How To Keep Seeds Warm (11 Ideas For Indoors & Outdoors)


Are you looking for a way to keep seeds warm for germination – without heating up the whole house?  Luckily, there are lots of ways to keep seeds warm without breaking the bank.

To keep seeds warm indoors, you can use a seedling heat mat, a humidity dome, an enclosed box, warm lights, warm water, a space heater, a boiler room, or even a refrigerator.  To keep seeds warm outdoors, you can use a cloche, a cold frame, a greenhouse, or a layer of plastic over the soil.

Of course, you can combine some of these methods to give seeds a little extra heat so they germinate quickly.  Just remember not to overheat them or dry them out – keep an eye on your seeds!

In this article, we’ll talk about 11 different ways to keep your seeds warm (both indoors and outdoors).  We’ll also explain the setup for some of the more complex ones.

Let’s get going.

How To Keep Seeds Warm

There are lots of ways to keep seeds warm, both indoors and outdoors.  That’s a good thing, because soil temperature is one of the most important factors for seed germination.

thermometer
Temperature is one of the most important factors for seed germination.

If the soil is too cold, seeds will germinate slowly, or not at all.  If the soil is too warm, seeds are more prone to drying out, and extreme heat can also prevent germination.

The ideal temperature for seed germination depends on the plant.  However, you really cannot go wrong with a soil temperature of 75 degrees Fahrenheit (24 degrees Celsius).

This soil temperature is warm enough for warm-weather crops (such as tomatoes), but cool enough for cool-weather crops (such as lettuce).

tomtao seedlings
Tomato seeds can germinate within a week at a soil temperature of 75 degrees Fahrenheit (24 degrees Celsius).

You can learn more about ideal temperatures and time to germination in this resource from the University of California.

To achieve these ideal temperatures, you’ll need to keep the soil warmer.  Let’s begin with ways to do that if you are starting seeds indoors.

How To Keep Seeds Warm Indoors

It is a bit easier to keep seeds warm indoors, since the air will be warmer than it is outside (at least during winter and early spring).

However, most people don’t keep their houses at the ideal seed germination temperature of 75 degrees Fahrenheit (24 degrees Celsius).

Furthermore, the University of Minnesota Extension suggests that potting mix in an indoor container can be as much as 5 degrees Fahrenheit cooler than the surrounding air.

Here are a few ways to keep seeds a little warmer than the air inside your house.

Put Seeds On A Seedling Heat Mat

A seedling heat mat works by providing heat for seeds from below.

seed tray
You can put seed trays on a seedling heat mat to keep them warm.

Here is how it works:

  • Plug in the seedling heat mat and lay it on a flat surface (table or bench).
  • Put the seed tray or container on the heat mat.
  • Fill the seed tray with seed starting mix.
  • Plant the seeds in the mix.
  • The heat from the mat will move up into the seed starting mix to warm the seeds.
  • Leave the heat mat on until the seeds germinate.
  • Unplug the heat mat after the seedlings emerge.

There are different sizes and types of seedling heat mats available.  Here are a few:

As you can see, there are lots of choices for a seedling heat mat – and not just the ones listed above.

If temperatures fluctuate in your house, you may want to choose one with a thermostat/controller combo.  The extra cost may be well worth it to avoid losing your seeds due to extreme heat or cold.

How Long Do You Leave Seeds On A Heat Mat?

Leave seeds on the heat mat until they germinate (sprout).  According to Horticulture magazine, seeds will sprout faster with warm and even temperatures (this is where a thermostat/controller comes in handy).

plug plants
Unplug the seedling heat mat after the seeds germinate.

After the seeds germinate, remove them from the heat mat.  Otherwise, the heat may encourage fungal diseases and damping off of your seedlings.

Of course, it is possible to start seeds and keep them warm without a seedling heat mat.  Let’s take a look at some seedling heat mat alternatives, starting with one that doesn’t need electricity: a humidity dome.

