Answer: Here are a few plants for you to consider:
Achillea (Yarrow) USDA Zones: 2 - 9, varies by species - Bloom Span: 3+ Months
Achillea will grow almost anywhere, but it actually favors dry, lean soil. If given too much moisture or rich soil, the plants can become floppy. Deadhead spent flowers for repeat bloom. After the second bloom, rejuvenate the plant by cutting back to new growth. Yarrow tends to die out in the center of the plant and should be divided every 3 years or so. GOOD CHOICES: Achillea 'Coronation Gold', A. millefolium 'Fire King', A. m. 'Summer Pastels'
Centranthus (Red Valerian) USDA Zones 3 - 9 - Bloom Span: 3-4 Months
Centranthus is often seen spilling over in photos of English gardens. Like Achillea, Centranthus prefers dry, lean soil, but it blooms longer in cooler climates. In intense heat it will bloom in spring and again as it feels up to it, throughout the summer. Centranthus rarely grows true from seed and is best propagated by cuttings. To be certain of what color you are getting, buy the plant while it is in flower. The plants don't live longer than about 5 years and they resent being divided or every relocated. GOOD CHOICES: Centranthus ruber 'Albus'
Coreopsis (Tickseed) USDA Zones 3 - 9 - Bloom Span: 3+ Months
Coreopsis are undemanding plants, but short lived. Either allow them to self-seed or divide the plants every 2-3 years and replant the newer, outer sections. Flower buds form all along the stems, making deadheading a time consuming challenge. Once the initial buds have completed blooming, sheer the plants back by 1/3 to encourage new flower buds. GOOD CHOICES: Coreopsis verticillata 'Zagreb', C.v. 'Golden Showers', C. grandiflora 'Early Sunrise'
Corydalis lutea (Fumewort) USDA Zones 4 - 9 - Bloom Span: 4 Months
Corydalis' ferny foliage and delicate flowers belie its fortitude. This is a plant that prefers partial shade and well drained soil and will find a home in cracks in rocks, on slopes in woodlands and along paths. Once established, Corydalis self sows wherever it can. However it can take years for the seed to germinate, so trying to start your own plants can be frustrating.
Gaillardia (Blanket Flower) USDA Zones 2 - 9 - Bloom Span: 3-4 Months
Daisies on caffeine. Gaillardia's yellow petals around a burgundy center are impossible to ignore in a garden. All they ask is full sun and they will keep on blooming all summer. Too mush shade and the stems begin to flop. In most cases, deadheading is not necessary for continual bloom, but it can make the plants look tidier. Gaillardia is another short-lived perennial and should be divided or seeded often. GOOD CHOICES: Gaillardia x grandiflora, Gaillardia 'Goblin' (dwarf), G. 'Burgundy', G. 'Monarch'
Kniphofia (Red Hot Poker) USDA Zones: 5 - 9 - Bloom Span: 3+ Months
The spiky, bottle-brush flowers of Kniphofia are beacons for hummingbirds. Although they look like tough customers, Kniphofia actually requires a bit of winter protection in cooler zones. They are also a bit fussy about liking moist conditions in the summer, but well-drained soil for the winter months. Full sun is generally necessary for ample blooms. Kniphofia does not divide or transplant well, although you can usually get away with removing and replanting the young side shoots of the plants. GOOD CHOICES: Any of the hybrids. Kniphofia ''Primrose Beauty' is especially hardy.
Best wishes with your garden!
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