Marigolds look lovely and cheerful in the garden at this time of year, and we need to pick them, but if the foliage of marigolds is submerged in a vase it creates an unpleasant odor so be very careful to strip off the leaves before conditioning marigolds in warm water.
Asters should be cut when about half of the flowers are open, and most of the leaves should also be removed. Split the ends of aster stems about one inch or dip them in boiling water for 30 seconds, and then condition in deep lukewarm water.
Snapdragons should be cut when about half of the blooms on a flower spire are open. Strip the lower leaves and place the stems in hot tap water to condition for 3 to 4 hours.
Zinnias are cut in full bloom and the stems should be dipped in boiling water before conditioning in cool water.
Yarrow is cut when more than half of the flowers are open. The stems should be dipped in boiling water and then conditioned in cool water.
And Queen Anne's lace also has its stems dipped in boiling water before being conditioned in cooler water.
You'll notice that stems with multiple flower heads or flat flower umbels always need the boiling water treatment because they need to take up a lot more water.