Reliable flowering can be expected from summer bulbs and tubers.

Early Summer Flowering
Alliums flower May-July, I like those that look like a burst of stars
Anemone start in April with the woodland varieties then come the blousy De Caen and larger corm types through summer

Dutch Iris look good for a short while in midsummer but I find I get poor results in containers.

Summer Tubers and Corms
Begonia tubers include trailing varieties or large dinner-plate blooms through to first frosts.
Dahlias vary in height upto 6 feet. In autumn lift and store with about 6 inches of stem left on the tubers
Gladioli are colourful additions to your garden and make excellent cut flowers.

Personal Favourites
In addition to the above I currently love Lilies. I have planted over 150 of the Oriental varieties this year.
Freesias would be a favourite for the scent alone if only they were easier to grow – this year I am not bothering!
Sparaxis can be quite vivid if they get enough sun and are protected from winter frost.
Chincherinchee are late spring flowers also sold under the latin name of Ornitogalum thyrsoides