If you wish to use real flowers, make sure you use sustainable varieties (e.g.: stay away from wildflowers unless they were cultivated, try to find flowers that come from draught resistant plants, etc.), use locally grown flowers, and buy from organic, sustainable growers. You might even be able to grow your own cutting flowers if you have the space, plan well (make sure you consider rate of growth, flowering time, etc.) and have a back up plan in case your harvest doesn't meet expectations. Alternatives to real flowers are available as well, but use safe, sustainable materials. Silk is preferable to plastic and has improved aesthetically over the years. Silk is a natural and renewable resource, and lasts a long time, so not only will not fade on a hot, sunny day, but can be used to decorate well after the celebration is over.