Thursday, March 21, 2024

Heat Mats for Indoor Seed Germination by Brenda Sterns

As March brings 50-60°F warm days combined with all the winter snows, our yards are showing slight signs of life. For many of us, this tinge of greenness ushers forth images of what our 2024 gardens will hold.  What will we grow – vegetables, annuals, perennials?   We know the last frost date is two months away and now is the time to start seeds indoors.  As you eagerly grab your seeds, pots, and soil to start your best garden ever, take a moment to think about heat mats.

Let’s start with some benefits associated with heat mats.

1. The consistent warmth provided by a heat mat increases the number of seeds that germinate and provides synchronized germination.

2. Seedlings started on heat mats can exhibit faster growth rates compared to those grown without heat. The warmth from the heat mats promotes root development and enhances nutrient uptake.​

3. Damping off, a fungal disease that affects seedlings, can be minimized by using heat mats. The heat mat creates a drier environment around the seedlings, reducing the likelihood of fungal pathogens thriving in moist conditions.​

However, there’s more to think about.   Let’s turn our attention to our house conditions.  Where are you going to start your seeds? By a window, on the table in a common heated space, or in the basement where it is cooler?  What type of seeds do you want to grow – warm season vegetables that prefer 80-85°F to germinate? Spring annuals that prefer 70-75°F temperatures? 

If you are considering a window for seed germination (which later will meet your seedling light needs), realize that the temperature by the window will fluctuate throughout the day from lows (below germination needs) to highs (good for germination).    I measured the daily temperatures at two windows over a cold week in February – one with low E glass and one with older double pane glass.   For the low E glass location, temperatures reach 75°F and above for less than 3 hours a day. For the older double pane glass, the infrequency of being above 75°F is not conducive to seed germination.

Low E Glass (Brenda Sterns)

Old Double Pane (Brenda Sterns)

Some online sources recommend putting seed trays above your refrigerator.  I measured the temperature above my refrigerator over several days.  The thermometer shows between 68-72 degrees.  This is not warm enough for seed germination in many cases.   Plus what a pain it is to make sure the soil is moist and check for germination every day.

Soil temperature is actually cooler than the ambient air temperature – up to 5°F cooler.   If you keep your house near 70°F during the winter months, this setting could translate to 65°F in the soil.

Heat mats can be a great aid to successful seed germination.  If you do want to purchase a heat mat, please read all the specifications listed for the product.  Is the manufacturer one who specializes in plant products and knows what to build?  Do not get trapped by keywords that can lure you into a poor purchase decision.  Not all heat mats are created equal.  Understand that mats come set to a small temperature range or are adjustable across a large range.  For fixed temperature mats, find out what the temperature range is.  Some specifically state a range like 70-85°F which is good for warm season seeds.  Some state a temperature range above ambient air temperature.  Pay attention to Celsius and Fahrenheit on packaging (5-10°C is about 10-20°F). Consider if the upper temperature range could be too hot for the seeds you want to grow.   Do you grow different types of plants that have different optimal germination temperatures - warm vs cool season plants?  Do you change what you grow year to year?  You may be better off with a variable temperature mat (or mats) where you can specify exactly to what temperature the soil is heated.