Put Seeds In A Humidity Dome

A humidity dome is a cover placed on top of a seed tray (or any container that holds the growing medium and seeds).  It traps moisture in the soil and air underneath, which improves conditions for seed germination.

A humidity dome is made of plastic or glass.  That means that is also acts like a mini greenhouse to retain heat under the dome.

plastic containers
In a pinch, a plastic container & lid serve as a humidity dome.
Image Courtesy of:
Stebulus via:
Wikimedia Commons: https://commons.
wikimedia.org/wiki/File:
Tupperware_translucent_
blue.jpg

Note that a humidity dome will not generate heat.  Instead, it will trap heat underneath, reducing the risk that your seeds will get too cold.

This can be helpful if you are relying on sunlight on a windowsill to warm up the soil and seeds.

A humidity dome has several benefits for seeds, including:

  • Faster Germination – seeds that are kept at the ideal temperature will sprout sooner.
  • Seed Protection – a humidity dome can protect seeds by sealing them off from any pests or diseases that might be present in your home.
  • Higher Germination Rate – keeping soil warm and moist will improve seed germination rates, leading to fewer wasted seeds and more plants for your garden.

Some humidity domes have vents, which allow excess heat and humidity to escape.  However, you can close these vents to keep the soil warmer.

You can find humidity domes in round, rectangular, or square shapes.  Of course, you can also find humidity domes that will fit perfectly on a standard 1020 seed tray, such as this one from Bootstrap Farmer.

You might also choose to make your own small humidity dome from a plastic clamshell container.  Be creative and you will think of all sorts of ways to make a custom humidity dome!

You can learn more about humidity domes for seeds in my article here.

Put Seeds In An Enclosed Box (Heated & Insulated)

You also have the option of building an enclosed box to keep your seeds warm indoors.  If you insulate the box, it will retain warmth from any heat source you use.

Here are some options for the insulated box itself:

  • Styrofoam – if you have a Styrofoam box, it will make the perfect place to keep seedlings warm.  Styrofoam will act as insulation to trap heat inside the box.  To get the same effect, you might also be able to use 5 separate flat pieces of Styrofoam to line the bottom and sides of a cardboard box.
  • Aluminum – aluminum foil is readily available at grocery stores.  You can use it to wrap the insides of a cardboard box or plastic container to trap heat within.  If you are using a light source to provide extra heat, the aluminum will also reflect more light back to seedlings after they sprout and start to grow.
  • Cardboard – in addition to lining with Styrofoam or aluminum, you can also use a blanket to line the sides of a cardboard box.  This will help to trap heat inside the box and slow down cooling.
  • Wooden Crate – a wooden crate is sturdy, and you can line it with either Styrofoam, aluminum foil, or a blanket to help trap heat inside.

Be sure to make a cover for your insulated container.  You can use any of the materials above, but a flat piece of cardboard covered with aluminum foil on both sides should work well.

styrofoam
Styrofoam insulates to keep heat inside a container.

If you prefer to keep an eye on things, you can use a clear piece of plastic or a sheet of clear plastic wrap to insulate the container and trap heat and moisture inside.

Once you have an insulated container, it’s time to find a heat source.  After all, an insulated box will not generate heat – it will only trap heat.

Here are a few options for heat sources in an insulated container:

  • Hot Water Bottle – you can use any container that will hold hot water and give off heat.  For example, a regular clear plastic bottle with hot water in it will work fine.  For a larger insulated box, you might need to replace the hot water more often.  Just be careful if you use this method together with any electricity (seedling heat mats, lights, etc.)
  • Hand Warmers – disposable hand warmers use a chemical reaction that involves iron powder, salt, water, activated carbon, and an absorbent material.  When you “break” the pouch, the materials mix, and an exothermic reaction gives off heat to warm your hands.  If you put hand warmers in your insulated box, they will do the same for the soil and seeds.  They can last for 10 to 13 hours, so you might be able to get away with changing them once in the morning and once at night.  Just keep an eye on things to make sure the soil doesn’t get too hot!

