Water them during dry periods, once or twice per week. Add mulch around the plants for appearance and to keep weeds down. Around mid-summer, your plants will begin to produce flowers and will continue to do so up to the first frost. You should remove dead flower blooms to encourage new growth and new blooms. Improve appearance of Giant Zinnias by trimming back stems that have grown long or gangly, but do not overtrim them.
Zinnias are annuals and are susceptible to frost.
They may survive the first light frost with only a little damage. They will not survive a hard frost or freeze. Giant Zinnias have long stalks for flower vases and other arrangements. Cut the stem low, check for insects hiding in the flower, on the stem and under the leaves. Bring them indoors and place in water immediately.