Native to the warm areas of Asia, where it's a perennial herb, basil is grown as an annual in the cooler Northwest and doesn't do well if planted too early. It won't do much of anything until the soil warms up to at least 50 degrees, usually not until June.
For best success, here's some advice from the Oregon State University Extension Service:
Wait until soil temperature is consistently above 50 degrees, then plant basil seed or starts into a well-worked, composted garden bed.
Cover seeds with fine soil, no deeper than 1/8 inch. Gently tamp down soil over seeds and water with a fine spray. Seeds should germinate in one to two weeks. Keep the soil moist; soil can dry out in a jiffy on warm days.
If using starts, plant them 12 to 18 inches apart.
Young basil seedlings wither easily in the hot sun. Provide shade for them the first week after emergence.
Thin plants grown by seed by cutting unwanted plants off at the soil surface with scissors. Plants should eventually be 12 to 18 inches apart.
Water and fertilize your basil plants frequently. Basil leaves can be harvested off the plants periodically throughout the summer.
Pinch off the flowers to keep the plant from setting seed.
-- Homes & Gardens staff
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