I don't use them. Cincinnati area and I've started all my pepper seeds, in fact, one variety (Bhut Jolokia) that has close to a 150 day until ripe period I started a month ago. This week I also started Alpine Strawberries, Pink Angels Trumpet (flower) and Kenaf Hisbiscus (flower).

Since I like larger plants - ones with five-six sets of leaves to transplant, I will start my tomatoes next week and cucumbers in early April. Still experimenting with Green Beans but I'm thinking late mid-late March.

Unless a plant is over 18" tall, I can get it in the ground when it is time to transplant.

Mike

It all depends on the climate. I tend to start things like cabbages and head lettuce earlier than recommended because they need spring temps to grow. These start around mid-march continue through April. If I followed the recommended practice, I would be setting out my plants in mid-April. Too late for harvest as we start having temps rising quickly in May.

It's good to go by your Extension service in your area or a local weather almanac to judge some average temps for certain days to help you decided when to start.

Hope that helps.

I start my seeds from previous years experience. For example least year I started geraniums late and they grew okay so this year I started them first and they will be blooming when its time to plant. I pay attention only to the frost date I can keep them healthy till then

Here is a link that might be useful: Gardening by the Seat of my Pants

I know what my last frost date and what previous years taught me. I start seeds indoor and turn and twist dates depending on the weather, my mood and time availability.