Petunias and marigolds thrive on summer conditions. With a little care once in a while we can keep them blooming until the first frost.
The only reason plants make flowers is to produce seeds. Seed are what keeps the plant surviving years after year.
Once the plant starts to form or forms seeds, new flowers production starts to slow.
That is why it is important to deadhread and remove the old flowers.
Simply pinch or snap the dead or dying flower at the first leaf on most annuals. This pinch should be really easy on most annuals because their stems are succulent and not woody. After deadheading the flowers place them in the compost.
Do not leave spent or dead blooms around the plant as diseases such as botrytis can develop.
This is most true with geraniums.
Perennials such as coral bells, hostas, iris, daisies, peonies and daylilies should have the flowers removed, also.
When flowers remain on a perennial plant it reduces the amount of food stored in the roots.
Once seeds start to form the plant put all its energy into their formation. Roses should be pruned to the first leaf containing five leaflets when deadheading.