The following is a seed starting schedule for the Dallas area from John Jeavons' outstanding book How to Grow More Vegetables.
This schedule assumes an average first frost date of November 13th. Note that this is an average date, and different sources claim an average frost date of anywhere from November 11th through November 30th for the Dallas / Forth Worth area.
Here is a link to a useful Interactive Map showing first and last frost dates.
If you are outside of Dallas, adjust the dates accordingly.
Plant seeds in garden for:
pumpkins (assuming 120 day maturity)
July 1st - 15th
Start seedlings for:
fall tomatoes (assuming 75-100 day maturity)
Plant seeds in garden for final corn planting (assuming 100 day maturity)
Start seeds for:
August 14th (14 weeks before first frost)
Start seeds for:
You are probably better off starting the forgoing indoors and then transplanting them later, but you must slowly harden them before transplanting. Nevertheless, I am a lazy gardener and prefer to just take my chances by direct sowing them into the garden with regular waterings for germination.
August 28st (11 weeks before first frost)
Start seed for:
fava beans (for fall harvest)
September 18th (8 weeks before first frost)
September 25th (7 weeks before first frost)
October 1st-October 15th
Plant strawberry crowns for Spring
Plant potted fruit bushes
garlic cloves (for spring harvest)
shallots (for spring harvest)
onions (for spring harvest)
fava beans (grown as a cover crop)
Late December after numerous frosts
Harvest horseradish after numerous frosts. Divide the roots using the largest to make horseradish sauce. Transplant smaller roots for next year.
The items in red are not listed in John Jeavons' How to Grow More Vegetables.
Some of these vegetables like beets and radishes can be succession planted.
Remember, when you are starting seeds directly in the garden. you need to supply steady, daily moisture to these seeds for them to germinate and thrive. Also remember to consider the soil depth for the particular variety of vegetable when planting and mulching.