Using Heat Mats For Seed Starting (Pros & Cons)

You do not need a heat mat for most indoor seed starting but there are circumstances where they come in handy. Learn best practices and what to look for when purchasing.

For a complete guide on growing from seed, see Seed Starting For Beginners with all my top tips and recommended low-cost supplies.

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Start Seeds Indoors With Heat Mats

While most seeds sprout fine indoors in average household conditions without the need for heat mats, there are some circumstances where they can help.

Find out why soil temperature matters for seed starting, when to use a heat mat, what to look for when buying one, and how to use one.

While often referred to as “seedling” heat mats, they are usually just needed for seed germination not the seedling growth stage.

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Contents

• What Is A Heat Mat?
  • Why Soil Temperature Matters
• When To Use a Heat Mat
  • Buying Tips
• How To Use a Heat Mat
• Heat Mat Pros & Cons
• Frequently-Asked Questions
• Resources

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What Is A Heat Mat?

Heat mats for seed starting are much like the electric heating pads we use for muscle aches but instead of cloth they are encased in a water-resistant plastic. You can see one here at Amazon.

To help with germination, the mat is placed under trays or containers of newly planted seeds where the warmth radiates into the tray and soil. We say “soil” but usually this is soil-less growing medium made for seed starting.

The idea is to provide the right amount of warmth for the particular type of seed—they each have different preferences—to achieve faster germination rates than we would in colder conditions.

These mats are only used indoors or in greenhouses, because, while they can take some water spills, they cannot be submerged in water.

You can also use heat mats for propagating various plant cuttings.

Why Soil Temperature Matters

To germinate, most seeds need water, air (oxygen), and warmth to break dormancy. There are exceptions, but most seeds do not need light until they have sprouted.

We know from research that each type of seed germinates best if, along with adequate air and water, the growing medium (soil, potting mix, seed starting mix) is within a certain temperature range. This varies for each type of plant.

Most of us never think about temperature in particular for seed sowing because it often works out by circumstance. Seed packet instructions may say to sow seeds when the risk of frost has passed and that happens to be in late spring when the soil is warming up naturally. Or, we start seeds indoors in our homes where we have consistently comfortable ambient temperatures (around 70°F or 21°C)—that happen to suit our seeds as well.

Because of this, you may very likely get through a lifetime of gardening without ever needing a heat mat. Where they come in handy is situations where conditions are not quite right or we want to sprout seeds as quickly as possible—although the difference, as you’ll see, may be trivial.

Some also claim heat mats produce stronger seedlings but I cannot find any evidence of this.

When To Use a Heat Mat

There are certain circumstances where a heat mat may be beneficial but it’s certainly not necessary for most indoor seed starting.

A heating mat can help if you have one or some combination of these:

• Your seed starting room is too cold (consistently in mid-60s F or lower) and seeds take too long to sprout.
• You are growing seeds that specifically require higher temperatures to sprout (e.g. eggplant, watermelon, rosemary). I’ve linked to soil temperature charts in the Resources section.
• There is a time crunch and you need to get some annuals established asap.
• You are growing temperamental seeds and want optimum conditions.

I started seeds indoors for years before ever buying a heat mat. I finally decided to try using one when I started experimenting with fussy seeds. It’s really just one more tool in the kit to help provide the right conditions if they are not occurring naturally.

There are a few species like rosemary seeds that like soil temperatures much higher than we’d ever have indoors and a boost of heat—at just the right level—can work wonders. Alternately, it’s much easier to root rosemary from cuttings.

Time-wise, the advantage may not be worth it unless there is a real time crunch. Sometimes “faster” is really just a matter of a few days or perhaps a week.

One common rookie mistake is continuing to use the heat mat after the seeds have sprouted. Again, unless the room is unusually cold, that extra warmth is only needed for germination. Most plants grow best with a temperature at least 5-degress below their seed sprouting temperature, and seedlings really struggle (or die) if over-heated.

