I know it's hard to believe since the summer solstice is still 11 days away and if like me, you are still harvesting spring broccoli, cauliflower, cabbage, kale and kohlrabi, but it's time to start planning for the fall garden. Fall, cool season vegetable transplants broccoli, cauliflower, kale and collards will need to be sown indoors in the next few weeks in order to have transplants ready for the garden in the first weeks of August. Brussels sprouts should be sown now, since they take longer to mature and should be transplanted in mid July
Referring to the Home and Garden Information Center's publication HG 16 and using broccoli as an example, broccoli transplants can be planted in the garden in late July through the first three weeks of August. Your actual transplanting date depends on the first frost date for your area. First frost date for Maryland can be found by accessing this link. I use October 15 as my first frost date for Clarksville Md. I usually transplant my fall cool season vegetable the first week of August, since I'm usually at the beach during the second week.
Determining when to start your transplants is a fairly easy calculation. First, pick a date when you want to start harvesting your fall broccoli or use the first frost date in your area, say you want to start harvesting on October 10 and continue to harvest side shoots throughout the fall. From October 10, subtract 14 days for so called short day factor (after the summer solstice, the period of daylight each day shortens so plants don't grow as fast). You also need to take into account the suggested number of days it takes to raise the broccoli from transplant to maturity. In the case of the Packman broccoli I'm growing this fall, it's 55 days from transplant to maturity. So that means the transplants should go in the ground 69 (14 + 55) days prior to my October 10 harvest date or about August 2. Since it takes five to six weeks to get broccoli from seed to transplantable size, subtract that number of days from the August 2 date (I'll use six weeks or 42 days in my example), so you should start your seeds on June 21. The formula is, harvest date minus short day factor (14 days) minus days to maturity minus days to grow from seed to transplantable size equals seed starting date.
On the top right hand side of the Grow It Eat It web page, there is a link to a useful Excel spreadsheet which can help with calculating seed starting dates for spring and fall gardens.
For more information on starting vegetable seeds indoors, access this link. This web page will also allow you to view videos on seed starting.