If you’ve ever wondered when to plant lettuce, watch for dandelions. Every spring and autumn, these urban wildflowers signal the start of temperate periods when the region has warmed up or cooled down to the point that it’s salad season.
Emily Green in LA Times Home and Garden

To Emily’s advice I’d add plant your seeds when you see volunteer seedlings appear. The soil temperature, day length and other conditions must be right.

This arugula seedling appeared in early September. I’ll be planting arugula soon now that the weather is more seasonal.

Ditto for peas, and in this case, sweet pea seedlings which also showed up about six weeks ago. Robert Smaus, former garden writer for the LA Times coached readers to plant sweet peas when the kids go back to school. (That was before July and August starts).

Good gardeners attend to subtle seasonal changes.