Soil Warming Cables are handy tools for the gardener to have when he wants to manipulate nature a bit. 
I first used them last winter when I needed to get a bed of carrots to germinate in January. Germinating most directly sown seeds out in the garden would be nigh impossible here in January, as our night time lows are typically between 25 and 38 degrees Fahrenheit. And as most of you know, you cannot sow carrots in a nice warm flat in the greenhouse and transplant them out in the garden. Veggies with tap roots just don't tolerate that. Things like radishes, carrots, beets, turnips, salsify, burdock, daikon, all need to be sown where they will stay put.
The cables are placed several inches under the soil surface, about 4 to 6 inches apart in any arrangement (straight lines, squiggle lines, etc.) just as long as they don't touch or cross each other.
You can dig mini trenches and insert the cable into them, or you can do what I do and place the cables, affix them with landscape staples, then add a good two inches of soil on top. Here, we've just started adding compost, and will finish topping it off with garden soil.
Once the cables are placed and covered, they are plugged into an extension cord, with the connection wrapped in waterproof tape, and carefully protected from moisture intrusion. The cables are supposed to keep the soil surrounding them at 70 degrees, which is a good average temperature for most seed germination. Truth be told, though, the cables do not quite warm the soil that much. Fortunately, though, they do warm it enough to get some germination going on this long bed of carrots (a little wispy and hard to see in the foreground of this picture), beets, and at the far end, turnips. The cables can no longer be seen, but my black irrigation lines are clearly present on top of the soil. 
We moved the white row cover to the right in order to take the photo, but we usually leave the bed covered with the lightest weight floating row fabric from seeding to harvest, as it keeps the carrot maggot fly from laying it's nasty little eggs on the soil surface and burrowing into the roots. We do remove the cover, though, after the seeds germinate, and we need to thin. As you can see here, these gorgeous beet shoots are way too close together, and we'll need to spend some good time carefully removing the excess. What's nice is that we have little waste here, the chef loves to see these thinnings in the harvest delivery. They make tasty and unusual garnish.
Once germination has been achieved to my liking, I unplug the cables and leave them in place. The roots easily grow around the cables. When the bed is completely harvested, I can remove the cables and use them again.
So would you use precious electricity to get a bed of veggies going? Well, if your mortgage payment depended upon it like mine does, then you might. If you were trying to get a jump on tomato growing season by warming your soil a month earlier, then that might entice you to use them as well (more on that later). The cables themselves can be pricey, but I've been lucky to get mine off of ebay (brand new) for about a third of the price. Click here for a link to where you can get them full price.
It's nearly winter in the UK and the weather always affects my vegetables so I'm sure to have to look into this idea, thanks for the information regarding the heating.
Posted by: Mandy Boilers | November 03, 2010 at 05:20 AM
We have 9 months of above freezing weather, however it does dip to the 20's at night. Since we have 20 hours of daylight during the Summers we grow them fast & big. I'd like to exend our growing season by 2 months on each end (April- November). I'm hopeing to build a small greenhoue or coldbox but (since I'm new at this)...I may need a small reality slap from someone who has more expeience than I do....Thanx
Posted by: Alaska Grower | May 02, 2008 at 11:36 PM
What a wonderful asset! I do challenge you on one small point - I never have any trouble planting out beet seedlings and getting them to grow to maturity.
Posted by: Keith | February 03, 2008 at 11:17 PM
Hmmmmm, baby beets. I love eating all of those things you grow when I go to Manresa!
Posted by: Baroque | January 24, 2008 at 11:24 PM
What a great find! Thank you for sharing this tip for extending the gardening season. Thanks also for the tip about watching for them on eBay.
Posted by: local fruits and vegetables in Asheville, NC | January 24, 2008 at 04:30 PM