Recycle Christmas Lights to Make a Heat Mat for Seed Starting

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Here I explain how to make an inexpensive heat mat for seed starting from Christmas lights. I went into Goodwill after Christmas and was astounded to see how many people donated Christmas lights. My first thought was: I’m glad they didn’t just throw them away. My second thought was: Upcycle them!

However, it’s important to be diligent about safety. This is why I likely wouldn’t buy strands of lights from a thrift store. The exception is if I was able to plug them in first and verify they were in great condition.

If you have LED holiday lights, they aren’t ideal to use for a heat mat because they don’t heat up like incandescent lights do.

Keep reading to learn about safety and how to make a heat mat using holiday lights.

Safety is Paramount

When using Christmas lights to make a heat mat for starting seeds, safety is essential. It’s crucial to ensure the lights are in good condition and not damaged. Please read all of these safety tips before DIY-ing a heat mat.

  1. Old or defective lights can overheat and ignite. Electric shock is another danger with using lights.
  2. Do not use any strings of lights that have frayed wires, flickering bulbs, missing bulbs, hot cords or outlets, or broken bulbs. Be sure there is nothing that would pose a fire hazard.
  3. Plan to monitor the heating mat to ensure it doesn’t overheat. Use a thermostat or thermometer and check it regularly.
  4. Avoid electrical risks by setting it up in a dry area, away from contact with water or excess moisture.
  5. Unplug the lights before watering the seeds. Be sure to not get any part of the lights strands wet.
  6. Be extra careful if you are using a plastic tray or any plastic as the base for the lights.
  7. Always follow electrical safety guidelines. Consult a professional if you are unsure about the safety aspects this DIY project.
  8. Use a thermostat to monitor the temperature; it will function as an on/off switch for the lights.
  9. Be sure to turn off the lights when you aren’t around to monitor them. Just as you wouldn’t leave your Christmas lights on for days at a time, the same is true as when you are making a heat mat.

You can use a thermostat to monitor the heat. It will turn the lights off when the mat gets hot. You can set it so the soil is around 70 – 75°F (21-24°C), a good temperature for most seeds to germinate.

You can also turn off the heat mat lights before you go to bed. Turn them on again when you wake. Throughout the day, periodically check on them to ensure they aren’t overheating.

According to the Consumer Product Safety Commission (CPSC):

  • Don’t string together more than 3 sets of incandescent lights together.
  • Don’t overload electrical outlets.
  • Use only lights that a well-known testing lab has checked for safety. It will be on the label. If the label is removed, do not use.
  • Inspect all new and used lights for broken sockets, damaged wires, or loose connections. Throw away sets that are damaged.
  • Make sure the extension cord is suitable for its intended use and in good shape. Don’t use cords with frays or cuts.

Using Incandescent Lights vs LED Lights

If your Christmas lights aren’t LED, they are typically incandescent lights. Incandescent Christmas lights are the traditional, older style of lights that people often string on their trees.

Incandescent lights also generate more heat compared to LED lights which is why they are better to use when making heat mats to germinate seeds.

These lights work by passing electricity through a wire filament, heating it until it glows and produces light. Incandescent Christmas lights are known for their warm glow and are less energy-efficient than LEDs.

Incandescent lights also tend to have a shorter lifespan than LED lights so it’s very important to monitor them regularly.

You can use Christmas LED lights as grow lights but they don’t produce enough heat to be used to germinate seeds.

How to Tell if You Have LED or Incandescent Lights

  1. Feel for heat: Incandescent lights heat up because they use a filament to produce light. LED lights don’t heat up in the same way because they use energy bands to create light. If your lights stay cool after being on for a while, they’re probably LED.
  2. Check for glass bulbs: If your lights have glass bulbs, they’re likely incandescent. Glass LED bulbs exist but are rarer.
  3. Look for a filament: In smaller bulbs, like mini lights, seeing an “old-fashioned” filament usually means they are incandescent. However, there are some LEDs made to look like retro, incandescent lights.
  4. Look at the color: Incandescent lights may have a softer glow. LED lights tend to have a more vibrant, “electric” look, especially in the red, green, blue, and bright white lights.

