Why is my Christmas tree beginning to grow?

Bert Cregg<cregg@msu.edu>, Michigan State University Extension, Departments of Horticulture and Forestry, and Jill O'Donnell, Michigan State University Extension - December 10, 2020

Under the right conditions, some Christmas trees may think it’s springtime inside your house.

Decorated Christmas tree with new growth. — In some years, species prone to early break bud like Concolor fir, Douglas fir, Balsam fir and Black Hills spruce are likely candidates to possibly break buds once displayed. Photo by Doug Thalman.

Often, Michigan State University Extension educators receive calls from homeowners in December because their Christmas tree has broken bud and started to grow while in the house. To understand what is going on, we need to talk about how conifers develop and survive the winter. Each year, trees follow a cycle of dormancy in the fall. This process helps them survive through winter until spring when they will come out of dormancy, de-harden and resume growth.

The two most critical environmental factors that trigger the dormancy process are the reduction of light, or photoperiod, and low temperatures. Conifers will stop growing and set terminal buds as days become shorter even though the day temperatures are still relatively warm, but the nights are cool in August and September. The dormancy process first begins because of decreasing photoperiod, but continues as trees respond to low temperatures around or just below the freezing point. This dormancy or chilling period is needed before normal growth will resume in the spring.

As a general rule, most conifers need to accumulate at six to 10 weeks of exposure to temperatures below 40 degrees Fahrenheit in order to meet their chilling requirement to overcome dormancy. The chilling requirement is an evolutionary adaptation that protects trees from starting to grow anytime they experience a brief warmup during the winter. It’s the same reason bulbs don’t start to grow as soon as you plant them in the fall.

Some tree species require a relatively short chilling period to overcome dormancy. If we have a cold fall and early winter, trees may accumulate enough chilling hours to satisfy their dormancy requirement before they are harvested from their field or during shipping and display at the tree lot. Once the chilling hours are met, the only thing keeping the tree from growing is that outdoor temperatures are too cold. Once trees are placed in a warm, favorable environment, they can and sometimes do begin to grow like it’s springtime.

Close-up of a Christmas tree breaking bud. Photo by Doug Thalman.

More information on selecting and caring for your Christmas tree from MSU Extension

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Michigan Christmas Trees

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