Dahlias are often grown for cut flowers. If you cut them while they're in full bloom, you will be able to enjoy them indoors. The practice of cutting--or harvesting--flowers will actually encourage more blooms. For this reason, you should also cut off spent flowers as well. Dahlias typically are planted in the spring and bloom in the summer.
Harvest dahlias in the morning. At this time, they are well hydrated and plump. Avoid harvesting in the middle of the day when the flowers are more likely to be limp and dehydrated from the sun. They will have trouble recuperating in the vase and will have a short life thereafter.
Harvest dahlias when they are completely open. Only one flower grows on each stem and it's best to harvest when they look their best.
Cut dahlia flowers about 1 inch from the bottom of the stem with a sharp knife or clippers (not house scissors). Cut at a 45-degree angle, which will expose more of an area for the flowers to drink water.
Place your dahlias in a bucket of lukewarm water (100 to 110 degrees F) as you harvest them. A plastic bucket is better than a metal one since metal can change the water's acidic levels.
Transfer dahlias to a vase mixed with preservatives as soon as possible. Dahlias have hollow stems so before you stick them in water, fill their stems with water and place your thumb over them, turn over quickly and stick them in the vase. You can also plug up the stem with a piece of cotton ball.