It's been proven that warm soil will usually sprout seeds and promote root growth faster than cool soil. Our experience has been that germination is often within three days. Greenhouse owners spend tons of money to install bottom heating. You can spend much less.
Anything you do to warm the soil when you plant seeds will be beneficial. Anything from a room Radiator, to a warm Composting bed, to electric Heat Cables can be utilized. The home gardener can also buy waterproof heating mats made for this purpose.
The problem with most of these devices is that they are inefficient, since they do not contact the soil. Of course, if you use it in a greenhouse, the lost heat helps to heat the greenhouse.
This little DIY device is efficient and inexpensive. The one drawback is that after the seeds sprout, they prohibit the sprouting of other seeds unless the original sprouts are removed first.
The way around this is to use two containers; the bottom one will contain the heater covered with enough sand so the second container will touch the warm sand and have the heat conducted up to the second container which holds the Seed-starting "soil" and seeds.
I like the 12-quart, $2.00 container made by Sterlite. A three-foot rope light cost me $3.00, and will warm it nicely. Be sure you have a Rope Light that is labeled for outdoor use and in good condition. Always follow the manufacturers instructions.
Note that Wal-mart and Lowes sell a 12-foot Rope Light for $7.00 which may be used on a seed-starting benchtop to warm multiple flats or containers.
Some Rope Lights can be cut to shorten them; some can not. If you remove one bulb and they all turn OFF (like Christmas Lights), then that light string can NOT be shortened. If the rest stay ON, then that string may be shortened. Additionally, each bulb may be marked more than 100VAC.
Color makes no difference in heat output. They produce heat in direct proportion to their length, and are rated at about six watts per foot. So a three-foot light is about 18 watts; very close to the 20-watt matts sold to heat one greenhouse flat (for $30.00+ shipping).
To make yours:
First make 1/4" drain holes at all four corners of the container to prevent the Rope from standing in water. Then make a 1/2" hole as shown to insert the 3-foot light. Curl it around the container about 2" from the sides. It can be held in this position by wire. Several dabs of Silicone Bathroom Caulk will hold it permanently. A bead of caulk around the inside of the 1/2" hole will secure the cord.
In use, a temperature of 75 F. to 80 F. has proven best. Mine are in that range when the container lid is secured. An indoor-Outdoor thermometer is a great aid in determining soil temperature. The downward glow is a pleasant night time bonus.
Longer Rope Lights can be used
to heat a bench top for seeds, as shown here:
Try to have approximately three feet of lights under each standard flat or 12-quart container as described above. To adjust the container soil temperature, the flats can be reoriented to touch more or less rope .
As with all greenhouse plug-ins, you should use a GFCI-protected receptacle. And be sure to read our Disclaimer !