The USCCB issued a highly controversial document in November of 2000 called
Building Stones: Art, Architecture & Worship, (
nccbuscc.org/liturgy/livingstonesind.htm )
which offered some guidelines but did not carry with it the rule of law.
One of the suggestions for creating a worship atmosphere is to use “natural” flowers.
§ 123 § The tradition of decorating or not decorating the church for liturgical seasons and feasts heightens the awareness of the festive, solemn, or penitential nature of these seasons. Human minds and hearts are stimulated by the sounds, sights, and fragrances of liturgical seasons, which combine to create powerful, lasting impressions of the rich and abundant graces unique to each of the seasons.
§ 124 § Plans for seasonal decorations should include other areas besides the sanctuary. Decorations are intended to draw people to the true nature of the mystery being celebrated rather than being ends in themselves. Natural flowers, plants, wreaths and fabric hangings, and other seasonal objects can be arranged to enhance the primary liturgical points of focus. The altar should remain clear and free-standing, not walled in by massive floral displays or the Christmas crib, and pathways in the narthex, nave, and sanctuary should remain clear.
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In short, artificial flowers can enhance worship space, but regardless of whether they are live or silk, the person placing the flowers does need to have a sense of the sacred and a critical eye.