Being a new or returning gardener can be daunting-there is a lot to learn and remember. As a teacher and friend to many new gardeners, one of the things I get asked again and again is the timing of planting.
There are a few great resources out in the world to help with this question. The first is seed catalogs. I use a few catalogs here in the U.S. to guide my plantings. For instance High Mowing (East Coast) and Territorial (West Coast) both have "cultural information boxes" which are little sections in the catalog for each crop detailing when to start indoors, when to transplant, what fertilizer the plants prefer, what diseases to watch out for and more. If you don't already have these catalogs-get them. I'm sure there are other seed catalogs that also put this in, but these two are off the top of my head (and at the top of my stack). The nice thing is I can still get seed from other sources as well, but use these catalogs as an information resource.
The second thing to know about timing is when your average last frost date is. Many catalogs will tell you to "start seeds indoors 6-8 weeks before the
last frost date" or "transplant after danger of frost", so you need to know when that date is for where you live. There are many sources on the internet that will tell you based on the city you live in-for many countries around the world. My current favorite for the U.S. is Plant Maps.The trick with this one is to scroll down and find your state on the left hand side column. Click on it, and when that page loads, look to the right side column to find some interactive maps. Choose your state's last frost map. Voila! It really helps to zoom in, because it might highlight certain micro-climates that have a different last frost than the area five miles away.
If you are buying starts from a farmer's market or nursery, it helps to know when you can put those babies outside. Use both of these resources to find the best time. Here is an example:
I currently live near the Seattle area. According to the seed catalogs, I should start my tomatoes indoors 6-8 weeks before the last frost, and I shouldn't transplant tomatoes outside until danger of frost has passed. This means I need to go to the map and find out when my last frost date is so I can plan when to start my tomatoes, and safely plant them in the garden. For my area I can transplant tomatoes outside the last week of April... which means I've got work to do this weekend!