C. J. Gould
Although bulb growing has been attempted at various times in most parts of the world, major production has become centered in certain temperate countries with comparatively mild climates. Generally these countries are in the north temperate zone between 30° and 55° latitude, where the extremes of winter and summer are tempered by winds from nearby oceans or other large bodies of water. For example, our bulb 'strip' in Washington (WA) extends from about 46° to 48° 30'.

Thus, it is no accident that the Pacific Northwest, and especially western WA, has gradually become the largest producing area for Narcissus, Tulip and Iris bulbs in the U.S. Adjacent areas (Oregon and British Columbia) also produce bulbs of the same types but trail in production (Gould, 1957a; Obermire, 1989; Ormrod, 1989).

Temperature and precipitation vary somewhat from one local area to another but, in general, the entire area is cool and moist in winter and root growth usually continues all winter long. Summers are usually warm and dry, conditions that are very favorable for digging and curing. For example, average temperatures and precipitation at the Seattle-Tacoma Airport have been 3.8°C and 149 mm for January and 18.1° and 20 mm for July, respectively.

Gould, C. J. (1990). HISTORY OF BULB GROWING IN WASHINGTON STATE. Acta Hortic. 266, 15-24
DOI: 10.17660/ActaHortic.1990.266.1

Acta Horticulturae