Note: Some LED lights are made to mimic incandescent bulbs, including having a synthetic filament look or being made of glass. The best test is to see if they warm up after being on for 10 minutes or so.

How to Make a Heat Mat for Seed Starting Using Christmas Lights

Follow these steps where I explain how to make an inexpensive DIY heat mat with Christmas lights.

Level of difficulty:

Moderate. It requires some basic understanding of safe electrical practices and careful assembly.

Equipment needed:

  1. Incandescent Christmas lights in great condition
  2. Flat, waterproof surface (such as a large tray)
  3. Aluminum foil
  4. Electrical tape or duct tape
  5. Strongly preferred: Thermostat or thermometer
  6. Seedling trays or pots
  7. Soil and seeds

Examples of what you can use for a waterproof surface:

  • Seeding tray, baking sheet, cookie trays
  • Plastic trays or seedling trays
  • Baking sheets or cookie trays
  • Glass or plastic cutting boards
  • Waterproof table mats

How a thermostat will help

A thermostat will control the temperature of your heat mat, ensuring it’s warm enough for seeds to germinate but not too hot. You should place the thermostat’s sensor directly on the heat mat surface, near where the seed trays are located.

While a thermostat cannot change the temperature of the Christmas lights, it will function as a controller for the power supply to the lights. You will connect the thermostat to the Christmas lights. It will as an on/off switch based on the temperature setting, usually between 70 – 75°F (21-24°C), for optimal seed germination.

This on/off cycling maintains a more consistent temperature, which is crucial for seed germination. Without a thermostat, the lights would continuously produce heat (unless you turned them off yourself), potentially leading to overly warm conditions.

Choose the right thermostat: Ensure you have a thermostat that will be compatible with your setup. Some thermostats are designed specifically for seedling germination or heating mats. You can buy one from Home Depot, Lowe’s, ACE Hardware, Amazon, etc. for around $20.

If you don’t have a thermostat

A DIY heat mat project using Christmas lights really works better if you have a thermostat to monitor the temperature and turn the lights on and off. If you don’t have a thermostat, you can use a thermometer:

  • Indoor-outdoor thermometer
  • Aquarium thermometer: Ideal because it’s waterproof
  • Regular room thermometer: Can give you a good idea of the temperature when placed on the heat mat
  • Meat thermometer: Not ideal but you can check the temperature of the soil

How Many Strings of Lights Do I Need?

The number of Christmas light strings needed for a heat mat depends on the size of the mat you’re creating and the size of lights you’re using. Here are some general guidelines:

Size of the mat: Measure the area of the surface you’re using for your heat mat (like a tray or a shelf). This will determine how many lights you need to cover the area effectively.

Size of lights: For this project, I recommend strands of the smaller incandescent bulbs and not the large bulbs due to the amount of heat the large bulbs produce.

Spacing: You’ll want to lay out the lights so heat is evenly distributed. Avoid overcrowding but ensure there are no large gaps.

A rough estimate:

For a small mat (like a standard seed tray, approximately 10×20 inches), one string 25 – 50 incandescent lights should be enough.

For a larger area, like a shelf or a bigger tray, you might need multiple strings or a longer string of lights.

Test your heat mat before using it to ensure it provides consistent warmth without overheating. Safety is essential so monitor the temperature regularly.


