If you have definite killing frost in winter, things are different. In these areas (most of the country) spring and fall are both fine for planting, and each has its advantages.

If you decide to plant in fall, wait until after killing frost. The timing is roughly the same as when tulip bulbs are planted-late enough to be sure your seeds will not sprout before winter. The point is to wait until the soil is so chilled that seed cannot sprout, but stays dormant until warming soil and moisture trigger germination in spring.

The idea is to clear the ground, do not sow seed, but instead begin immediately to encourage weed growth as quickly as you can. This means watering if it's dry, and watching closely. After about two weeks, you'll see green seedlings popping up, and you'll know at least the early germinating seed population of your soil. Wait as long as you can (this usually depends on weather, and how early you got started), and once you have a good idea of what you're dealing with, you're ready to kill those young weeds and spread your flower seed.

here are several ways to proceed. Many use a herbicide like Round-Up. Others have been known to lay wet newspaper on the weeds to smother them, but this is not surefire and takes longer. At this point, you must resist heavy raking or tilling again, because if you do, you'll turn up fresh weed seeds which will begin their sprouting process, starting the whole cycle over again. In other words, at this point, you must kill the weed seedlings you see, but NOT disturb the soil again.

Annuals are the flowers that normally sprout quickly, grow fast, and are the first to bloom. They bloom heavily, then drop seed from fading flowers, and are killed by the first frosts. Annuals are the plants that live only one growing season.