How To Use A Seedling Heat Mat The Right Way

You may already know how to use a seedling heat mat. Not only does a heat mat help seeds germinate at higher rates, but it’s essential for certain types of seeds. Having the correct soil temperature is one of those things that can change your game completely. 

Let’s say you already have our Epic 6-Cell seed starting trays and our Universal Bottom Tray with the accompanying Epic 1020 Germination Dome. Our Epic Seed Starting Heat Mat provides you with yet another useful tool to ensure your success in the seed germination process! 

Maybe you aren’t familiar with any of this, and there’s no problem with that at all! In this piece, we’ll cover seedling heat mats, why you would use them, and how to use them. You’ll find the addition of a heat mat will completely change the game for you. 

Why Use A Heat Mat?

Heat mats offer your seedling trays warmth that they may not otherwise access prior to the growing season. Because many gardeners have to start seeds indoors in late winter or in early spring, they may use a heat mat to maintain a steady temperature or achieve a specific temperature range. 

Warmer temperatures in the soil promote active microbiology and reduce the likelihood of molds, damping off, and diseases that are common in starting seeds indoors. They also help your seeds sprout faster and help seedlings develop stronger roots as they grow.

When To Use A Heating Mat For Seed Germination

Do you always need a heating mat in the seed-starting process? You don’t, but there are plenty of times when having the extra warmth will help get those seeds sprouted. As a general rule, anytime you need to boost soil temperatures to support germinating seeds is a good time to use heat mats. If your ambient room temperature is 70-75 degrees already, you probably don’t need one, but if it’s colder, you might consider it!

For instance, if you are starting seeds in a basement or garage, a heating mat is going to help a great deal with maintaining the soil temperature. While the ambient temperature in these spaces is warmer than the outside temperature in winter per se, it may still be too cold for seed germination. 

Use a heat mat when you’re sprouting seeds that need warmth to germinate. Solanaceous plants like tomatoes, tomatillos, and peppers all require heat to germinate. Some of your favorite summer crops, like zucchini and basil, need a heat mat if they’re germinating in a cooler-than-temperate space. 

Let’s face it. We’re all busy, and we aren’t always on time. A heat mat will help your seeds sprout faster and get you back on track. The warm soil assists those stubborn seeds that are more finicky. Having a consistent temperature is great for emerging seedlings who rely on extra heat in soil for good growth.

How To Use A Seedling Heat Mat

Now that you have some idea of why you would want to use a heat mat let’s discuss how to use one to help your seedlings grow. We’ll discuss seedling mat types, seed germination, and using them with your seed trays. This should set you up for a really successful start to the season.

As you’re using your mat, know that it will be in operation 24 hours per day at around 75° to 80°F (24° to 27°C) unless you deliberately turn it off. You’ll heat the soil at all times to help your seedlings grow. Now it’s time to get all your materials together for seed starting – including your light, pots, and of course, your heating elements.  

Types of Seedling Heat Mats

Before we get into this, note that we have a great piece on the best seedling heat mats and thermostats that gives you a good rundown of some of the options on the market – including our own Epic seedling heat mat. 

The Epic heat mat is a sheet of waterproof PVC plastic with a heating element inside. It warms the trays by 10-20 degrees above the ambient room temperature, which is usually perfect for germination and early seedling development. While our mat does not have temperature controls, some other heat mats do. You can find some that get very granular with these controls, and for most plants, this isn’t necessary. 

Another type of heat mat you can find is in the form of a seed tray, heat mat, and germination dome combo. It’s usually sold as a kit, and the dome often has a vent at the top. This creates a greenhouse effect that supports early plant development. 

In the past, I’ve used one that has a probe for monitoring the soil temperature, and the thermostat keeps the soil consistently warm at a particular degree. This one plugs into an electrical outlet near a seed starting station. 

It’s possible to make a temporary DIY heat mat by using a heating pad set on its lowest setting. However, remember that most heating pads, electric blankets, and the like are not made to work with moisture and don’t have water-resistant electronics, so you’ll have to be very careful not to get it wet. We don’t recommend it due to the risk of water coming into contact with the wiring, but if you’re in a pinch, it can be a short-term solution until you get a water-resistant one – and it’s a surprisingly common recommendation on the internet these days!

If you’re working in a small space indoors or in a greenhouse, you may have luck maintaining temperature in a sunny window. Your grow lights may be able to keep the soil warm without needing a heat mat, depending on how they’re placed. Sometimes the ambient temperature in the room mixed with light conditions will keep it hot enough to help each seed sprout. 

General Seed Germination Time Frames

In order to use heat mats appropriately, you need to know how long it usually takes to germinate most seeds. (We’ve got a cheat sheet on the Epic heat mat!) Most germinate in 3 weeks or less. Some sprout in less than a week. A few species of herbs take longer than 3 weeks to germinate.

In the right conditions, basil and mint will germinate in a couple of days. Rosemary can take up to two weeks. Marigolds take less than a week. Chives, catnip, parsley, and hyssop take anywhere from 10 days to two weeks. Overall, most herbs take longer than other plants to sprout.

Beans take about a week to sprout, being one of the plants that takes a little longer compared to other annual crops. Leeks, parsnips, broccoli, carrots, and kale all take between 10 and 14 days. Beets, turnips, lettuce, and most cucurbits take just a few days at most.

