to double-check your circuit to ensure it's at least 15 amps before you plug it in.  We do not recommend going over four mats in a chain, as the controllers are only designed to run four mats each. 

Keep in mind that in many cases, there may be something running on household electrical circuits that you are unaware of. This becomes doubly true in grow rooms. 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 simultaneously.

So let's say you have 20 amp circuits in your house; you don't want to overload them, so eight mats is a comfortable level. You can do 10-12 mats, but if you're going to do 10-12, get three thermostats and divide those mats up evenly. This way you can also have three distinct temperature zones.  

Getting the ideal seedling heat mats for your growing space.

The best heat mat for your setup will be the one that fits within your budget and accomplishes what you need it to do.

• For those propagating delicate flowers, having the optimal warmth provided throughout the process by a commercial-grade heat mat with a 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.