Particularly in the spring as you raise loads of seedlings, you will get
far more reliable results if you use some sort of heated propagator. These
are low wattage electrically heated trays usually with seed trays on.
Often the seed trays have covers to help keep the soil conditions warm and
damp - ripe for germination. Some of the Propagators come with thermostats
so you can control the temperature exactly as you want it, others tend to
just stay warm (i.e. On all the time). You can also use heat mats to just
create a generally warm surface in your greenhouse - then you can just put
whatever seed trays or pots you are germinating straight on to the heat
Soil warming cables are ideal if you want to make a fixed propagation bed
- usually, you would zig-zag the soil warming gable on a bench then cover
with sand to create a warm propagation bed. Separate thermostats can be
wired in with these.