Sunflowers (Helianthus annuus) grow as annuals, with some varieties only living for two months. They thrive in hot weather with full, all-day sunlight, but they require 34 inches of water annually for best growth. Some of that water comes from rain and natural water in the soil, but most is dependent on regular watering.
Once established, sunflowers tolerate brief periods of drought, but best growth occurs in consistently moist soil. Soil quality and the amount of natural moisture in the soil affects how much water sunflowers need, so monitor the soil moisture in your garden and water when it begins to feel dry. Generally, watering deeply once a week and providing at least 1 inch of water or enough to keep the soil moist to a 6-inch depth is sufficient if there isn't comparable rainfall in the preceding seven days. Don't allow the soil to dry out completely in the 20 days before and after flowering, if you are growing sunflowers for seeds.
Overhead watering wets flowers and foliage, but very little moisture makes it into the soil and to the roots, so water the soil directly instead. Sandy or quick-draining soils may not hold enough moisture, so the sunflowers may need more frequent watering in these locations. Working compost into the site and covering the soil with mulch can help maintain consistent moisture.