Fertilizing is key. Get your soil tested at your local agriculture extension office and apply fertilizer as recommended. Zinnias like soil that is slightly alkaline. Always apply liquid fertilizer at ground level. The leaves are sensitive to liquid fertilizer.
Zinnias make excellent cut flowers.
Cutting a bouquet a couple times a week brings color inside to brighten any room. Doing this will be good for the remaining plants, as they will not have to compete as vigorously for resources. Cut long stems for more elegant or striking arrangements. These tips have been established from our own success growing beautiful Van Dyke Zinnias for over a decade. He who cultivates a garden, and brings to perfection flowers and fruits, cultivates and advances at the same time his own nature.