Except where noted, these annuals prefer mostly sunny locations. Keep old flowers picked off to prolong bloom.
Annual mums. In hot climates, choose a spot that gets some afternoon shade. Sow seeds outdoors after weather warms for blooms in summer and fall. (If you live in a mild-winter climate, you can also sow in fall for spring and summer bloom.) You may also plant from nursery containers. Summer mums aren't fussy about soil. Space plants about 8 inches apart. Water deeply and frequently where soils are porous, less in heavy soils. Feed mums two to three times during the growing season.
Cosmos. Sow seeds in open ground from spring to summer, or set out transplants from cell-packs, 4-inch pots, or 1-gallon cans. (Yellow cosmos are easiest to start from nursery-grown plants.) Cosmos will flower best in poor, sandy soil; heavily amended soils and lots of fertilizer result in fewer flowers. Space plants about 12 to 18 inches apart. They can tolerate some aridity, but for best bloom, water them regularly (once a week or so), especially in hot inland valleys.
Dahlias. Provide light afternoon shade in hottest areas. Plant tubers in spring after soil has warmed and danger of frost is past. Dig holes 1 foot deep in loose loam high in organic matter. Space largest kinds 4 to 5 feet apart and smallest ones only 1 to 2 feet apart. Drive a stake into the hole; place the tuber horizontally, 2 inches from the stake, with the eye pointing toward it. Cover tuber with 3 inches of soil and water thoroughly. As shoots grow, gradually fill the hole with soil. Start watering regularly after shoots are above the ground. Dahlias planted in soil enriched with compost rarely, if ever, need supplemental fertilizer.
Marigolds. Plant in full sun. Marigolds are easy to grow from seed and sprout in a few days in warm soil. Or set out plants from nursery flats, cell-packs, or 4-inch pots. Slugs and snails are especially fond of young marigold foliage; use traps or ring the planting with horticultural diatomaceous earth (available at nurseries).
Sunflowers. Sow seeds in spring. If you use young nursery plants, space them 8 to 12 inches apart in soil well amended with compost. After true leaves appear, water plants deeply once a week. Fertilize once when plants are actively growing, using a controlled-release fertilizer. Large-flowered kinds need rich soil and lots of water.