The timing of starting seeds in an unheated greenhouse depends on your climate and last frost date. Most seeds need enough consecutive warm days and nights to germinate. Even in sunny weather, the overnight temperatures may be too cold for germination! As a rule of thumb, you will get better germination rates in the house or a propagator until the last frost in your area has passed. After this, overnight temperatures will increase, and you can start sowing seeds into pots in an unheated greenhouse.