- 1). Look for an area of the garden that has well-drained soil and direct sunlight for six or more hours per day. If you live in U.S. Department of Agriculture plant hardiness zones 9 through 11, choose a location that has afternoon shade to protect the plants from excessive heat.
- 2). Spread 2 to 3 inches of compost on top of the area and mix it into the top 6 to 8 inches of soil using a garden tiller.
- 3). Spread a slow-release, 20-20-20 granular fertilizer over the top of the soil per the dosage amounts specified on the package. Retill the soil to mix the fertilizer in well.
- 4). Dig a hole that is the same depth as the root ball on one of the zinnia plants and insert the roots into the hole. Ensure the top of the roots are level with the top of the hole before backfilling it completely. Space additional giant zinnia plants at least 12 to 18 inches apart.
- 5). Water the soil until it is saturated to 6 inches deep immediately after planting. Thereafter, reduce watering to only when the top 2 inches of the soil feel dry.
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