Cold-germinators are still referred to as frost-germinators, although
this isn’t quite correct. The sowing must be kept warm (about +18 to
+22°C) [about 64 to 72°F] and moist for the first 2–4 weeks. After this
period the sowing must be kept at a cold temperature (between –4 and
+4°C) [between 25 and 39°F] for another 4–6 weeks. Colder temperatures
of –5°C [23°F] are only advantageous for most species of the Ranunculus
family. It is not so important if the temperature is higher or lower
during the cooling period, but the cooling period has to be prolonged
because the synthesis of the germination inducer, hormon-like acid,
slows down or comes to a standstill.
It is beneficial to cover the
sowing with snow during the cooling-period. The temperature below it
usually keeps in the optimum range of –4 to 0°C [25 to 32°F]. The sowing
is kept moist, and the melting snow helps to destroy the shell, which
is advantageous for the germinating seedling. After this cooling-period
the sowing may not be immediately exposed to high temperatures. The most
effective temperatures are between +5 to +12°C [41 to 54°F], even if
germination has started. The best location for this sowing, even in
March, April and May, is the open field, the cold frame or a cold
greenhouse.