For many gardeners, browsing through plants in a garden center is a favorite spring pastime, but some find that bedding plants can be pricey and only come in limited varieties. Starting seeds indoors a few weeks before transplanting them in the garden saves money and allows gardeners to grow many varieties, but some seeds are slow to sprout at room temperature.
Heat mats are specifically designed to gently increase the temperature of the seed-starting medium and are a boon to home gardeners. When placed beneath seed trays, heat mats offer gentle warmth—just enough heat to mimic the temperature of outdoor soil on a sunny spring day. This heat encourages seeds to sprout. The best seedling heat mats are safe, effective, and increase the seed germination rate. Ahead, learn what to look for when shopping for a heat mat, and find out why any of the following would be a beneficial addition to an indoor gardening tool collection.
- BEST OVERALL: VIVOSUN Seedling Heat Mat and Digital Thermostat Set
- BEST BANG FOR THE BUCK: VIVOHOME 20W Waterproof Seedling Heat Mat
- BEST CAPACITY: iPower 48” x 20.75” Seeding Heat Mat
- BEST STARTER KIT: Super Sprouter Premium Propagation Kit
- BEST MAT SET: Propagate Pro Wrap Around Heating Pad Strip
What to Consider When Choosing the Best Seedling Heat Mats
Raising a growing medium’s temperature up to 75 to 85 degrees Fahrenheit helps trigger germination for many common seeds such as tomato, squash, basil, and other varieties. In most homes, however, room temperature is usually 68 to 72 degrees Fahrenheit. A standard heating pad—the kind for sore muscles—isn’t recommended because it could get too hot and keep the seeds from sprouting.
In addition, heating pads are not designed to withstand the moisture that often accompanies seed starting. A seedling heat mat is the best option, both for safety and for growing success.
Seedling heat mats are available in two main types: solitary flat mats and mats included in a more extensive growing system that provides a humid environment. Either type is suitable for starting seeds.
- Flat mats: For many, placing a flat seedling heat mat beneath a plastic grow tray filled with seed-starting medium and just-planted seeds is practical and cost-efficient. A single mat is not expensive, depending on its size and accessories, and it will raise the temperature of the growing medium by about 10 to 20 degrees Fahrenheit.
- Growing system: For those wanting a more inclusive approach, heat mats are also available with grow kits that feature humidity domes. The domes keep the medium and new seedlings moist and protected. Expect to pay about twice as much for an inclusive grow kit that comes with a heating mat and a humidity dome.
Today’s seedling heat mats are designed to resist spills, surface moisture, and the high humidity levels associated with starting seeds and growing seedlings. The mats feature multiple layers of flexible waterproof plastic or PVC that encase heating wires or heat-conducting film. The outer layer is sealed to protect the internal heating elements. The mats are easy to wipe clean with just a damp cloth and will resist rusting and corrosion.
Seedling heat mats come in a variety of sizes, with the most common being 10 inches by 20 inches, which corresponds with the size of a standard plastic growing tray. Indoor gardeners who want to heat additional seed trays can find a larger size—20 inches by 48 inches—which is sufficient for holding four standard grow trays side by side.
While those are the most common sizes, other sizes are available. For example, some come in long roll-out mats for heating many trays or in narrow strips designed to fit on a windowsill.
Seedling heat mats are either water resistant or waterproof and bear associated ratings. For most indoor seed starting projects, either type is suitable.
- IPX4: A seedling heat mat that bears an IPX4 rating indicates that the mat is water resistant and will withstand sprays of water, spills, and splashes without damage. It is not, however, designed to be fully submerged beneath the water.
- IP67: A mat with a IP67 rating is fully waterproof, and it has been tested to continue functioning even after being submerged underwater for 30 minutes.
Temperature Range and Controller
Unlike standard heating pads used to soothe sore muscles, seedling heat mats don’t get hot. Most heat pads will maintain a temperature between 75 and 85 degrees Fahrenheit, although some can maintain temperatures of around 90 degrees Fahrenheit.
Some heating mats come with controllers, while others turn on when they’re plugged in and must be unplugged to turn off. A controller, while handy, isn’t essential for most home seed starting. Controllers differ in function—some allow the user to choose an exact temperature, while others offer heat levels such as high, medium, and low. Still others come with attached soil probes that measure the growing medium’s exact temperature.
Typically, gardeners store their seedling mats once the seeds sprout, but the mats can be beneficial for other projects as well. For example, they can supply the gentle heat necessary for fermenting kombucha, yogurt, or even for brewing beer.
Pet owners can also slip a seedling heat mat under a blanket or a towel to warm up a pet bed. The supple nature of seedling heat mats makes them easy to store as well—simply roll up the mat and wrap the cord around it. When it’s time to start seeds again, it will roll out easily and perform its task.
Seedling heat mats don’t come with many bells and whistles, but some feature an auto-shutoff mode that turns off the heat after a predetermined time, while others can be programmed to turn the mat on and off to mimic cyclical heat changes that occur in nature.
A few mats come with storage bags. Others come with insulating pads that can go between the mat and the grow tray to reduce heat transfer by about 5 degrees Fahrenheit.
