Don’t cut forsythia branches for indoor flowers yet. Forsythia and other trees and shrubs are not ready for spring — indoors or outdoors. Woody plants can sense when winter is over and can’t yet be fooled into unfolding their blossoms.
Changes in temperature and length of day tell plants when spring has arrived. Some plants need both cues; others respond to either temperature or day length.
Apples, plums, and most other fruit trees — all of whose branches make beautiful, sometimes fragrant, indoor blossoms — respond to temperature alone.
They won’t grow until they’ve experienced a certain number of hours of cool temperatures, called “chilling hours.” Chilling occurs at temperatures near 40 degrees Fahrenheit.
Fruit trees need about a thousand total hours of chilling before they can resume growth in spring, but the exact amount needed varies with the particular plant.
Plants native to regions with very mild winters and early springs need little chilling before they can resume growth in spring.
Little chilling is also needed by plants native to regions where winters are long and steadily frigid.
Then chilling requirements are not fulfilled until late spring, at which time plants must begin growth quickly in order to ripen their fruits within the short growing season.
In many regions, trees and shrubs put some hours into their chilling “banks” in late fall, and in winter when temperatures fluctuate.
Unfortunately, such weather often awakens “low-chill” plants too quickly in spring, so their flowers and fruits are damaged.
Once days lengthen sufficiently or chilling requirements are fulfilled, plants can grow, and will do so as soon as temperatures warm.
Forsythia branches can then be forced, but you have to wait long enough to begin.
Even when plants are ready to grow, forcing demands patience. I
f you rush the process, blossoms open sporadically along the branch, then dry up and fall off.
So, first plump up buds by immersing cut branches in tepid water for a few hours.
Then recut their bases and put them in a vase of water in a cool room. Move the vase out for display just as the buds are about to burst into bloom. The time for this will be shorter the closer to the natural bloom time that you begin forcing.
If you’re impatient for blossoms now, try immersing branches for a few hours in 90 degree water.
“Chilling bank” rules are not all that strict, and sometimes you can awaken a branch from its sleepy state with a high temperature shock.