Another possible heat source is warm lights above the seed tray.  However, you might not want to enclose a light, or else you risk a fire.

water jugs
A plastic bottle filled with hot water will give off heat over time to warm soil.

Put Seeds Under Warm Lights

You can keep seeds a bit warmer by putting the tray under a light that gives off heat.  However, not all lights are created equal when it comes to heat generation.

LEDs are nice and bright, and they are super-efficient because they don’t produce much heat.  However, they won’t do much to keep your seeds warm.

Fluorescent light bulbs give off more heat than LEDs, but still far less heat than incandescent bulbs.

Incandescent bulbs give off lots of heat energy, which makes them inefficient for lighting (but great for keeping seeds warm!)

incandescent light bulb
An incandescent light bulb gives off heat, which can help to warm up soil and seeds.

Keeping your seed tray in an insulated container (without a top) will help to trap some heat from an overhead light.

According to Sewell Direct, light bulbs burn at the following temperatures:

  • LED: 87.2 degrees Fahrenheit (31 degrees Celsius)
  • Fluorescent: 179.2 degrees Fahrenheit (82 degrees Celsius)
  • Incandescent: 335.4 degrees Fahrenheit (169 degrees Celsius)

As an added bonus, you can use these heat sources as grow lights after seedlings emerge.  As with any seed heating method, keep an eye on things to make sure they don’t overheat!

Spray Soil With Warm Water

Eventually, you will need to water the soil to keep it moist enough for proper seed germination.  Why make it colder when you can make it warmer?

spray bottle head
If you spray the soil to keep it moist, use warm water instead of cold.

Cold water will cool down the soil.  Instead, run the water for a minute and let it warm up before you spray the soil.

As an alternative, just let the water sit out for a while.  This will bring it up to room temperature, which is better than using cold water from the tap.

There is one thing to remember, though: don’t make the water too hot!  Seeds don’t like extreme cold, but they don’t like extreme heat either.

An extreme temperature in either direction will slow down seed germination (or prevent it altogether).  So, use warm water (not cold or hot!)

Put Seeds Near A Space Heater

Before you decide to use a space heater, make sure you are using a model built for safety.  Some older ones are dangerous and can start fires without proper care.

If you have a small room in your house, you could put the space heater in there along with your seed trays.  Then, you can close the door and let the heat warm up the soil.

space heater
A space heater can keep a small space warm to help seeds germinate.
Image Courtesy of:
Roger McLassus 1951 via: Wikimedia Commons: https://commons.
wikimedia.org/wiki/File:
Fan_heater_2005.jpg

If you are nervous about a fire, you can sit in the room and “babysit” the space heater and seeds during the day while reading, writing, or researching seed catalogs!  At night, you can turn off the space heater and use a seedling heat mat to warm the soil.

As with any heating method, monitor the temperature to make sure you are not overheating your seeds.

Put Seeds In The Boiler Room

The area near your hot water heater or heating system will be warmer due to the heat given off.  You can put that heat to good use by putting your seed trays there.

Just set up a table or bench and put the seed trays on them.  Don’t put the trays right on the hot water heater or heating system.

This setup might work well with some incandescent lights to heat things up a bit more.  Just be sure to monitor the temperature to be sure the seeds don’t get overheated.

Put Seeds On Top Of The Refrigerator

According to the University of Maryland, a refrigerator, especially an older one, can give off a lot of heat on the top.  If you put a seed tray on top of the refrigerator, it can keep the soil a little bit warmer.

Since the refrigerator runs constantly, it should keep the soil at an even temperature.  You might also be able to get the same heat effect from a freezer.

refrigerator
Refrigerators: cold inside, but warm above!

The same could apply for any appliance that gives off heat, such as drying machines.  The only problem is that they aren’t running constantly, so the heat will be sporadic.

