The Time for Planting 157 
United States it is less than 4 months. Over a consid- 
erable area of the Rocky Mountain region and west- 
ward, the growing season for garden plants is not more 
than go days. 
Planting zones. Although certain cool-season crops 
can be planted in spring before the last frost, warm- 
season crops should not be planted until after frost. It is 
therefore very convenient, in planning for the spring 
planting of both seeds and young seedlings, to know 
about when the latest killing frost may be expected. One 
cannot tell in advance exactly on what calendar date 
this will occur, as it varies from year to year. But the 
beginner in gardening will be helped very much by the 
maps, prepared by the United States Department of 
Agriculture, which show the planting zones, based on 
the occurrence of frost. 
In making these maps, lines are drawn through the 
points where the average date of the last killing frost in 
the spring occurs on the 1st and the 15th of each month. 
Thus the line for killing frost in midwinter crosses central 
Florida and the extreme southern part of Louisiana. In 
a narrow belt below this line killing frosts are likely to 
occur each year, and below that they are likely to occur 
only once in several years. Killing frosts usually occur 
at points on or about this line about February 15. 
Two weeks later (March 1) localities much farther 
north experience their latest frost. Each two weeks 
sees the frost line move farther north (as shown on the 
map, Fig. 88) until about June 1, when the last killing 
frosts in the United States occur in the extreme north- 
ern parts of North Dakota and Minnesota. 
