Some Like It Hot.....And Some Don't

I decided to do a post today about the heat mat that I purchased a couple of months ago.  I have never had one before so I am just figuring out how to use it effectively.  But before I continue, let me just show you what I woke up to this morning:
Snowstorm in April - Just lovely (can you hear my sarcastic tone?)
We basically had a blizzard.  In April.  Are You Kidding Me??
Yesterday, we had temperatures in the 20’s (68°F+); this evening, it’s supposed to go down to -5°C (23°F) with a wind chill of – hold onto your parka – minus 16°C (3°F)!!  Enough already!
Ok – rant is over.  Back to our regularly scheduled program.
When I first got the heat mat, I wasn’t overly thrilled with the heat that it generated. I had placed it directly onto the plastic tray in my light stand & it barely got warm.  The regular temperature in our basement during winter is around 19°C (66°F).   The mat seemed to raise the temperature to only 21°-22°C (70°-72°F).  Then I read a few online reviews that said you should have some sort of insulation under it.  I placed a couple of folded up towels underneath the mat and – WOW!  The temperature shot up to 28°C (82°F)!

Heat Mat with my Handy Dandy Insulating Towels Underneath
I don’t have a soil thermometer (yet!) so my readings were taken with a regular digital thermometer lying directly on the mat.  Since soil temperature is always lower than ambient air temperature, I’m thinking that the soil would warm up to around 23°C or so (73°F).  That's a bit cooler than I would like, but definitely a big improvement over no heat at all.

When I started using the heat mat, I placed the cell packs on an uncovered seed tray on top of the mat.  I had big issues last year with damping off (that’s a whole other story), so I was really hesitant about creating an overly moist environment, even when it was only temporary.  But eventually, I broke down and started placing one of those clear plastic lids on the tray.  This worked very well for seeds that were sown in the soil BUT I soon discovered that it was WAY too hot for pre-germinating seeds.

When I started my tomato seeds (which I was pre-germinating), I had placed the baggies directly on the bottom of the covered tray.  Some germinated, but most of them didn’t.  After one week had passed, I started to get a bit panicky.  Images of a super late start to tomatoes (from the 2013 tomato fiasco I talked about HERE) began to pop into my head.

When I did my daily check for germinated seeds, I noticed that the baggies did seem very toasty – maybe a little too toasty.  So I checked the temperature by placing the thermometer on the bottom of the covered tray.  The reading came back at a whopping 36°C (96°F)!!  Now that’s great for most seeds when they are sown in soil, since the soil temperature would likely be about 30°C (86°F), but for pre-germinating – not so good.  Even heat lovers like tomatoes & peppers prefer temperatures that are quite a bit lower – around 27°- 29°C (81°-84°F).

Seeds Germinating on Bottom of Tray
--WAY too hot at 36°C (96°F)--
But what to do?  I don’t really have a moderately warm spot in the house where the heat is consistent.  There’s a cabinet above my fridge, so no room there.  Another suggestion I often see is the top of the hot water heater or near the furnace.  But both of these are only a few years old and really energy efficient - I can hardly feel any heat coming from either of them (other than the occasional waft of warm air from what I believe is some sort of vent).

Then I had an idea – what if I placed the seeds in the covered tray on the heat mat, but not in direct contact with the bottom of the tray where the most heat was.  I placed an empty plastic salad container (with its lid on) in the tray & put the thermometer on top.  A little while later I took a reading.  Bingo!  This gave me a nice, warm – but not too warm - 27°C (81°F).

Seeds Germinating on Top of Salad Container Inside Tray on Heat Mat --Just right at 27°C (81°F)--
I wasn’t sure if I had totally messed up the original tomato seeds and didn’t want to waste any more time, just in case.  So for all the seeds that had not germinated yet, I decided to start fresh with some new seeds.  Also, in addition to the change in location, I remembered that last year I had soaked the seeds in water for 8 hours. (I REALLY have to keep better track of what works/doesn’t work and actually LOOK at my notes in subsequent years to avoid these kinds of mistakes).  So I soaked this 2nd batch of seeds before placing them in their paper towels.  And guess what?  All the seeds that I started in my 2nd go-round germinated in 2-3 days!! Yeah!!

After this somewhat rocky start, I now have high hopes that the heat mat will help my seed starting efforts.  My seedlings will, however, still have to do without once they emerge from the soil.  I only have one heat mat and don’t plan on getting another, at least not anytime soon.  So I will use it predominately for pre-germinating & for the seeds initial growth in the cell packs.  Once they poke their heads out of the soil, they go under grow lights with no bottom heat.  Not the best situation for heat lovers like peppers and tomatoes, but oh well – you make do with what you have!

Oh – and one more thing – my son brought home a tomato plant from a school field trip to an agricultural center.  This plant is significantly further along than mine:  
J's Plant on the Right - Leafy & Stocky
My Oldest Tomato Seedlings on the Left - Can't even tell they're tomatoes yet
As the season goes on, it should be interesting to see how J’s plant does when compared to mine.  It may spur me to start my seeds earlier next year.

Till next time…


  1. Gees! I don't what I would do if I woke up to that this week. I had the same realization when I first bought my heating mat a few years ago. It's the only reliable why to start peppers and tomatoes in my opinion. Not everything likes to germinate at that high of a temperature so I try to be selective about what I use it for.

    1. That's definitely a lesson that I am very quickly learning! I never really bothered to check what temperature seeds germinated at before but now it's one of the first things I look at.

  2. I feel for you. It has been a brutal winter for many folks. Luckily for me, that white stuff was gone a couple of weeks ago. Wishing you some soil warmth and sunnier days.

    1. Thanks George - I'm keeping my fingers crossed!!


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