How to Use a Seedling Heat Mat for Smart Seed Starting

Seed-starting mats are designed to kickstart your gardening journey with efficiency and precision. Whether you want to nurture a select few seedlings or an entire garden’s worth, a seedling heat mat is your gateway to fostering robust and thriving plants from the very beginning!

Heat mats are incredibly beneficial for starting seeds indoors, IF they are used correctly. Used incorrectly, they can do more harm than good. If you’re shopping for heat mats or already have them, these tips will help you master using them.

New Beginnings

One of the most exciting parts of my year is starting seeds! I love the potential that every new garden beginning holds.

I want to help you begin this gardening season on the right foot with resources that teach you about using grow lights, choosing the best seed starting trays, crafting a starting mix for seeds, soil blocking, cold stratification, and even timing when to start seeds indoors.

Heat Mat Sizes

MIgardener’s premium seed starter heat mats are available in two sizes:

• Regular - Measuring 10x20", the regular-sized seedling heat mat fits seamlessly into any gardening setup, whether it's a cozy apartment balcony, a greenhouse garden, or a dedicated indoor gardening station. It doesn't take up unnecessary space, but fits one 1020 seed starting tray perfectly. 

• Jumbo - Our jumbo seedling heat mats are huge: 48x20”. You can fit four flats of seedlings on a single heat mat. You read that right, four entire 1020 trays on one seedling heat mat!

Pros and Cons of Using a Seedling Heat Mat

A seedling heat mat can provide ample warmth for your precious seedlings, promoting rapid germination and healthy root development. They help maintain even, consistent temperatures, especially at night, and eliminate uneven sprouting caused by cold pockets.

Heat mats are an important asset for a gardener, but if you use them incorrectly, they can absolutely be more of a liability than a help! They will dry out the soil faster than without, overheat certain types of crops, and cause leggy growth if not used correctly.

Types of Crops That Benefit From a Heat Mat

Heat mats are great because they warm the soil from underneath, creating a warmer environment that helps seeds germinate faster. They can expedite the germination process by up to 50%! 

MIgardener heat mats have a built-in temperature regulator that turns them on and off at around 85°F. It is very beneficial for warm-weather crops such as tomatoes, peppers, and other plants that prefer 65°F and warmer.

You should AVOID using a seedling heat mat for cold-weather crops! These crops sprout better at temperatures between 45° and 65°F. They might germinate fast, but they don’t like warm soil, and you will have a stressed seedling. This includes the following crops:

• Lettuce

• Spinach

• Radish

• Kale 

• Broccoli

• Cauliflower

• Arugala

• Mustard Greens

How Long to Use a Seed Starting Mat

Only continue using a heat mat until the plants are germinated! Regardless of what type of crop you are growing, once that plant sprouts, you need to take it off the heat mat.

A common mistake many gardeners make is leaving seedlings on a heat mat for too long. Most seeds germinate better in warmer conditions, but heat stresses out roots once they start to develop. If the roots get too dry or too warm, it will lead to stress and leggy seedlings.

This is even true of your warm-weather crops like tomatoes and peppers! They need the ambient temperature to be warm, between 70° and 80°F, but the soil itself should be 10° to 20°F cooler. 

Don’t Use a Timer With a Heat Mat

You do not need to plug your seedling heat mats into a timer. Keep them on continuously until the seeds sprout. The seeds will benefit from consistent temperatures, day and night.

I tried plugging my heat mats into a timer for quite awhile thinking that it would simulate natural daytime and nighttime temperature fluxuations. The result was inconsistent germination. You’ll get much better results by leaving the heat mat on all the time.

Using Heat Mats Is Optional

Seedling heat mats are a luxury, not a necessity, for starting seeds. I know, we sell heat mats with MIgardener’s name on them, so it would be in my best interest to tell you that you need to buy one; but I want to tell you the truth and let you make an informed decision!

I do love using heat mats, especially my jumbo-sized seedling heat mats, but seed starting can be done without one. (They make great gifts for gardeners!)

If you are on a budget like I was when I started gardening, absolutely invest in high-quality heirloom vegetable seeds (a must!), durable seed starting trays and good seed starting soil. I would even invest in grow lights before a heat mat, but once you have one, it is a wonderful luxury. 

You can put your seedlings in a warm area to simulate a heat mat, like on top of a refrigerator, near a radiator, or in a sunny window, and it can work just as well as long as it doesn’t get too dry or drafty. 

Seedling Heat Mats Are For More Than Just Seed Starting 

Heat mats are great for more than just starting seeds! MIgardener’s heat mats are durable and waterproof.

We've even used them for brewing kombucha, making sourdough, and proofing bread, so be sure to consider their other uses. You might find yourself using them year-round!