USDA Plant Hardiness Zones and ‘average first frost free dates’ explained

NMSo what are USDA Plant Hardiness Zones? And what is this ‘first frost free date’ we hear about as gardeners so often?

The Plant Hardiness Zone is a standard set by the USDA (United States Department of Agriculture) and is calculated by accumulating many years of data. Zones are used by gardeners to help determine which plants will most likely thrive in their area. “The maps are based on the average annual minimum winter temperature, divided into 10 degree-F Zones”. There are 13 Zones, the coldest being Zone 1 and the warmest is Zone 13.

In Santa Fe, we used to be in USDA Plant Hardiness Zone 5 but now Santa Fe is in Zone 6b (-5 to 0°F). The reason our Zone has been changed according to the USDA is because it has better models to follow with more information gathered throughout the years- not necessarily because it’s getting warmer (although I do believe that too). Some areas in our county could be Zone 6a (-10 to -5°F) if they are closer to the mountainous areas. If you are not sure what zone you live in (where ever you live) go to the USDA plant hardiness site and type in your zip code and it will tell you what zone you are in-it’s as simple as that. So when you go to a nursery and the plant tag says zone 7-forget about it—it won’t survive our winters but any tag with Zone 6 or lower number should survive.

The average first frost free date is the date that we can safely put plants outside in our gardens. Notice I said average because some years we are colder and some years warmer. You’ll just have to watch the weather closely in spring for deciding if you want to try to sneak them out earlier in your area if spring appears to be a warm one. In Santa Fe our first average frost free date is May 15. So plant away outside after May 15, unless you hear we are getting a hard freeze! Of course if you are starting them in a hoophouse or greenhouse, you can start seedlings much earlier.

Now, once you know your planting zone and first frost free date, you can use the many tools available on the web for calculating all kinds of things from seed starting to succession planting to harvest times. Many seed companies have web tools to help you calculate the dates. In my next post I will show one such tool I use.

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