If you have a yard that is sometimes dry, sometimes flooded, you'll have to plant flowers that take both conditions. Luckily, wet areas that dry out in summer are common in nature and many plants have adapted to fluctuating levels of moisture. Most prefer an occasional watering in summer, though some will go dormant until the next spring.
These are the kinds of spreading plants you should be wary of putting next to small, dainty plants but that grow easily and reliably into handsome patches of bloom if given the room they need.
Gooseneck loosestrife, L. clethroides, has spikes of tiny white flowers that curve gracefully on plants about 3 feet high.
Moneywort or creeping jenny, L. nummularia, is a flat ground cover with gold flowers and round leaves. Its long runners root at intervals along the ground, each one turning into a new plant.
Loosestrife, L. punctata, has upright stems, to 3 feet tall, with whorls of yellow flowers set along the top third of the stem.
Quite a few species of Iris are at home in wet soil, or places that are damp in winter and spring, but dry out later in the year.
Iris pseudacorus, the yellow flag, is quite happy in several feet of water, but also grows in places that are merely moist. It can reach as tall as 5 feet in height, likes acid soil and blooms in part shade.
Louisiana irises, hybrids of species native to the Southeast, grow 2 to 5 feet tall with flowers of blue, purple or rusty red. They are happy at the edges of ponds or in moist garden soil.
Siberian irises do well with lots of water in spring, but also tolerate some drought in the summer. Narrow foliage, to 3 or 4 feet in height, and delicate blue, purple or white flowers make this a useful perennial in the flower border or the informal border.
The cardinal flower, L. cardinalis, has spikes of brilliant red flowers in mid-summer and varieties with ruby red or purple flowers are available. It is a bog plant in its native habitat but will grow in rich garden soil with regular watering. Give it partial shade or sun and very wet soil.
Its cousin, Lobelia syphilitica, blooms a bit later with 3-foot spikes of blue flowers. It likes partial shade.
Joe-pye-weed, Eupatorium purpureum, is a substantial plant, 3 to 9 feet tall, with heads of pinkish purple flowers in late summer. A smaller variety, 'Gateway' stays about 5 feet tall and has darker flowers on purplish stems. Give either one sun or part shade. Though it is native to damp meadows, it tolerates wet to somewhat dry conditions.
Astilbe, sometimes called false spiraea, is a perennial with ferny leaves and plume-like heads of white, pink or rose-colored flowers. Some varieties grow 3 to 4 feet tall, others only 2 feet. All prefer moist, rich soil and, while they don't mind an occasional flooding, dislike a boggy soil with poor drainage. They prefer partial shade, but will take full sun with ample moisture.