Danita,
We have a 14"w x 5"d round pot on an old bird bath pillar in the front. Usually, I put just geraniums in it. This year, I put a young ostrich fern in the middle and bright red geraniums around the edge. The fern provided some nice colr, size and texture effects. )I'd post a picture, but the excessive heat took it's toll on the fern.
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Depending on the size of the container, I would use young fern so that you can enjoy the arrangement for a couple of years before the fern takes over.
