In my vegetable garden, the pink zinnias and some of the red ones came up “wild” from seed that goldfinches dropped last year. The parent plants are giant zinnias and mine usually get taller than 5 feet so this plant isn’t really unusual. Giant varieties need support and water (if it doesn’t rain enough). If they remain unsupported, they will fall over and continue to grow, thereby using a larger “footprint” in your garden. Thus, support is needed if you want them to stay in their designated areas.