Zinnias thrive in warm soil and hot weather, so plant them now, from flats or from seed. If the plants are to be incorporated into a border, use those from flats, because you can put them right where you want them.
A good reason for planting from seed is that you have a greater selection. The Burpee catalogue (300 Park Ave., Warminster, Pa. 18991) devotes five pages to zinnia seed. The taller zinnias come with ruffled flowers, quilled or cactus flowers, and the more classic shape often called dahlia-flowered or giant. Park Seed Co. (Highway 254 N., Greenwood, S.C. 29647), under 'Ambrosia' hybrids (dahlia-flowered), offers an array of colors. Among the many other types of zinnias are 'Peter Pan,' with its large flowers on semi-dwarf plants, and the dwarf 'Thumbelina,' with ball-shaped blooms.
Zinnias make for excellent cut flowers; planting in rows from seed is the ideal way, because more can be planted and weeding is easier. The seed should be barely covered and then pressed into the soil with a flat object. Leave furrows for irrigation; zinnias do better without wet foliage, which promotes mildew. If watering overhead is necessary, do it in the morning so the leaves dry by night.