The Right Place
Light - Zinnias like full sun and good air circulation. As soon as all danger of frost is past, put out seedlings or sow seeds in fertile, well-drained ordinary garden soil.
Soil - While they are not terribly particular about their soil, zinnias prefer it to be more alkaline than acidic (pH 6.0 to 8.0).
Starting Zinnias From Seed - Zinnias are easy to start from seed and you can find many varieties in the seed packet display in your local garden center. For details about starting seeds indoors and out, see Starting Plants From Seed and check out Seed Starting Equipment and Supplies in Yardener’s Tool Shed.
Buying Seedlings - Choose small, compact seedlings in six cell packs at the garden center or nursery. Check to see that their foliage is a healthy green and is crisp looking. Choose ones with buds just barely open to verify the color. Keep their soil moist until planting time. Space seedlings 6 to 12 inches apart, depending on the type.
Planting Zinnia Seedlings - Put nursery or home grown seedlings out in the garden when they are about 5 weeks old. Prior to transplanting, set home grown ones outside so they can become acclimated to the cool outdoors during the day for a few days, bring them in when evening temperatures drop. Transplant late in the day or on an overcast day so that seedlings struggling to get established do not have to cope with bright sun immediately.
Prepare the bed by clearing it of weeds, stones and other debris. With a trowel or shovel dig deeply into the soil to loosen it to a depth of 6 or 8 inches, turning it over and breaking up large lumps. Smooth it and level it. Dig individual holes about the size of the container the seedling is in. Gently pop each seedling from its container and place it in a hole. Be sure that it is no deeper in the soil than it was in its pot. Press the soil gently around the plant stem to support it and water immediately. Double-flowered zinnia varieties may temporarily revert to single flowers when transplanted to the garden and may suffer from shock, so it is best to start them as seed directly in the garden.