So a wedding arch is a really important centerpiece of any ceremony. Some factors to consider are, obviously the setting. If you have a wedding arch, you're typically going to be outside in some kind of a garden setting. Doing the flowers and the organic elements on the arch, you're going to want them to blend into the environment as much as possible, look like they are flowers and organic elements that have maybe grown on the arch.
You don't want something that's going to contrast too much with the environment. You don't want to take flowers that are very modern or very sleek and try and attach them to a garden-y, kind of romantic looking arch. You also don't want anything too severe to be framing the look of you and your beloved under the arch at the ceremony. So you want something quite soft typically, and something romantic.
You can really go in any direction with a wedding arch. You can do just vines and branches and things that are really not a lot of face flowers, or you can do elaborate clusters of things like roses and hydrangea and orchids, things that really stand out and look quite done. So there are a lot of ways to go with a wedding arch.
I think it's really important to think about the mood that you want to set, the tone that you want to set at your ceremony, and whether you want to have something that kind of fades into the background that's not too obtrusive. It's not a huge visual cue for people. Or whether you want to do something much more elaborate, like clusters of roses and hydrangea, something that looks like somebody really worked hard to make it your special place that really marks your ceremony.