Should consider the high initial cost of the bulbs, short life span of the lights....
2 basic design's 1 where light provides bottom heat & another where enclosed above the flat & then provides the heat. I use a heat mat specifically designed to provide bottom heat.. also has a thermostat control to keep temp steady day & night (it is expensive) but serves MY needs.

I found a thread about this from back in November, hope it helps.

Here is a link that might be useful: Christmas Tree Lights and cold protection

Just an FYI- Not sure what the 'winter' temps are down there, but all of the posters there are in Zone 9 and up. I don't know that it'd do a bit of good here in Z5 winters, unless you were using them inside a cold frame (which I also know nothing about, but figure it would make sense where they are in an enclosed environment.)

You don't need elaborate heating set-ups to keep seeds warm. I put seed paks that need warmth to germinate, under the warm part of the fluorescent lights or sometimes on top of the warm spot. Lights should be no more than one inch from seed paks/plants. I've been doing this for 25 years and never needed a heat mat. I keep the colder loving plants closer to the ground and farther from the warm spot.

Of course, people like to play with their hobbies...

Just dropped by for a visit and noticed this post-- after I'd responded in the earlier thread. Anyway, I'll repost here...

The following "light-bulbs-in-a-box" system is a cheap and simple way to get variable bottom heat. The heat source is a string of christmas lights placed inside a box to heat the air within.

There is a platform inside the box that supports the cellpaks or flats. The box can be wooden or sturdy cardboard about 12" x 18" x 18". Cut a few holes in each side a few inches from the bottom. These are for ventilation so the box doesn't overheat.

Now for the heat source. Find a string of christmas lights. I tried using a 20-watt light bulb in a "trouble light" housing, but it tended to create a hot spot. Christmas lights help keep the heat evenly distributed.

Put an insulating, heat-proof liner on the bottom of the box. The idea is to keep the bulb(s) from touching the bottom of the box. You could use an old plate or a ceramic trivet-- so long as it does not conduct heat well and it keeps the box bottom from heating up.

Now make the platform on which the seed trays rest. Cut a piece of cardboard to fit inside the box just trace the bottom and trim it up to fit. Leave a small gap between the edge of your platform and the box wall-- for ventilation and to allow "finger access" to remove the platform. The setup I'm using these days is a glass cutting board that happened to fix the box.

Support the platform with your choice of heat-proof "risers". Try a mason jar in each corner. Empty cans or old glass tumblers would also work. There should be several inches between the top of the bulb(s) and the platform. On top of the platform put a waterproof cover.

Set the seedling trays on the platform. Now measure and monitor how hot the seeding mix gets. Use a meat thermometer set to just below the surface of the mix. To decrease the temperature, cut more holes, raise the platform, unscrew a few of the Christmas light bulbs, or pull a section of the lights out of the box.

For seeds that need light to germinate, get a clamp lamp and use a compact fluorescent bulbÂ-or two, depending on how many trays are underneath. Rig up the clamps so the light is a few inches from the tops of your trays.

That's about it. Cheap and easy. Here are some photos:

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For this setup I used a wooden crate that held wine bottles. I screwed in some 6" x 1/2" boards to make the overhead "beam" to which the lamps are suspended.



The photo above shows the glass platform-- actually a cutting board that happened to fit pretty well-- and the mason jars supporting it.

Hi. Portions of this are crossposted in the garden experiments forum. I'm inexperienced with starting seeds and I don't have much disposable income. I got the idea of using Christmas lights for bottom heat and did a search. bev_w's above post was the affirmation I was looking for. I have not followed through using seeds yet, but this is what I have come up with so far...

Yesterday I had a string plugged in while being wrapped around the plastic rack they came on. My thermometer stuck in the middle read 90.2 degrees. That sounds perfect to me. I tried it somewhat similar {{gwi:211852}}. The seeds can go on a {{gwi:211853}} just above the lights. I guess I could wrap more or less around the rack to get more or less heat. The heat is evenly distributed too, since it is a series of small bulbs evenly distributed over any shape/area you choose. Since they are indoor/outdoor lights, they can take a little dampness without fear of electrocution. Also, since we trust them wrapped around a dead and drying tree in our living room, the fear of fire should not be too great. Although wrapping them can concentrate their heat, I think 100ish degrees should not cause a meltdown. 44 watts for this specific string sounds reasonable.

I imagine that people with cold frames could use more than one string, each being on its own timer, so as the night goes on, getting colder, additional strings could turn on.
They could then turn back off in sequence as the sun comes up so as not to get the coldframe too hot. I imagine it would take some testing to get things right, but it sounds like a good idea to me.

Thanks.

isn't this kind of like buying an airplane to get free peanuts. it'll work but there are better ways to do it.

My main point is that I don't have to buy a heat mat since I already own Christmas lights. I understand that to build a nice box like bev_w would quickly off set the money saved by not buying a heating pad. I don't plan on doing that.

No one uses heating cables for this purpose? Mind you, I sow my seeds in seeding flats to be transplanted into cell packs later. The heating cables keep the SOIL at a dependable 75-ish (which is plenty warm), and depending upon the size cable you get, you can string several flats together. Once the seedlings have developed a set of true leaves I pull the plug on the cable, allowing the plants to develop nice, stocky stems with no heat induced etiolation.

Remember, you should focus on warming the soil, not the surrounding air (unless cold temperatures are a serious issue where you are propagating).

This method is good for rooting various cuttings, too. Speeds up the process.

The cables last for years when stored properly and I wouldn't use anything else!

Here is a link that might be useful: {{gwi:211854}}