Still, if your laundry room is a little bit warmer than the rest of the house, it could help seeds to germinate just a little faster.

How To Keep Seeds Warm Outdoors

Some seeds need to be planted directly in the soil, such as carrots.  It is a bit more difficult to keep seeds warm outdoors, since you don’t have control over the weather.

Here are a few ways to keep seeds a little warmer outside.

Put Cloches Over Seeds

A cloche is a great way to keep soil and seeds a little warmer in the great outdoors.  In addition to trapping heat, a cloche will protect plants from wind and pests.

A cloche is a clear plastic or glass cover that goes over the soil.  It acts like a mini greenhouse by trapping heat in the air and soil underneath.

You can make your own cloches out of plastic bottles.  Just cut out the bottom of the bottle (leave the top intact).

water bottles
Make a cloche out of a plstic bottle by cutting out the bottom.

Then, put the bottle (cut end down) on top of the soil over the spot where the seed was planted.  You can remove the cap on top of the bottle to act as a vent if it gets too warm under the cloche.

Of course, you can also buy ready-made cloches if you like (check out my cold protection resource page to learn more).

If you want, you can also wrap a frost blanket or row cover around a cloche to provide extra insulation.

Put Seeds In A Cold Frame Or Greenhouse

A cold frame is bigger than a cloche, but smaller than a greenhouse.  It follows the same concept, though: it uses plastic or glass to trap heat and keep soil warmer for seed germination and plant growth.

You can learn more about cold frames from the Colorado State University Extension.

A cold frame is a better option for tall plants that you will transplant later in the season (such as tomatoes.)

A greenhouse is a better option for plants with shorter growth (such as lettuce) or plants that do not transplant well (such as carrots).

greenhouse wedding
A greenhouse will keep retain heat to keep seeds warmer.

You can find a link to cold frame and greenhouse plans on Etsy on my cold protection resource page.

Put A Layer Of Plastic Over The Soil

If you don’t have space or time to build a cold frame or greenhouse, there is another option to keep soil and seeds warm.

Instead of using cloches to warm individual seeds, you can use a layer of plastic to warm an entire area in the garden.

plastic wrap
A piece of clear plastic wrap will help to warm up your soil to help seed germination.

There are a few different options for warming up the soil outdoors to germinate seeds:

  • Clear Plastic – a clear plastic sheet will trap heat underneath, but will also allow sunlight through.  This might be a good option if you want to keep plants warm and give them sunlight after the seeds germinate.
  • Black Plastic Mulch – black plastic mulch will absorb more heat from sunlight than lighter materials.  However, it won’t let sunlight through (this can be a benefit, since it also suppresses weeds).  This is a good option to heat up cold soil after the sun starts shining.  You will need to remove black plastic mulch after seeds start to germinate, since all seedlings need light to grow.
  • IRT (Infra-Red Transmitting) Plastic MulchIRT plastic mulch suppresses weeds just like black plastic mulch.  However, it allows infrared light through, which helps soil to warm up faster.  After seeds germinate, you could cut holes in the mulch to make spaces for the plants to grow and receive sunlight.

No matter which type of plastic you use, you can use soil, mulch, or rocks to weigh down the edges and keep heat trapped underneath.

Just be sure to take off any plastic covers before it gets too cold. You can learn more about when you can leave plants covered all day in my article here.

Conclusion

Now you know how to keep seeds warm for germination, even without a seedling heat mat.  Maybe you also got some ideas and inspiration for alternative ways to keep your seeds warm.

You can learn about more ideas for making your own seed trays in my article here.

You might also be interested in learning how to grow seeds in sponges.

I hope you found this article helpful.  If so, please share it with someone who can use the information.

If you want to read some of my most popular posts, check out the “Best of GreenUpSide” page here.  Enjoy!

~Jonathon

jonathon.david.madore

Hi, I'm Jon. Let's solve your gardening problems, spend more time growing, and get the best harvest every year!

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