Step 2. Plant short day onions by June 20 at the very latest in this example. The entire growing period is 135 days and ends September 15. Short day onions require 85 days for the onions to reach maturity. Counting back from September 15, 100 days brings the date to June 20.Adjust the planting date to reflect the fact that onions require temperatures between 45 and 60 degrees to germinate and between 60 to 75 degrees to bulb and cure.ong day onions need about 120 days to mature. Short day onions need 12 hours of sunlight to bulb; they do the best in southern regions and mature in about 85 days. Additionally, leave both kinds of onions in the ground for an additional week to two weeks, to cure.