Pretty much everything has different needs. Most seed packets or web sites will tell you when to start them.

6 weeks before planting out is the common suggestion with tomatoes, (I do 7-8)

Eggplants at least 8, possibly 10.

Peppers at least 10, some go for 12-14 (hot peppers are usually slower)

Broccoli 4 weeks - (to early and they will stunt and not produce well)

Most people direct sow Peas. I start them inside but plant them out when they are only an inch or so - maybe one week?

Annuals can be started as early as 12 weeks, but 6-8 is common, it will depend on how much room you have under lights, as you will have to pot them up to larger containers if you start them early. And some grow faster than others. With most flowers you can pinch them back when they gain some height which will make them bushier. Last year I started Petunias in mid-January and had blooming , trailing baskets of petunias in May, but it took a lot of babying and a lot of space to bring them through 4 months of growing under lights.

Perennials really differ plant by plant, some have long seed germination times, so you need to take that into account. Many don't bloom the first year, so it hardly matters when you get them planted out. I often don't start perennials until after my annuals are in the ground due to space under the lights.

I start pretty much everything indoors, except for root crops. But I have a very short growing season.

I'd suggest doing searches for each specific plant that you are going to grow. What I do seems to work for me, but you are going to get different answers from different people with smaller lighting set ups and in different climates.

Pretty much agree with mandolls. I do peppers and eggplant 8 weeks (hot peppers 10 weeks) before plant out date, tomatoes 6 weeks before.

But May 15th isn't your plant out date for everything. Broccoli and peas are cool weather crops and get planted out much earlier than summer crop planting date anyway. Peas should be direct seeded as soon as weather allows.

Strawberries - I'd never do them from seed.

Zinnia, Angelonia, Petunia, Viola, Verbena all have very different starting dates due to much different germination times. Zinnas should be direct seeded. They grow too fast. Petunias I start in Jan. as I want lots of cuttings to root off them before planting out. The others 6-8 weeks depending on how many potting ups you want to do.

And perennials are a whole different ballgame. For first year blooms I start them back in Dec. and baby them through till plant out.

Dave

Perennials (start indoors):
Agastache, Daisy, Delphinium, Osteospermum

IMHO these are better off being winter sown, they won;t bloom the 1st year anyway and it saves space under the lights.

Here is a link that might be useful: WInter sowing forum

Thanks for the info so far. All good stuff.
I have some followups for all three of you.

Broccoli: Mandolls recommended 4 weeks but Dave mentioned that they get planted out much earlier than summer crops. For my zone (6A/6B) what would be the approximate plant out time? How large should be broccoli seedlings typically be? What is the lowest temp they can tolerate once planted? Do they need to be hardened before going in the ground?

Peas: Mandolls suggests most people direct sow, but again Dave mentioned it's a cool weather crop (I didn't know that, I actually thought there were more like beans). If direct sowing, relative to my typical planting date of May 15 for summer crops , when should I considering sowing these? Also what kind of temps can they tolerate without having to cover them?

Annuals: Both Mandolls and Dave mentioned these are commonly started at 6 to 8 weeks (I could always start them sooner next year if need be). So I guess I should start these in the March 15 to March 30 time frame (except for Zinnias which Dave mentioned grow really fast and its best to direct seed them). For the Zinnia, when would I direct sow them? Before May 15 plant out?

Perennials: I didn't know that most don't bloom the first year. Quite disappointing. Ontheteam recommends winter sowing. In a nutshell is winter sowing similar to planting tulip bulbs? Mandolls on the other hand indicates that he doesn't start perennials indoors until annuals are ready to go out. When do you put the perennials out in that case, mid-summer? Do you keep them indoors until then? If so, how do you harden them? When do you know that they're ready to go out?

Strawberries: Dave indicated he would never do these from seed. Why not?

Starting at the bottom: Strawberries are normally grown from transplants not seed and bare root transplants are readily available for planting. Strawberry seeds are only available for a couple of small wild-derived varieties ie Alpine and are slow and difficult to germinate. Strawberries produce their own runner plants which are easily transplanted.

Planting times in your zones I can't help you with but that info is readily available on the web from all sorts of sources. On average cool weather crops (chard, beets, peas, broccoli, cauliflower, cabbage, fava beans, lettuce, spinach, etc.) are planted out at least a month prior to summer crops. For me that is early to mid March depending on soil temps as my last frost is Apr. 15th.

When it comes to direct seeded plants you need to learn to measure and plant by soil temps. Each has an ideal range of soil temps required for germination and that is what determines your planting date. Charts are available for all vegetables and most flowers. I linked one of the many below. EX: lettuce, spinach, peas require minimum of 40 degree soil, beans 60 degree soil, broccoli requires 50 degree soil, tomatoes 70 degrees, peppers 75, etc. Broccoli transplants are best transplanted prior to the 3rd true leaf which is normally approx. 4" tall.

Perennials blooming the first year varies depending on zone. In the warmer zones, if started early enough, many will bloom that summer.

Most annuals seed packets will give you an approx. direct seed date on the packet. They are divided into hardy annuals, half-hardy annuals, and tender annuals classes and require different soil temps for germination and survival so you need to know which class your choice fall into. hardy annuals will tolerate light frosts but tender annuals are only planted after all danger of frost is past.

Zinnas - there are lots of posts here from folks who have tried to start them indoor and discovered the problems. Direct seeding them in early May should be fine for your zone.

Dave

Here is a link that might be useful: Soil temps for direct seeding vegetables.

I just bought a couple of bags of starter soil today to get the ball rolling and I'm thinking of starting the broccoli, perennials, and annuals today in my basement. Sounds reasonable? This would give the broccoli at least 7 weeks before putting them outdoors.

Regarding lettuce, peas (both of which I'll direct seed) and broccoli (started indoors) under what conditions would I need to protect these cool weather crops once they are in my raised beds? I'm guesstimating that they'll be in my garden in mid-April, a month before my summer crops go in.

I really want to give the strawberries a shot so I might start them tonight too. Even if there is a low probability of success, at least I'll know I tried.

By the way, thanks for that link to the germinating temps. Neat thing to add to my seed storage container for reference.