Seeds need sufficient warmth to germinate. If they are planted in frozen soil they will not sprout. You can plant them outside after the ground begins to thaw, but as you can see in the table below it takes much longer for seeds to sprout at lower temperatures. I order to speed up the sprouting process, SLUG recommends sprouting seeds indoors and transplanting them in a few weeks when they are strong enough to be put outdoors. See this page for more information on when to start seeds indoors.

Germination Times According to Temperature of a Few SLUG Crops:
Crop
32°F
41°F
50°F
68°F
Beets
Not tested
42.0 days
16.7 days
6.2 days
Cabbage
Not tested
Not tested
14.6 days
5.8 days
Carrots
0.0
50.6 days
10.1 days
6.9 days
Lettuce
49.0 days
14.9 days
7.0 days
2.6 days
Radishes
0.0
29.0 days
11.2 days
4.2 days
Spinach
62.6 days
22.5 days
11.7 days
5.7 days
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Boston is in Zone 6A of the USDA Hardiness Zone map. This means that the average last frost date is usually between March 30th and April 30th. You should not plan to have plants outdoors without protection before this time period. For more information on setting up greenhouses or season extenders, see the wikispace.