Tomatoes, peppers, and most solanaceous plants are heavily reliant upon heat, humidity, and sometimes light to sprout. If you’re growing seeds of any of these plants, employ a heating pad, a dome, and a grow light. These take variable times and ideally sprout in about 10 days. Sometimes you’re looking at 3 weeks or more, though.  

By knowing these seed germ rates, you’ll know if you’re on track and if you need to add heat to the mix – that is, if you haven’t already.   

Using Heat Mats With Bottom Trays

While your seedlings don’t necessarily need a bottom tray, we recommend using one. You don’t want water leaking out of your seed trays and onto your heat mats, or worse, onto the electrical outlet below. In this same vein, you want your seed starting area to be flat and dry before you lay your mat on it. 

If you are growing in a cell tray that has holes in the bottom for ease of extraction, having a tray below these and on top of your heat mats is ideal. 

If you’re working with a large tray, you may find yourself using multiple mats at once, with one for each tray. As you water, ensure you’re not spilling it onto the heating pad. Remember that if you have too much heat, you may have to use more water as the heating pad evaporates the water.

In this case, an option for temperature control that will keep your seeds warm but not too hot will put you in a better place. Another important tip: double-check that your outlets are not close enough to the trays to take on water that spills. 

Using Germination Domes With Your Heat Mat

Do you need germination domes when using a heat mat? Not necessarily. However, most seeds benefit from humidity as much as they do a heating pad and grow lights. Your seedlings will grow more reliably with enough heat from a mat and enough moisture.

Whether vented or not, the greenhouse effect created inside a humidity dome will help you develop healthier plants. You want to remember to get each seedling sprouted and let each seedling grow to the point where it’s right to the dome’s limit. 

At that point, you can remove the plants that are tall enough and put them in pots. Let the remaining seedlings grow within. A seedling tall enough to surpass the limits of the dome doesn’t need the warm temperatures from a heating pad anymore. This gives you the option and the room to plant more seeds in your germination dome.    

Thermostat or No?

As mentioned before this section, there are plenty of mats that don’t have temperature controls via a thermostat. While you may want to spring for one that has the option to control temperatures fully, it’s not something you’ll need all the time.

Strictly speaking, you really only need to have your seedlings on the mat until they have germinated and put out their first true leaves unless you’re starting them in a really cold location, like an uninsulated garage in a snowy area (that can be really cold!) or even outdoors on a covered patio. Very cold environments may need longer exposure to the warmth, and at that point, a thermostat can be beneficial to ensure you’re not giving too much heat to the tender roots.

In some cases, the thermostat not only gives you temperature controls but also lets you determine when cutoffs occur. That means with a thermostat, you don’t have to monitor the heat as often in the room or greenhouse. But most of us are going to need to check the moisture levels daily anyway, so it’s easy to flip on and off the mat!

Some thermostat types made by specific companies are highly specialized, with the ability to control multiple mats at once and measurements in both Fahrenheit and Celsius. You can buy a thermostat that tells temperature based on a soil probe attached to it, but there are also those that measure humidity and control fans. 

Whether or not you want a thermostat is determined by how you’re gardening. Do you want expert controls, or do you even need them? These are questions to ask before you start your plants and purchase your starting supplies.  

Which Seeds Need Heat To Germinate?

We discussed how solanaceous plants need light and heat to grow. There are other seeds that need heat to grow a seedling too. Remember, basil seeds, zucchini seeds, tomatoes, eggplant seed, and pepper seed need warm soil to grow into a plant. 

A good rule of thumb is that a warm-season plant needs a heating pad or some form of heat to grow. Some of these require light, and others need the heat. Here is a short list that gives you an idea of those seeds that need at least 75°F (24°C) of heat. 

• Corn
• Cucumber
• Eggplant
• Squash
• Pomegranate
• Watermelon
• Lisianthus 
• Petunia
• Zinnia

This is not an exhaustive list, but it gives you a hint of what to give your seeds when you start gardening for the season. 

Frequently Asked Questions

Q: How long do you use a heat mat for seedlings?

A: You will use your mat 24 hours a day until the seedlings are large enough to subsist independently. This usually occurs when they are ready for hardening off or transplant to a larger container (with the roots starting to hit the tray directly on top of the mat). 

Q: Should seedlings stay on heat mat?

A: They should stay there until the seedlings are mature enough to do their own thing without supplemental heat. 

Q: What do you put under a seed heating mat?

A: You don’t have to put anything under the mat, but if you do, you could opt for a towel or something to diffuse the heat on sensitive surfaces. Ideally, it’s best to keep the surface under the mat completely flat and place it on a surface that isn’t going to be harmed by constant warmth.

Q: When should I take the dome off my seedlings?

A: Remove the dome when your plants are large enough to be just below the limit of the dome. 

Q: What is the best temp for seedling heat mat?

A: A good general base temperature is 75° to 80°F (24° to 27°C). Our Epic heat mat increases soil temperatures by 10-20 degrees above the ambient room temperature, putting most seedlings right into the optimal zone for growth. If your ambient room temperature is around 70-75° already, you don’t need a mat.

Q: Are heat mats just for germination?

A: Most of the time, yes. However, you can use them to support the growth of warm-weather plants that need soil warmth to thrive. 

Q: Do heat mats use a lot of electricity?

A: They use about the same amount of energy as a small lamp does. Our Epic seedling heat mat uses an energy-efficient 20W of power, making it very inexpensive to use.

Q: What surface do you put a heat mat on?

A: The surface should be flat and dry.