Plant hardiness maps updated: Austin shifts zones

AUSTIN (KXAN) -- The USDA recently updated their Plant Hardiness Zone Maps (PHZM) to reflect the latest 30-year averages published by the National Weather Service. This map looks at the average coldest temperature of the year over a 30-year period to figure out what plants should be able to survive and thrive there.

KXAN Meteorologist Nick Bannin spoke with the USDA about the changes to the zones in the map and what that means for planters here in Central Texas.

KXAN Meteorologist Nick Bannin: The USDA recently issued a Plant Hardiness Zone map update. First of all, what does the map show in general? Who's the map for? And how does it help casual and professional planters and farmers alike?

Peter Bretting, USDA Agricultural Research Service: Well, the the map shows what the average annual lowest range of lowest temperatures are for a year. And that's a critical measurement for understanding what perennial plants those woody plants might survive in a particular area. So it's primarily a tool for gardeners, state and local agencies, plant breeders, growers, etc. to identify trees and shrubs that are adapted for specific areas.

Bannin: The map was most recently updated to change Austin from zone 8B to 9A. What caused this change and what does it mean for people who live here?

Bretting: It means that the average minimum low was as mentioned it shifted from one to five degrees, which would move you from one zone to another. And the shift can be caused by several different things. One is that we are in a period of global climatic warming and also one of the key differences between this map and the prior map is that we had 68% more data points. So it's more precise for estimations.

Todd Rounsaville, USDA Agricultural Research Service: Our general advice to folks that have asked similar questions about maybe being moved into a new half zone, as the Austin area has, is that they shouldn't really be considering major changes to what they're cultivating. As Peter alluded to, at the end of his last remark, these... half zone bands are five degrees. And if you were very close to the edge, even a very minor change could have put you into the next zone. So for the most part, while there may be a trend overall, that there is some warming. trying new plants still comes with some caution. And the plant hardiness zone that you're in is that 30 year average, [it] can still be a volatile statistic as Texans probably know well. You could still have a very cold winter that is more reflective of your previous zone.

Bannin: Now, these zone numbers, do they show up when you're buying plants?

Rounsaville: Yeah, the zones have really become the standard for nurseries and growers and consumers that when they go shopping online, or they go visit a nursery in person on on the tag or on the plant pot. They've done a very good job listing recommended hardiness zones so that people that are cultivating these plants can try to match the appropriate material to their local conditions.