Frost dates and zones

I would expect that last frost dates would track somewhat with climate zone numbers, however, it seems many people in many zones, I've seen as low as 5a, say their date is mid-April. I'm in zone 8 by the map, oddly if I was to go 12 miles straight south, I would go from 2700 feet above sea level, to 6800 feet above, where it definitely gets much colder, and stays cold much longer, but according to the map, would be in zone 9.

Anyway, I've always been told April 15 was our frost free date.
The tool here shows, for 93543:
Each winter, on average, your risk of frost is from November 8 through April 3.
Almost certainly, however, you will receive frost from November 25 through March 5.
You are almost guaranteed that you will not get frost from May 2 through October 22.
Your frost-free growing season is around 219 days.

Would you take that to mean my frost-free date is April 3?
I am seeing that around April 15 is when there is a 30% chance of a 32F temperature after, maybe that's where the April 15 came from? there's only a 10% chance of a 28F after April 6

I just don't see how someone in zone 5 could possibly have anything close to mid-April.
Of course Lowes and Home Depot had vegetables in up to 5" pots and ready to plant out in January...At least they didn't have blueberries, Walmart has them this year, and they really don't grow here, probably a combination of alkaline and too hot in the summer.

This message was edited Feb 25, 2011 12:46 AM

California has so many microclimates caused by elevation changes, etc that you need to be careful with info you get from zone maps, plugging zip codes into things, etc. That area south of you is at higher elevation, but the maps may not be sensitive enough to realize that so they map it into the same zone as nearby areas that are closer to sea level. Sunset has more detailed climate maps and has developed their own system of climate zones which do a much better job of of accounting for all the different climate variables we have out here http://www.sunset.com/garden/climate-zones/

As far as last frost dates, I looked up last frost dates on the Farmer's Almanac site for some cities I know are in zone 5 and the last frost date is coming out to be the end of April, so the people who are saying it's mid April are probably being a little optimistic, although in a milder winter they could probably be safe getting things going a few weeks earlier. So your last frost date of 4/3 is almost a month earlier than zone 5.

If you were in zone 8 in the eastern half of the country it would be a little bit earlier (looked up Atlanta and theirs is 3/24) but our winter average temperatures tend to be lower than places in the equivalent zone east of the Rockies and we tend to have more frosty nights as well so it's not surprising that the last frost date would be a little later (zones are based on the absolute lowest temperature you would expect to see in any given winter, but out here we tend to approach that lowest temperature much more frequently than other parts of the country in the same zone)

The problem is that the traditional USDA zones have to do with minimum temperature. The are there to tell if a perennial will survive where you are. Areas ove maybe a 150 mile path near me will vary from zone 3 to zone 6 depending on their minimum average annual temperature. However I don't think the average last frost date varies much more than 2 weeks. The reason is that the Great lakes prevent some of the coldest drops in temperature.

Kevin, I am in zone 5 here in St. Louis, our last frost date is April 15. However, my front yard is warmer than my back yard because of wind protection, heat retention from the house, etc. So the zones are averages, with time you will learn how your property warms. We even have a place against a wall that stays so warm that tender bulbs that should not be left out in the winter survive year to year.

That said, I don't push the time, I plant at the beginning of May unless we are having an early, consistently warm spring. Others plant earlier but have materials ready to cover plants.

Here in central Iowa the frost date is around the 7th of May. I have planted a week earlier, but also have had a light frost happen.....it should be really safe after the 1st week of Mar......