Our Top Picks
To qualify as a top pick, a seedling mat should raise and maintain the temperature in a grow tray to between 75 and 85 degrees Fahrenheit. It should also be water resistant and well sealed. The best seedling heat mats will vary depending on how large a heating surface is desired and how many grow trays the gardener wants to warm.
If the mat is part of a humidity-controlled unit, the dome should come with ventilation to keep the seedlings from becoming overheated in direct sunshine. Any one of the following mats is well suited for helping encourage seed germination.
With the ability to maintain temperatures in the optimal range of 68 to 86 degrees Fahrenheit, the VIVOSUN heat mat is well suited to home gardeners who want to start bedding plants indoors from seed. The flat mat features a thick external layer of supple, flexible PVC, plus a layer of infrared heating film in the center. It also comes with double insulation around the edges.
The mat measures 10 inches by 20.75 inches—just the right size for slipping beneath a standard growing tray. It features an attached controller and a thermometer probe for adjusting the mat temperature while monitoring the growing medium’s temperature. The VIVOSUN is IP67-waterproof rated, so it is safe to use in wet environments, and it’s simple to clean—just wipe with a damp rag.
Increase seed germination without spending a lot of money. The VIVOHOME heat mat doesn’t come with many bells and whistles, but it raises the seed-starting medium’s temperature 10 to 20 degrees Fahrenheit above room temperature, which is usually suitable for encouraging seeds to sprout. Measuring in at 10 inches by 20.75 inches, the mat is just the right size for holding a standard rectangular growing tray, and it comes with a water-resistant IPX-4 rating.
There’s no controller with this heat mat, but users need only plug it into an outlet to start the process. The mat is made from flexible, sealed PVC plastic and features an inner layer of insulated heating wires. It wipes clean with a damp rag and rolls tightly for easy storage. Best of all, it increases seed germination at an affordable price point.
If you’re looking to start multiple trays of seedlings indoors, consider this option from iPower, which is capable of accommodating four standard grow trays placed side by side. The mat measures a beefy 20.75 inches by 48 inches and will raise the temperature of the growing medium 10 to 20 degrees Fahrenheit above room temperature.
The mat features a supple PVC plastic outer layer with an inner layer of iPower carbon heating film. The iPower mat is IPX-4 water resistant but doesn’t come with a controller. Plug the mat into a household outlet to start heating and unplug it to turn it off. As a bonus, users also can place this mat under fermenting kombucha, yogurt, or beer.
For an all-inclusive seed starting kit, check out this kit from Super Sprouter, which comes with a seedling heat mat, a plastic grow tray, and a large clear dome that covers seedlings until they reach 7 inches high. The kit also comes with a grow light that will provide the light seedlings need until they’re ready to transplant outdoors. The Super Sprouter kit can help sprout seeds in homes that lack a sunny window in which to place the seedlings once they sprout.
The flexible heating mat measures 10 inches by 20 inches, is IXP-rated for water resistance, and it turns on by plugging the cord into an outlet—no controller is included. The dome is made of durable clear plastic and supports a T5 fluorescent light tube in its center (included). The plastic grow tray is also 10 inches by 20 inches, and it can hold growing medium, peat moss grow pellets, or plastic growing containers. The heating mat raises the growing medium’s temperature 10 to 20 degrees above room temperature. The dome will retain humidity and reduce the need to add water.
At just 3 inches wide, this set of two windowsill heat mats make it possible to start seeds next to a window. Each mat increases the growing medium’s temperature 10 to 20 degrees Fahrenheit, and a single controller allows the user to select the desired temperature for each mat individually. The mats are 20 inches long and can also be used on a countertop or a table if desired.
With the dimensions of these strip-style mats, they also are suitable for wrapping around containers of fermenting kombucha or home brew to speed fermentation. The mats are IP67-rated for waterproofness, and buyers will also receive a color-coded fermentation thermometer strip.
FAQs About Seedling Heat Mats
Starting seeds indoors to later transplant to a garden or pots is growing in popularity, and seedling heat mats speed the process. Still, new gardeners will likely have a few questions.
Q. What is the right size for a seedling heat mat?
For starting seeds in a standard grow tray, purchase a seedling heat mat that’s 10 inches wide by 20 inches long. For starting seeds in multiple trays, consider a 20-inch by 48-inch mat. If only a windowsill is available, purchase a windowsill mat that measures approximately 3 inches wide by 20 inches long.
Q. What soil temperature is needed for seeds to germinate well?
Read the back of the seed packet. Most annual flowers, herbs, and vegetable seeds need temperatures between 75 and 85 degrees Fahrenheit to sprout.
Q. What type of growing medium should be used for germination?
A soilless seed-starting medium or sterile potting soil are the best options for encouraging germination. These formulations are germ-free and contain various ingredients such as peat moss, perlite, and coconut coir. Avoid starting seeds in garden soil, which can contain contaminants.
Q. When should seedlings be removed from the heat mat?
As soon as they sprout. Often, even if only a handful of seeds sprout, the others will sprout after the grow tray is removed from the heat because they already have benefited from the added warmth.