All except the most extreme north west tip of Harris County fall within zone 9a. The people who tell you that you are in zone 8b are probably going by the old data. The new map released in 2012 that dave links to above pushes our zone further north by several miles. Even within a particular zone, temperatures are going to vary, and you can expect temperature in general to be colder further north, even within the same zone. The USDA map is base on the most extreme cold temperature that you should expect in any given year. So in zone 9a you can expect that the lowest temps you we will receive in this zone are between 20 and 25.
