Answer: The major considerations are how much direct sun the beds get so you can choose plants with a similar preference, and choosing plants that have different bloom times (early, mid, or late summer) so you always have something in bloom. Plant tags can tell you what a particular plant needs in terms of sun exposure and should also tell you when the normal bloom time is.
Here's a list of some of my favorite sun loving perennials for summer bloom:
Rudbeckia - Black-eyed Susan is well known to millions of gardeners by its bright yellow petals surrounding a dark black center. Rudbeckia fulgida 'Goldstrum' is probably the best all-around cultivar in this family because it produces compact and bushy 2 to 3 foot tall plants that are covered with blooms for at least a month during July and August. It will self sow and you may find yourself surrounded by this wonderful plant.
Echinacea - Purple coneflower is native to the prairies but it blooms quite well here in the Northeast. It grows 2 to 4 feet tall and produces purple daisy-like flowers with dark orange-brown centers. It is a magnet for orange/brown fritillary butterflies. It is the root of this plant that is used to make the medicinal herb. E purpurea 'White Swan' is a very pretty white version of this purple plant.
Monarda - Bee balm or Oswego tea is a member of the mint family and the leaves do make a fine herbal tea. It grows 2 to 3 feet tall and is topped with pink, red or purple flowers depending on which variety you choose. It is very attractive to butterflies, bees and hummingbirds. It spreads like crazy and you may find yourself ripping it out of your garden to keep it under control. Another fault it has is the plaque of powdery mildew some varieties get each summer. New varieties including 'Marshal's Delight' are not prone to mildew and they are very pretty with their bright red/scarlet flowers.
Physostegia - Obedient plant gets its name from its supposed characteristic of staying in place if you bend its flowering neck. I haven't seen this attribute at work, but I do love this plant. It grows 2 to 3 feet tall and is topped with spikes of pale blue flowers in August that resemble snap dragon. There are white flowered varieties of this easy care plant as well as variegated selections that can add to your garden design.
Boltonia - Sometimes known as false aster, B. asteroides is a late summer bloomer that can grow up to 4 feet tall. It is like a big overgrown daisy with tiny white flowers on top that are so dense and sparkling, one can see why it resembles an asteroid to some people.
Buddleia - Butterfly bush looks like a shrub but in our area is a herbaceous perennial that needs to be cut back every in the early spring. It will grow up to 6 feet tall and 6 feet around can will produce scads of purple, white or pink flowers that look a lot like lilacs. There is a bank of Buddleia at Riverbank State Park in Harlem that attract so many butterflies it is actually difficult to walk past the plants on a warm summer day.
Hemerocallis - Day lilies are the most reliable of all summer blooming perennials. Once their bloom cycle starts in early July, they just bloom and bloom until they are stopped by frost. There are more than 2,000 registered colors of these hardy lilies but most are in the yellow, red, pink and orange range with lots of variations. A lot of people plant daylilies as ground cover around swimming pools and along embankments, but one well-placed is just as pretty. .
Aster - Both New England aster (A. novae-angliae) and New York aster (A. novi-belgii) are also known as Michaelmas daisy because they come into full bloom near the feast of St. Michael in late September. These are the premier late summer flowers for your garden. Most asters are quite tall, usually from 2 to 6 feet in height and covered with 1 to 2 inch wide daisy-like flowers in shades of red, blue or purple.
Phlox - Garden phlox (P. paniculata) is an old-fashioned summer flower that produces clusters of blossoms in red, pink or white on 3 to 4 foot tall spikes. They love full sun but will tolerate a little bit of shade. They can be susceptible to powdery mildew but keeping the clumps thinned to no more than five stalks per clump will help as will choosing those varieties such as "David' that are not as prone to the disease.
Hope this short list helps you get started!
Q&A Library Searching Tips