Watering: Zinnias need moist soil to grow their best. This is especially true of young plants. Water deeply a few times a week so the soil stays moist 6 to 8 inches deep. Don't overwater because zinnias can also succumb to rot diseases especially on wet, clay soils. Once established, zinnias a relatively drought tolerant.
Fertilizing: Amend the soil before planting zinnias with compost. When flowers start to form, side dress with an organic 5-5-5 fertilizer to get more and bigger blossoms.
Mulching: Mulch zinnias once they are established with a 2 inch layer of straw or bark mulch. This will help preserve soil moisture and prevent weed growth.
Trimming & Pruning: Deadhead (trim) spent flowers regularly to promote more flowering and to keep the plant tidy. To create bushier plants on tall varieties, pinch the growth tip of the plant when young. This will stimulate more side branching, a shorter plant, and more flowers. However, it may delay flowering, especially in cool summer areas.