If you've ever considered or have planted seeds outdoors, you've read the directions on the seed label that says something, sow seeds as soon as soil can be worked or sow seeds after all danger of frost or 8 weeks before last frost date. But what does all that mean? How do you know when the soil can be worked?
The reason they say as soon as soil can worked because if you try working in a garden plot that's not ready, you risk turning that dirt into hard clumps that takes considerable work to repair. That's why it's important to know when the soil can be worked.
Dig down about 4 - 6 inches and grab a handful of soil and squeeze. Yeah, it's dirty, but someone has to do it. If the ball won't crumble easily, it's probably too wet. If it crumbles easily, it's good to go. If you can't make a a good clump and the soil just crumbles, then the soil is too dry and will need some supplemental water.
The frost free date is a specific date to your area and it is the last date of the last spring frost over the last several years. Some seeds can be sown before this date, and others on this date, and some after this date. So that frost-free date is a marking point to make your calculations for seed planting time.
Another qualifier is soil temperature, and this is usually specified by the seed packager. A soil thermometer is the most accurate way to know that number is. Soil thermometers are inexpensive and a good thing to have around if you do any serious gardening. Plant when the soil reaches your crop's ideal temperature.
Know the ideal soil temperature for the plant to be planted and what kind of temperatures they can survive. Knowing what the average last frost date is important too. It is this date that most seed manufacturer's use as a guide for the ideal planting date. For example: "plant seeds 6 weeks before the last frost-free date."
Adding compost o your garden is always a god idea every spring. This helps keep microbe activity good, adds tilth to the soil and makes it easy to drain excess water and still hold moisture necessary for the plants to absorb.
The 2012 USDA Plant Hardiness Zone Map is the standard by which gardeners and growers can determine which plants are most likely to thrive at a location. The map is based on the average annual minimum winter temperature, divided into 10-degree F zones. The 2012 map is the first revision since the 1990.