The cooler part of the day is better. If it is hot and uncomfortable for you, you can bet the flowers are thinking the same thing. Picking them in the heat means they won’t last long. Mornings at the orchard are best
Pick flowers that haven’t fully opened will last longer
Don’t pick flowers with floppy stems
For taller varieties like zinnias, cosmos and sunflowers, cut stems the length of your forearm. You can trim them shorter later, if needed
Remove the leaves from the stems as you pick
Replace water in your vase every 2-3 days