Here's some info from Jekkas Herb Farm-
"Sow seeds in early spring under protection at 18°C
(65°F). Sow thinly and cover with perlite. Germination
usually takes 5-10 days. Alternativley, sow seeds in
late spring or early summer, when the air temperature
at night does not fall below 7° C (45° F), the soil has
warmed and all danger of frost has passed."

Full content-
http://www.jekkasherbfarm.com/HerbWildRocket.asp