Most zinnias thrive in full sun, particularly in cool climates, but in hot climates they may benefit from light afternoon shade. Space smaller varieties as close as 6 inches apart, intermediate ones about 12 inches, and tall kinds up to 18 inches. During dry weather, water with a soaker hose or drip irrigation to avoid wetting the foliage. If growth is slow or leaves look pale green, fertilize with a soluble or controlled-release fertilizer. Choose a formulation with modest or low nitrogen so that excessively tall growth is not forced, and follow label directions.
Cut flowers can last a week or more.
Harvest the flowers when they are fresh but fully mature, ideally when pollen begins to form. Take a bucket filled with water out into the garden, submerging the cut stems immediately, then recut the stems underwater before using them in arrangements. To double the vase life, add a commercial cut-flower preservative, or a teaspoon of bleach and sugar to the water. Mildew is the primary zinnia troublemaker. It can attack susceptible kinds, which include most garden zinnias, whenever the weather is warm and rainy or humid.