This is Erigeron annuus, or daisy fleabane, a native annual that pops up in my garden most years. Usually there are one or two plants in otherwise bare spots in a garden that gets about half a day of sun. The leaves look very much like those of the volunteer goldenrod in the same garden (not sure of the species, it’s hard to differentiate the many goldenrod species). This Erigeron may be a mere 1′ tall or it may grow up to 4′, depending on what’s around it. This year it’s tall, because it’s competing for sun with the sweet joe pye weed (Eupatorium purpureum) that has spread through the same bed.
Why are there so few native annuals? I know of only a few: this Erigeron; two lovely Impatiens species, orange and yellow spotted touch-me-not, that grow in wet places; and two species of ragweed (Ambrosia). I’m sure there are more, but the vast majority of our native forbs (herbaceous plants) and grasses are perennials.
The answer is rainfall. An annual is a plant that completes its entire life cycle, from germination to seed production, within a single year. Annuals must work fast: the time from germination to death may be only a few weeks. Annuals tend to be plants of dry climates, where rainfall is sudden and unreliable. Typically, annual seeds remain dormant until it rains; then, under favorable conditions, they germinate and grow, and for a short time, the desert blooms. Annual plants have small root systems–they don’t have time to grow big ones. In contrast, perennials develop large root systems that allow them to wait out short periods of unfavorable weather. They don’t need to wait for favorable conditions to germinate or grow.