Starting seeds should be a simple thing, right? Just poke some seeds into the soil, wait a few days and…voilá! You have plants.
That’s true of a lot of seeds, but some seeds–like most nightshades and some flowers–like to be extra warm before they’ll germinate. So unless you keep your house really, really warm, you may find yourself in need of a seed warming tray.
What’s a seed warming tray?
As the name implies, it’s just a simple tray that you put underneath your seedling flats that keeps the soil a constant 70-75°. You can buy some like this one. It costs about $11-17 and is 10X20. A standard seedling tray is 10.5″ X 23″, so this mat is just a little shy of what you need for one seedling tray.
Or you can make one yourself for $10 or less and it is long enough to warm two standard seedling trays (12″ X 48″.) It can cost you as little as $2-3, depending on where you get your materials.
Gather your materials
You know I like to shop the boulevard during Spring Clean Up Week. If I can’t find what I need during Spring Clean Up, there are always the thrift stores. But even if you have to shop for all new materials, it will still only cost about $10 for everything. All you need is wood and Christmas lights.
The lights are the rope-style Christmas lights. I’m not sure if color matters, but mine are white.This is what will create the warmth under your seedling trays. I bought mine on clearance after Christmas for $5 each. But you can find them at rummage sales and thrift stores all the time for $1-2. If you get the lights second hand (at a thrift store or rummage sale) they probably won’t have the clips, so you’ll need to buy a package of clips for coaxial cables. That will cost about $1.50 for about a dozen.
For the wood you need one piece of plywood or 1″ X 10″ or 12″ and five strips of 1″ X 1″ or 1″ X 2″ that are 40″-42″. I often see wood like this being tossed during Spring Clean Up week or listed on Craigslist for free.
When I made my warming tray, I found some shelving on clearance at the hardware store, so I got two shelves the perfect dimensions for $3 each. And the 1″ X 2″ was leftover from another project.
Put it together.
Use chalk to measure and divide your base piece into fourths. Attach the wood dividers, using screws or nails,



whatever works for you. Put two on the outer edge of each side of the board and space the other three evenly in between. Then attach the clamps. Now just lay the Christmas lights in the channels, in an S or Snake-shape pattern.
And it works!
With these lights the soil remains a constant 75-77° which is perfect for all my nightshades and most of my flowers.
You may find a warming tray is just the thing to ensure good germination of all your seeds. And this DIY is both affordable and super easy to make.
