Remember that the flower buds on forsythia are killed as temperatures reach –10 degrees F. If your area has had temperatures this far below zero, use one of the other woody plants.
Choose a day that is above freezing for collecting branches for blooming. Keep the stem length to 3 feet or less. As you cut, place the stems in a bucket of water. Once you have the number of branches you want, bring them into the house and soak them in warm water for several hours -- a bathtub works well for this. This ensures that the stems and buds are fully hydrated. Next, place them in a container that has a warm, preservative solution and place them in an environment with high humidity and plenty of light.
Make your preservative solution by dissolving packets of floral preservative in water. These packets can often be obtained from your local florist. You can also make your own preservative by adding a tablespoon of Listerine per gallon of water, but commercial preservatives are preferred. Floral preservatives accomplish two functions; they prevent bacterial growth in your water and provide nutrients and energy for the life processes of the plants.
Many times our houses have a very low relative humidity during the winter. These low humidities can lead to dehydration of flower buds and blossoms. To raise the humidity around your plants, mist the plants or drape a dry cleaner's bag over your stems. If a cleaner's bag is too small, use a painter’s clear plastic drop cloth. Humidifiers can also help raise humidity levels.
Normally, forsythia will take about nine days to flower, quince will require between 12 to 20, and pussy willow needs from five to 15 days. The time required will vary depending on indoor conditions and how late in the winter the branches were collected. Most woody plants should be in flower within three weeks of collection and will remain in flower for about a week before blooms start to fade. (Ward Upham)