phantom spring
n. Exceptionally warm fall or winter weather that causes plants or animals to act as though spring has arrived.
Les Hazelgrave, the gardener at the Airds Hotel, at Port Appin, Argyll, has also recorded changes in the weather.

He said: "It's warmer, the growing seasons are getting longer and I am cutting the grass earlier and later in the season. There were still midges about the other day.

"We are not getting the frosts we used to get and the winters are getting wetter."

He added: "The plants think it's spring now. It's called a phantom spring where the plants start to die back but, because it's so mild, they start to give fresh growth.
—Moira Kerr, “A world of difference as weather plays tricks,” Aberdeen Press and Journal, October 30, 2006
The weather really is going haywire. Britain's gardeners are reporting the first signs of a "phantom spring" in the midst of one of the warmest Octobers on record.

Shoots of spring flowers are pushing out of the soil in England and in the even-warmer climes of the Channel Islands primroses and dog violets are blooming.

Botany experts say it is likely that lilac and apple trees will be blossoming next month.

Tony Kirkham, head of the arboretum at Kew Gardens, southwest London, said: "This kind of weather is very confusing for plants. Some trees will blossom as a last-minute fling before the winter and some flowers come up because they are getting such mixed messages from the weather. It makes it look like spring has come early." …

Last year, gardeners reported a "phantom spring" in early November. Reports on a BBC website included red admiral butterflies, trees in blossom and flowering violets. Meteorologists say that the warming climate means the phenomenon is likely to become common.
—Jon Ungoed-Thomas & Yuba Bessaoud, “It's so warm plants think spring is here,” Sunday Times, October 22, 2006
1998 (earliest)
When the Big Snow hit Dec. 30, some wildlife watchers worried how the deer and other critters would fare in deep snow followed by bitter cold.

When temperatures in the 40s and 50s triggered the Big Meltdown last week, worry whipped around 180 degrees. Suddenly, people worried the balmy new year might fool plants and creatures into responding to a phantom spring.
—Jim Reilly, “Animals enjoy respite during winter thaw,” The Post-Standard, January 11, 1998