Most plants will "self sow" in the area that they grow in naturally. So when we grow plants that are not "wild" in our area it is best to research what they need to germinate. Zinnias (Zinnia elegans) germinate at a temperature of 70 degrees in 5-7 days. And they need it to be dark (seed covered with dirt).
I collect my Zinnia seeds and plant them in spring, this way the birds and bugs don't eat them over winter. Some plants need the cold period in order germinate, but Zinnias don't need this so you will get more plants if you put them in, in spring.