Floating Flower Tip - Most flowers float pretty well on their own until they start to pool with water and slowly sink. For a longer lasting centerpiece, use bubble wrap to make little skirts for your flowers.
21 DIY Greenhouses with Great Tutorials: Ultimate collection of THE BEST tutorials on how to build amazing DIY greenhouses, hoop tunnels and cold frames! Lots of inspirations to get you started! - A Piece of Rainbow (outdoor flower planters)