Answer: Most plants need a soil temperature of about 70 degrees to germinate. For this reason, many growers will start the seeds indoors under very controlled conditions. Next, the seedlings are moved to individual pots to grow larger. At this stage, many plants will grow well enough with daytime temperatures of about 65 degrees and nighttime slightly lower. Most annuals and especially vegetables are cold sensitive and will not do well at the cool temperatures you are describing. Some perennials will tolerate that low a temperature but will not actively grow. If you do not have them, you will need a min/max thermometer and a method to vent excess heat build up on sunny days in order to regulate the temperature. (An automated vent is a necessity if you are not at home during the day.)
A schedule would be determined based on what you are growing and when it is to be set in the garden. Most annuals and vegetables are set out at about the average last spring frost date. Therefore, you would count backwards from that date a certain number of weeks to start the seed. The number of weeks is usually listed on the seed packet.
Seeds can be ordered year round. Most suppliers will ship plants at the proper planting time for their destination's climate.
Since you are new to this, you might want to read up on seed starting a bit before you order and launch into full scale production. The following web page is full of helpful information through January: http://nationalgardening.com/sponsor/january01.htm
Have fun with your new greenhouse!
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