My neighbours and I grow a considerable number of zinnias. They are beautiful, useful for cut flower bouquets, and quite drought tolerant once established. Over the years we have found that larger transplants are preferred, so we start them inside and then plant them out once they're at least 5cm(2") tall.
The ones pictured below were seeded on March 25th, using seed that I saved from exceptionally pretty blossoms from last summer's 'Cut and Come Again.' I highly recommend this variety from Botanical Interests, as it has consistently put on a spectacular show for us for several years running. A dozen seedlings were kept in tiny 2" take out cups for a month, and their root development was incredible. Next time I will aim to prick them out at three weeks, as the roots were so robust they became a bit of a challenge to poke into the 1 1/2" soil blocks for growing on.