Are your tomato or basil seedlings struggling to grow? A seedling heat mat may be just what you need to help get your new seeds off to a good start.
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Using a Seedling Heat Mat
Adding a seedling heat mat to our seed starting operation has made a great improvement to how quickly our seedlings germinate and has also improved the health of many of our warmer season crops.
I have had some questions over the years about using a seedling heat mat so I thought I’d write a quick article to explain the benefits and why this small added expense can really improve your seed starting efforts this year.
Check out this video that I did on using a heat mat:
What is a Seedling Heat Mat?
Seedling heat mats are little mats that you can use under your seedling pots and flats. They raise the temperature of the soil between 10 to 20 degrees compared to the ambient temperature of the surrounding air. Commercially purchased seedling heat mats come in 3 different sizes meant to warm 1, 2, or 4 traditionally sized nursery flats.
These heat mats are made specifically for growing seedlings and have been designed to be safely used in the moist environments of seed starting setups. The same can’t be said of many DIY options that really pose a risk of electrical fires.
Why Do You Need a Seedling Heat Mat?
The short answer to this question is, warmer soil means faster germination! By increasing the soil temperature where you have planted your seedlings they can germinate in half the normal time. The warm soil temperatures give your seeds a boost and get them growing quickly.
The first few years that I started my own seedlings I didn’t have a heat mat. My seeds seemed to take forever to germinate and I would have very uneven germination times. One seed might germinate a week sooner than the rest of the seeds.
When dealing with warm-season crops like tomatoes, peppers, and many herbs the added bottom heat will also help to stimulate the warm weather conditions that they like to grow in. This makes for stockier, healthier plants.
When Should You Use a Seedling Heat Mat?
Heat mats are perfect for starting seedlings in the cooler spring weather. This is especially true if you are growing your seedlings in areas that are cooler like a garage, garden shed, or even a greenhouse. The seedling heat mat will give you good bottom heat and help get those seedlings growing.
If you are growing your seedlings in an area that is warmer (above 75 to 80 degrees Fahrenheit) then you need to be really careful using a seedling heat mat. At that air temperature, the soil is usually warm enough for germination on its own.
The added bottom heat will cause your soil to dry out quickly. I have also found that using a heat mat when the weather is warmer causes your seedlings to grow a bit too fast and they become leggy even if you have your lights close to them.
This can also be the case if you are growing your seedlings indoors in your home, but an indoor temperature of 68 to 70 seems to still benefit from a seedling heat mat.
How Long Should You Keep Your Heat Mat On?
How long you keep your heat mat under your seedlings will depend on the type of seeds you are growing and the time of year. For most cool-season crops I only leave the seedling heat mat under the pots until all of the seedlings have germinated. So depending on the ambient air temps, the heat mat may only be under the seedling pots for a week to 10 days.
For warm-season crops, I have found that the plants benefit from the added heat. So once the seedlings have germinated I will leave the heat mat under the pots for another week to 10 days.
What Size Mat Should You Buy?
I only have one of the single-sized seedling heat mats and that is usually enough for us. My planting is staggered during the season, so I can simply move the heat mat from one flat of plants to another.
However, I will admit that during the busiest time of the year for seed starting I have wished for another mat. I think someday I will either buy another single seedling heat mat or I might buy one of the larger ones that can cover 2 flats at a time.
Expected Cost and Other Gadgets You Might Like
Expect to pay around $20 for a single heat mat. The ones that cover 4 flats can be as much as $70. They also make a handy heat mat temperature controller. I don’t own one of these, but I could see a use for them.
The controller monitors the soil temperature and keeps it at a constant temperature. If things get too hot where you are starting your seedling the controller will turn off the heat mat to keep you from overheating your seedlings and killing your plants.
I hope this article helps your seedlings to be more successful! The supplies you need for starting seedlings are pretty simple and you can get away without having a seedling heat mat. But I have found that the benefits of having a heat mat, outweigh the small upfront cost.
Heat mats do seem to last a long time. The one I have is 8 years old and still going strong! So, go buy yourself a heat mat today!!
Would you like to learn more about seed starting? Try my Seed Starting Simplified video course on The Online Gardening School.
What I am trying to figure out is if I have raised gardens and pvc and plastic sheet greenhouses. How much earlier can I put plants outdoors? I am in zone 7b north of Raleigh NC.
Those should help you to get your cool-season crops started at least a month early. I would still be careful with warm-season crops, on a cold night, it might still freeze inside those unheated greenhouses.