"higgledy" wrote:

If the last frost date for zone 5 is April 30, can I SAFELY sow seeds
two weeks prior to April 30? Or is there some reason that I don't
realize why I need to wait till April 30?

First, it isn't. It depends upon where you are in zone 5. I am in zone
6 and our last frost date is May 15. Some people put tomatoes and other
things that will freeze out before then, but they are prepared to cover
them and protect them from frost. If you can't protect the things you
want to plant, and they are subject to frost, you may want to buy two
packs of seed and plant one early and then plant the other after the
frost free date or buy more seed if you get hit by frost.

The plant hardiness zone map is at:

http://www.usna.usda.gov/Hardzone/ushzmap.html

There is no last frost date. But there are dates giving certain
probability of being after the last frost. The most common one is the
one that gives 90% chance of being after the last frost. It is called
the 10% chance of last freeze date. The NOAA map for 10% chance of last
freeze date is at:

http://www.ncdc.noaa.gov/oa/document.../Spring32F.pdf

By comparing these maps, you can see that there is only a slight
correlation between them. Many crops can be put it at the 10% chance of
last frost date. Corn needs warmer soil and is planted later. Some
very tender crops are planted 2 weeks after this date to take less of a
chance.
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