Direct seeding or transplanting by soil temperature is much more dependable than a date or counting back from the ‘frost free’ date in your area. Published ‘frost free dates’ are based on the average last (or first) date that the temperature is expected to fall below 32° in your USDA zone. Unfortunately for plants, frost can settle on a localized area even when the temperature is well above 32°. Low lying areas are more susceptible and cities less so. Sign up with a local garden center for email frost alerts, watch the local evening weather forecast, and get up before the sun rises on a suspected cool overnight (frost occurs most often in the early morning hours). Protect your plants with a floating row cover or even a bed sheet suspended on patio chairs. If you find that your plants have been touched by frost – all may not be lost. Get out into the garden before the sun is high and start an overhead sprinkler. Water will often ‘melt’ the frost before it can burst the cell walls of the plant. Water the foliage with kelp too.
Having written all that, here’s the best I can recommend for Connecticut where our average TRUE FROST FREE date is around May 20th or the last full moon in May, whichever is later.
Check the instruction on the seed packet first!
Start Inside:
Onions 16-20 weeks before last frost (plant out 4-6 weeks before last frost)
Celery & Parsley 16-20 weeks before last frost (plant out 4-6 weeks before last frost)
Pansies and most Perennials 16-20 weeks before last frost (plant out 4 weeks before last frost)
Geraniums 12-14 weeks before last frost (plant out after last frost)
Peppers (Hot) 12 weeks before last frost (plant out 2 weeks after last frost)
Peppers (Sweet) 8-10 weeks before last frost (plant out after last frost)
Eggplant 8-10 weeks before last frost (plant out 2 weeks after last frost)
Tomatoes 6-8 weeks before last frost (plant out after last frost)
Lettuce 8-10 weeks before last frost (plant out 4 weeks before last frost)
Cole Crops 8-10 weeks before last frost (plant our 4 weeks before last frost)
Cucumbers & Melons 2-8 weeks before last frost (plant out 2 weeks after last frost)
OR
In January — Plant Inside:
Onions, Most Perennials
In February — Plant Inside:
Onions, Leeks, Celery, Parsley, Hot Peppers, Eggplant, Geraniums
In March — Plant Inside:
Broccoli, Cabbage, Cauliflower, Chives, Eggplant, Lettuce, Hot Peppers, Sweet Peppers, Early Tomatoes
Plant These Seeds in the Garden (when ground can be worked and soil temp is above 40 degrees):
Leeks, Lettuce, Shelling Peas, Spinach, Turnips
In April — Plant Inside:
Basil, Cucumbers, Melons, Peppers, Spinach, Squash, Tomatoes
Plant These Seeds in the Garden (when soil temp is above 45-50 degrees):
Arugula, Beets, Dill, Endive, Kale, Lettuce, Parsnips, Edible Pod Peas, Radishes, Spinach, Swiss Chard
Transplant Seedlings into the Garden (after they have been hardened-off):
Broccoli, Cabbage, Cauliflower, Celery, Onions, Pansies, Most Perennial Flowers
In May — Plant Inside:
More Basil, Cucumbers, Melons, Squash, Nasturtiums
Plant These Seeds in the Garden (when soil temp is above 50 degrees):
Snap Beans, Beets, Carrots, Collards, Kale, Lettuce, Radishes, Spinach, Swiss Chard
Transplant Seedlings into Garden (After danger of frost is past and plants have been hardened-off)
Cucumbers, Melons, Squash, Tomatoes
In June — Plant These Seeds in the Garden
Basil, Brussels Sprouts, Corn, Kohlrabi, Okra, Pumpkins, Squash, Morning Glories
Transplant Seedlings into Garden (when night-time temps remain above 60 degrees)
Basil, Eggplant, Hot & Sweet Peppers, Tomatoes
Continue to direct seed many short term crops through the summer and cool crops again in August for a fall crop.