This story was originally published in March 2021.
Seedling season is almost upon us. Because of the short growing season, many gardeners in Maine germinate seedlings indoors before transplanting them outside.
But seedlings can be finicky, so gardeners sometimes go to great lengths to make sure their seedlings thrive. Heat mats are a great tool for starting seeds, but they need to be used in a certain way in order to be effective.
Heating mats are plastic waterproof mats that apply a steady, warm temperature to the soil while seeds are germinating. Seeds germinate best at certain temperatures, and some heat mats will allow you to adjust the heat accordingly.
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“Commercial growers have been using this forever to germinate [seedlings],” said Melissa Higgins, wholesale manager at Sprague’s Nursery in Bangor. “Most of them heat to about 85 degrees or so. It really seems to help jump those plants right up quick.”
Seedling heat mats only need to be used when you have freshly sown seeds.
“They’re only for germination,” Higgins said. “It’s a short term thing. They’re not meant to grow on the mats all the time.”
For gardeners starting seeds indoors, this consistent temperature is especially helpful given the inconsistency of heat in the home.
“Heat mats can be a very helpful tool for those of us who tend to keep our homes on the cool side or allow temps to fluctuate greatly between day and night,” said Kate Garland, horticultural specialist from the University of Maine Cooperative Extension. “Gardeners using heat mats will want to move seedlings off the mat when they begin to develop true leaves. This process of gradually decreasing temperature will help the seedlings become more well adapted going out into the garden.”
The pros and cons of heat mats
Heat mats for seedlings are useful and easy to use.
“For the most part as long as you’re using it appropriately and getting the rooting zone where your seed is germinating to the temperature that whatever crop you’re growing germinates best at, it’s usually very very helpful,” said Caleb Goossen, organic crop and conservation specialist at the Maine Organic Farmers and Gardeners Association.
Heat mats are especially useful in a place like Maine for warm-loving plants, like tomatoes, peppers and eggplants.
“We start those always on the mats because they just like a warmer temperature,” Higgins said. “Even if people have lights on, I’m not sure the lights would be hung low enough to heat that soil. We find that germination rates for seeds, especially if people are using old seeds, you’ll get a better germination rate with the mat.”
Cold-loving plants that germinate best at cooler temperatures — including vining crops like cucumber and squash, brassicas like cauliflower and broccoli, leafy greens and even early season flowers like pansies — may not benefit as much from the tool.
“It’s important to make sure and take that heat away after germination,” said Tom Estabrook, vice president of Estabrook’s Garden Center which has locations in Yarmouth and Kennebunk.
If used improperly — for example, keeping plants on the heat mats past germination — heat mats can cause problems for seedlings, like legginess.
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“They improve rooting in the young states [but] you have to be careful not to warm them up too much in a household environment,” Estabrook said. “A lot of times it’s a combination of the heat from the grow lights and the heat from the soil that will make the plants stretch.”
Even though they are only used for a short period, Garland said that seedling heat mats also use a fair amount of energy, and, like any electronic equipment, can pose a fire hazard if used irresponsibly.
When using heat mats, gardeners also need to be extra vigilant about the moisture of their soil. More water will evaporate from the soil, so humidity should be carefully monitored to prevent fungi. Gardeners may have to adjust watering to compensate for the loss of moisture.
Choosing a heat mat
Heat mats are relatively inexpensive. They can range in price from $25 to $75, depending on the size and the features included.
“These things will probably last you 10 years anyway,” Higgins said. “They’re pretty basically built.”
Estabrook recommended one with a thermostat to adjust the temperature of the heat mat for the ideal germination temperatures of whatever crop you are growing. Thermostats aren’t necessary for home gardeners, though, unless you want to get really technical.
“As long as you know it maintains a constant temperature that’s within the range you want, that’s probably all that a home gardener would need,” Goossen said. “That’s more important for a commercial farmer where they’re buying thousands of seeds. It’s a bigger investment to be concerned about protecting. If you have to reseed your peppers in your home garden, it’s not that much of a loss.”
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If you are really on a budget, though, there are some steps you can take to warm your soil without heating mats. Keep seedling trays off of the floor to take advantage of the rising properties of heat. Place freshly sown seedling trays in warmer spots of your home.
“A trick could be to place it on top of your refrigerator,” Estabrook said. “If you feel the top of your refrigerator a lot of times it’s a pretty constant warm temperature or on top of another appliance that keeps a constant temperature [but] you don’t want to put it on top of a toaster oven or something like that.”
First-time gardeners should know, though, that heat mats aren’t a requirement to successfully grow seedlings.
“I’ve gotten away with growing many seedlings without heat mats,” Garland said. “If you don’t have one, don’t let that get in the way of starting seeds indoors. If you’re trying to figure out where to invest your money in your seed starting set up, I’d start by investing in good grow lights before investing in a heat mat.”