hankandrose, thanks for your question.
It does depend on what you use for the center, i.e. large glitter, rhinestones, pompoms, paper or craft sand. I like the sand centers best, but the other can be attractive also.
Your favorite rhinestone, glitter glue is fine for them and you buy whatever is available at a craft store. I do use a dimensonal type 'glue' for adhering the sand, i.e. Aleene's Paper Glaze or even her Jewel-it type. Another I have used is JudiKins Diamond Glaze. These all are crystal clear thick glues that are good for adhesion and coating, so they dry fairly quickly and retain the 3-D look. They are more expensive than say, Elmer's white glue which I use on the papers, but the Elmer's does not retain the 3-D after drying. There are other brands of these clear thick glues and any would be acceptable, though drying times might vary. I have also used some acrylic dimensional 'crack filler' type products, but some also have a strong odor which I try to avoid.
In my floral hub, I give instructions for the sand-centers gluing process, but I have been intending to make a video to show this.
You do have to work fairly quickly in 'sanding the glue globs' so that you will achieve the 3-D coverage.
I know your floral crafting efforts will be beautiful. Would be glad to see some of them too. Best wishes, and enjoyment!