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 90 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 ist and the i5th 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 i) 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 i, when the last killing 

 frosts in the United States occur in the extreme north- 

 ern parts of North Dakota and Minnesota. 



