Figuring out a seed sowing calendar is a huge challenge. Seed companies tell you one thing. Established farmers tell you another. Your gut tells you something else. In the end, you know that your situation is unique and no one else's advice is actually going to be "spot" on for what you need to do. It can feel confusing and frustrating. Be sure to write in pencil!
We start the old fashioned way. Paper calendar from The Old Farmer's Almanac, pencil, and a big eraser!
The essence of a good calendar begins with record keeping the year (and years) before. We keep a detailed, tabbed spreadsheet that outlines where seeds have been sourced from, when they are sown, how many are sown and under what circumstances, what type of space they are started in, how many germinate, if any die off and for what reason, and include any notes to remember for next time. Phew-- that is a lot to get down, but being able to look back and see "start later next year" or "start earlier" or "protect from mice" is really tremendous.
But, if you're new to growing flowers you won't have much to go on. To start, look up your last frost date. There should be a somewhat reliable record online for when that was last year and when it is on average. The date was seared in my mind last year and not for a good reason! May 20. Argh.
Next, determine which plants will be planted out without cover. You'll have to use the average last frost date to determine your start time for those varieties if they are sensitive to frost damage. If you have row cover (frost protection cloth) you can plant some of your crops (or all depending on how much cloth you have) a couple of weeks before your last frost date. If you have row cover AND low tunnels or high tunnels, you can plant out even earlier. Of course, the suggestions above are not fail-safe. You have to monitor your plants and the weather in your area.
My calendar is below-- and it changes often int eh spring as I'm working and monitoring the seedlings! It is based on my plan for some plants that will be planted out under cover and some that will not be protected. I also tend to lean on the early side for things that I am trying or testing for the first time. Seeds tend to grow slower for us up here. We have less light, and well, it's pretty cold. If those early-planted seeds fail, I have a little cushion to start again. It happens. -Even to the best growers out there. You have know that some seeds will be "sacrificed" in the process.
We have 3 different seed-sowing areas. Our basement (fieldstone cellar that is consistently about 50 degrees), my office (about 65-70 degrees), and a small heated glass house at a school in town (where the temps swing between 45 and 85+ degrees F--it can get up to 100F if I'm not careful!). Bulbs and cold loving plants are sown in the basement with or without heat mats, finicky plants (lisianthus) are looked after in my office, and hardy things and heat lovers can go in the school glasshouse. My goal each year is to look for more ways to move my schedule earlier and extend my season further. Our first frosts can arrive early September. Note- the date precedes the plant variety listed. I give myself a few days around each item as leeway for planting and I try very hard not to fall behind.
4 Sweet Annie - Direct sow away from gardens-- it spreads!
4 Tansy - Direct sow away from gardens -- it spreads!
Please share questions and comments. I'll try to help as best I can. Remember every grower's situation is different. I really discourage folks from trying to replicate what we are doing exactly. After all-- we're still figuring it out ourselves!