Seed selection is one of the most important parts of growing a successful crop. Considering the length of time for maturation is key in managing the time in the growing season for the Philadelphia area. Using tomatoes as an example: If started indoors, they can take up to two weeks to germinate then another 4-6 weeks until they are ready to be transplanted outdoors. You sow the seeds indoors to coordinate the planting of the seedlings of the right age at the right time.  They have to be planted outdoors no sooner than about the 19^th of April to avoid the chance of frost. The Philadelphia area has an approximate 24 week growing season and gives time for the tomato plants to mature and produce fruit. Tomatoes will keep producing fruit as long as the temperature are still warm, so the earlier you are able to plant them outdoors, the larger amount of fruit you will be able to harvest.  If you sow the seeds outdoors after April 19^th, you will end up losing several weeks of your growing season.

Here are some tips on starting tomato plants:

http://www.tomatobob.com/seedstarting.htm

Find out your frost dates here:

http://cdo.ncdc.noaa.gov/cgi-bin/climatenormals/climatenormals.pl?directive=prod_select2&prodtype=CLIM2001&subrnum

Seed Companies:

 Click here to get on the catalog mailing list for Rohrers. They are located in Lancaster County, PA.

www.rohrerseeds.com/gardenseed.php

www.highmowingseeds.com  - Organic

www.stokesseeds.com

Fall peas

Here is an information sheet on seed packets:

http://consumerhorticulture.psu.edu/files/seed_packets.pdf