Hi, John. It's a low-temperature limit, in many cases a conservative guess, and mostly in response to customers who virtually demand some quantification of cold-hardiness. Lots of plants that thrive in zone 9 in California fail in zone 9 in the southeastern U.S., just as many that flourish in zone 5 in Colorado perish in zone 5 in Massachusetts. Combine the hardiness rating with information on a bulb's native climate, and it's somewhat more useful. A species from a Mediterranean climate will likely do well in coastal California; if it has a zone 5 hardiness rating, it should also do well in a region (such as the Great Basin) with cold winters and relatively dry summers. It might be iffier in the far damper Midwest and Northeast, however. The same goes for heat-tolerance ratings. The cold-hardiness and heat-tolerance ratings for most plants will differ markedly in different areas of the country, so an estimated hardiness range is essentially worthless for many or most locations. Seasonal patterns of temperature and rainfall in the plant's native region -- this is by far the most useful information. Russell At 10:07 PM 2/14/2010, you wrote: >Russell, > >A question about your catalog: Is the USDA zone you give with each >species or cultivar the minimum temperature? Or is is a high >temperature limit? For instance, would a Mediterranean species >listed for zone 6 also be suitable for Zone 9? Most of your bulbs >are unusual enough that they do not occur in the Sunset garden >books. Wouldn't it be more helpful if you indicated a range of zones >where each bulb could be cultivated--not just the northernmost? > >John C. MacGregor >South Pasadena, CA >USDA Zone 9 >Sunset Zones 21/23 Russell Stafford Odyssey Bulbs PO Box 382 South Lancaster, MA 01561 508-335-8106 http://www.odysseybulbs.com/