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In Search of Bottom Heat
by John Harmon
January 19, 2003

I've decided to ignore the weird winter weather we have been having along with the weather forecasts for a cold wet spring and plant a bunch of geraniums and begonias anyway. Every year I get requests for information on bottom heat and just what that means. 

Most seeds will germinate faster and you will get a higher percentage of seeds sprouting if you use some kind of bottom heat. I have always recommended Low Wattage Propagation mats to provide bottom heat but they are a bit pricey and most folks don't start enough seeds to warrant spending that much. A Low Wattage Propagation mat is still the best way to provide bottom heat and they are available from most of the seed companies in sizes that will take one flat up to mats eight feet long. They are on a thermostat and can be adjusted for whatever seeds you are planting. For more information check out

Bottom heat can also be accomplished in many other ways. The oldest is of course the "hotbed". Fresh manure was placed in a pit, watered in and then covered with soil. With a cold frame placed over the bed the rotting manure created heat to help seeds germinate and kept them from being damaged by light frosts once they were up. Getting one to work is an art in itself and not suitable for apartments. Here's some other ways to get bottom heat to work for you.

If you are just starting a few plants and have an older refrigerator you can just set them on top. With the older appliances the top is warm. The top of a water heater will also work and is what I use for small pots of seeds. The idea is to keep the soil at 22 degrees Celsius more or less. Some tropical seeds require higher temperatures but 22 degrees Celsius is about right for most things. Place a seed flat or pots with a cover and moist soil in place on the water heater and check the soil temperature for a couple of days before you seed to make sure it's right. If you need to you can raise the flat or pots on blocks for adjustment. 

A soil thermometer is a great little gadget to have for this chore. If you can't find one locally you can order a good one from for just $6.99 US. Another option is an indoor/outdoor digital thermometer. I got one locally for less than $20.00 and just stuck the little "outdoor" probe in the soil. It's accurate and easy to use. Once you can tell what the soil temperature is remember to measure it where your seeds are. If you are planting on the surface that's where you should monitor the temperature. If you're planting a half-inch deep that's where you want your temperature probe.

Many folks who have or have seen some of the older greenhouses will remember a seedling box or bench. They were usually made of wood, filled with sand and covered with glass. They had a lead heating cable buried in the sand and plugged in to a thermostat for control. They were always a problem to use and hard to control. They would get hot enough too cook seedlings if you weren't careful. These days things are easier and lead cables are no longer used which is probably for the best since the lead could make it's way into the soil and plants. Today's soil heating cables are coated with plastic and preset. Just bury them in your box of sand or perlite and plug them in. They are safe and reliable as well as non-toxic. They are available from most of the seed company catalogs for less than $50.00 Canadian. 

Another option I saw last year is just an oil pan heating pad buried in the sand. They come in a wide range of sizes and wattage's and they are waterproof as well as tough and reliable. They are available from any auto supply and cost less than $20.00. They do not have a thermostat so you have to do some testing to determine the right depth to place the pad to give you the soil temperature you want. Test over a number of days before you plant and check daily after you plant your seeds.

Regardless of which method you use bottom heat will give you better seed germination and healthier happier seedlings. 

John Harmon owns and operates Tropicals North. Write to John at The Real Dirt, c\o 211 Wood St., Whitehorse, YT., Y1A 2E4 or e-mail

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