  1. Inspect the lights and wires: Make sure the Christmas lights are in good condition. Check for any exposed wires or broken bulbs. Do not use the string of lights if any are not in good condition.
  2. Prepare the base: Use a flat, waterproof surface like a large baking tray as the base. It’s important to ensure it’s big enough to lay out the lights and fit the seedling trays.
  3. Lay the lights: Spread the Christmas lights evenly on the base. Try to make them as flat and even as possible.
  4. Cover with aluminum foil: Cover the lights with aluminum foil. Wrap around to the bottom of the tray. Secure the edges of the foil with tape.
  5. Connect the lights to thermostat: The thermostat will have an outlet where you plug in the light string. First, plug the thermostat into your power source, then plug the lights into the thermostat. The thermostat controls the power supply to the lights, turning them on or off based on the temperature setting.
  6. Set the temperature: Most seeds germinate well at temperatures between 70-75°F (21-24°C). Set the thermostat to maintain this temperature range.
  7. Set the temperature probe on the aluminum foil: The thermostat usually comes with a probe that senses the temperature. Place it on top of the foil covering the lights. After running the lights for around 30 minutes, you can adjust as needed.
  8. Test the heat mat: Test the setup for a day to ensure it maintains a consistent temperature. Make adjustments to the placement of the probe or the temperature setting if necessary.
  9. Place seedling trays or pots: Set seedling trays or pots filled with soil and seeds on top of the foil-covered lights. The warmth from the lights will encourage germination.
  10. Monitor temperature: Regularly check the temperature, especially if you’re not using a thermostat.
  11. Watering: Be VERY CAREFUL not to let water seep into the electrical components. Always disconnect the lights before watering to avoid any risk.
  12. Wait for them grow
  13. Regular monitoring: Even with a thermostat, periodically check the temperature and ensure everything is working safely.

Using Christmas lights for warmth will hopefully promote faster and more consistent germination. By using a thermostat, you add a level of safety and precision to your DIY heat mat, promoting safe seed germination.

Heat Mat for Seed


What Is a Heat Mat

Think of heat mats for starting seeds as warm pads that help seeds sprout faster by keeping the soil warm. This warmth helps to speed up the germination of seeds, especially for plants that need higher temperatures, like tomatoes and peppers.

Heat mats are especially useful in cooler climates or during early spring when the natural soil temperature may be too cold for seeds to germinate.

Can you use LED lights to make a heat mat?

LED lights aren’t suitable to make a heat mat for germinating seeds. This is because they are made to be energy efficient. They remain cool when on and don’t produce heat like incandescent bulbs.

Can I use LED Christmas lights to grow plants?

Seed germination typically requires a consistent and moderate heat source to encourage growth. While the light from LEDs can be beneficial for the photosynthesis process once the seeds have germinated, they do not provide the necessary warmth to aid in the initial germination of most seeds.

For seed germination, it’s more effective to use a dedicated seedling heat mat or another controlled heat source designed for this purpose, which maintains the proper temperature range for germination.

What temperature does the soil need to be for seeds to germinate?

Ideal temperature for seed germination is between 70 – 75°F (21-24°C) for most seeds.

How to Build a Grow Rack for Seed Starting
Grow rack for seed starting ~ This is what I was using before I made a heat mat from unwanted Christmas lights
Recycle Christmas lights
Homemade ornaments for Christmas tree ~ No electricity required!

Repurposing Christmas Lights to Grow Seeds

Rather than donating or throwing away unwanted Christmas lights, consider making this easy DIY heat mat. After the holidays and all the busyness, it’s a fun project to get you thinking about and ready for spring planting.

When making a heat mat, only use lights that are still in good condition. If there are broken lights on the string or frayed wires, do not use them. You can recycle them in other ways but don’t use them for this DIY heat mat.

Remember to use incandescent bulbs, which give off heat, not LEDs. Glass bulbs are usually incandescent. Turn them on. If they generate heat, they are incandescent, not LED.

Setting up a thermostat for this project makes it safer to ensure it doesn’t overheat. Hopefully that will be the only expense if you don’t have one already. Then, when the seeds germinate, you can unplug it and use it for something else.

It’s So Easy & Fast to Make

Did you make it? I hope so!

Thanks for learning how to make inexpensive heat mats from Christmas lights. Using recycled lights is a great example of upcycling. It’s a way of repurposing an everyday item for something new. It’s about minimizing waste and making the most of what we have.

I always look for innovative and cost-effective ways to garden and improve my plants’ growth. Yes, you can buy heat mats from a nursery, garden center, hardware or home improvement store, but here I hope you’ve learned how to DIY one easily.

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