I live in Enumclaw, WA which is USDA hardiness zone 8a, and am hoping to grow pomegranate trees as a fruit crop. Can they be grown here or would they need the aid of a greenhouse?
Most of the Pacific Northwest nursery sources for pomegranates (Punica granatum) say that their varieties are hardy in zones 8 through 10. That being said, they may survive our conditions, but they might not have sufficient flowers and fruit set if you are trying to grow them as a crop outdoors. For basic cultural information, see the following, from California Rare Fruit Growers:
“Pomegranates prefer a semi-arid mild-temperate to subtropical climate and are naturally adapted to regions with cool winters and hot summers. A humid climate adversely affects the formation of fruit. The tree can be severely injured by temperatures below 12 degrees F. In the U.S. pomegranates can be grown outside as far north as southern Utah and Washington, D.C. but seldom set fruit in these areas. The tree adapts well to container culture and will sometimes fruit in a greenhouse.”
Purdue University has additional information about growing pomegranate.
According to Trees and Shrubs for Pacific Northwest Gardens by John Grant (Timber Press, 1990), pomegranates need the warmest possible site in our area to flower well (such as near a south- or west-facing wall), and are unlikely to develop ripe fruit.
If you want to grow them for fruit rather than their ornamental qualities, you may need a greenhouse setting. There are some varieties which are more likely to succeed in the Pacific Northwest than others. Check out the selection at Raintree Nursery in Morton, WA and One Green World in Molalla, OR.