How is it possible that Portland, Oregon and Longview, Texas have the same plant hardiness zone of 8?  Our weather is definitely not similar in the months of July through September.  The USDA developed cold hardiness zones which you find on most plant descriptions and plant tags.  These are based on the average annual minimum temperature for a given area – not the maximum temperatures.  This can be misleading if you live in a hot summer climate like we do.

Portland and Longview have similar minimum temperatures, but a wide discrepancy in maximum temperatures. The American Horticultural Society developed the Plant Heat-Zone Map which indicates the average number of days with temperatures over 86 degrees. It lists Longview as 9 and Portland as 3. Unfortunately, this information is not found on all plant sites or plant tags.

Using the 2 zone maps together helps you determine if a plant should perform well in your area. However, both maps note that results may vary depending on abnormal weather events that fall outside of the zone boundaries – abnormal cold or heat for extended periods of time.

We try to stock plants that are rated for East Texas based on both cold hardiness and heat tolerance. Some years our weather doesn’t stay “true” to our cold hardiness zone but the heat zone is more accurate.