Old fashioned string lights are a bad choice for outdoor trees. A better choice – trade your old-fashioned lights for LED lights when decorating outdoor trees.
Old-fashioned string lights heat the needles, buds and bark when the lights are lit. Then, when the lights are shut off, the cold winter air rapidly cools the sap in the cells to below freezing.
When a person uses old-fashioned string lights on young trees or trees that are just getting established, it causes ice crystals to form before the cell’s natural protective mechanisms can kick in. This can damage or kill the tree. A better alternative is LED string lights because they don’t conduct heat.
In addition, in young trees that are just getting established, new growth becomes very brittle in the low temperatures associated with Minnesota winters. Putting string lights on and then later taking them off can cause the brittle branches to break off. Trees grow during the winter, but at a much slower rate than in the summer. If a tree has a damaged branch, its healing won’t be totally complete until next summer, leaving the tree at risk for diseases and tissue damage from the cold. Best to keep the lights on until spring to avoid breaking brittle branches.
Wait until the new growth on your evergreen trees is hardened before you string lights. Wait to remove the lights in spring until the temperatures are consistently above freezing.