Alternate Ideas

• How to Use an Instant Pot to Sprout Seeds
• How to Use a Slow Cooker to Sprout Seeds

Buying Tips

If you have decided to try heat mats, here are a few tips.

Check

• Product reviews
• Size (dimensions)
• Temperature range
• Thermostat (included or buy separately)
• Return policy

Be sure to check product reviews and the return policy before your purchase. Some mats seem to be faulty from the start.

Heat mats are available in various sizes and shapes (square or rectangular). The one I use is 10 x 20-inches, the same size as a basic plant flat or cell tray.

Because I just use it for specific circumstances and just for the germinating stage (not growing), one is all I need.

Check the expected temperature range for the mat (highs and lows). Make sure it’s suited to the seeds you start indoors.

If room temperature is an issue—older basements are notorious for this—there could be more practical ways to warm up your seed starting area than trying to compensate for cool air with a tiny mat.

Basic heat mats come with an attached electrical cord and that’s about it. Some also have a kit with various seed sowing accessories.

The most important tip is to be sure to get a thermostat attachment. This may be an extra purchase unless it’s included as a set. I would never use a heat mat without one.

The whole point is to be able to warm your soil to a specified, consistent temperature to make the seeds sprout.

Without a thermostat to regulate things, some of these mats will heat way too hot. The one I tested recently went just beyond 112°F (44°C) when I left it running. That not only dries out the growing medium but wipes out any emerging seedlings.

How to Use A Heat Mat

Supplies

• Seed starting heat mat with thermostat
• Thermometer with probe (digital, for kitchen or soil)
• Plant mister (for water)
• Moisture meter
• Seeds in trays with growing medium (follow proper seed starting methods)

Set Up

1. Place heat mat on dry shelf or table in seed starting area with access to electrical outlet.
2. Place sown seed tray on mat.
3. Insert thermostat probe into growing medium in an inner cell of seed tray. Use dome cover to retain some warmth and humidity.
4. Place thermostat beside mat (not on top), away from any water.
5. Plug heat mat into thermostat.
6. Plug thermostat into electrical outlet.
7. Use product instructions to program thermostat temperature at recommended temperature for the seeds you are growing.

After allowing a few hours for mat to heat, use a thermometer to double check the temperature of your growing medium. Adjust thermostat as needed.

Care

Check your seeds once or twice a day.

• Use a kitchen or soil thermometer to check the temperature of the growing medium—that’s where it counts. Do not rely solely on the thermostat for readings.
• Never let the growing medium dry out: mist as needed to keep it moist just below seed depth.

Keep heating mat dry.

When several seeds have sprouted, remove the heat mat, and provide light instead. If the first seedlings get too much heat, they will get really leggy and may not survive. The slower seeds should still sprout later.

Heat Mat Pros and Cons

Heat mats, if reliable and used with a thermostat attachment, can deliver optimum heat levels to encourage seed germination. The question is whether it’s worth the time and money.

When using heat mats, you have to monitor things more closely than with regular indoor seed starting to be sure the mat does not overheat or dry out the growing medium.

Cost is another factor to consider. It may be reasonable to use one or two mats for small batches of seeds, but it gets expensive to use them for larger volumes.

The product directions say not to use extension cords and it’s not always possible to have suitable places for the mats within reach of electrical outlets or have several running at once.

Because the mats are water-resistant but not waterproof, they need to be kept dry and never sit in water. This means the seed trays need to be misted from above rather than the preferred method of watering from below.

If your seed starting room has consistent temperatures around 70°F (21°C) without drafts, you may do fine without ever using heat mats. The mats may speed up germination but sometimes it’s only faster by a matter of days.

Frequently Asked Questions

Resources

Read More

• Best Soil Temperatures For Flower Seeds
• Best Soil Temperatures For Vegetable Seeds
• Houseplants & Humidity: Tips That Help (& Those That Do Not)

Shop

• Heat Mat With Thermostat | Amazon

~Melissa the Empress of Dirt ♛