If you have bought your seeds, line them up and look at the packages. They will say whether the seeds can be planted before the last frost. Here's a hint.
The following can be planted before the last frost: kale, collards, spinach, mustard greens, broccoli, cauliflower, cabbage, green peas, snow peas, beets, turnips, parsnips, rutabagas, some onions.
The following have to be planted after the last frost: corn, green and wax beans, squash, peppers, tomatoes, okra, black eye peas, herbs, most flowers.
I'm forgetting some things, I know; these are the ones I have planted. Again, go by the seed packet.
If you plant hardy veg at the same time as the warm-weather veg, you won't get good results. They will bolt, which means they'll shoot up and go to seed. The only exception to this is peas and carrots. I usually split my packs of peas in half and plant some early and some later.
The only other exception is if you start warm-weather veg inside before the last frost and then transplant them to your compost. Seed packages usually recommend this. However, where I live the growing season is long enough that I can plant seeds for tomatoes and peppers in the compost outside and harvest on schedule, by Columbus day. If you live north of me, you should start them inside.
This is my schedule in an ideal year.
1 March: plant hardy veg;
1 May: plant warm-weather veg;
1 June: finish harvesting peas;
1 July: finish harvesting roots and greens;
1 August: green and wax beans almost done, summer squash going strong;
1 September: plant hardy veg for autumn, take out beans and squash;
1 October: take out summer veg;
12 October: take out tomatoes and put peppers in pots to winter in the house;
Thanksgiving: finish harvesting hardy autumn planting.
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