Why Conifers turn Red in the Spring
Winter can take its toll on trees. In the spring time, you may notice red needles on white pines and other conifers that are growing along highways. Although the needles look terrible, the buds, twigs and trees are not dead. The needles had a rough winter and they were discolored by winter injuries. Resist the urge to prune them away or remove discolored trees. Chances are very good that these trees are live and healthy beneath their mask of red needles. Buds are well protected during the winter and will grow once spring arrives. Winter injury was enhanced by strong, dry winds, many days of bright sunshine and by low relative humidity. Humans also added to the injury equation by applying road de-icing salts which are toxic to plants. Even during the dormancy of winter, tree needles need and use liquid water. When water, stored in twigs and needles, is gone, cells and tissues become progressively more dehydrated and start dieing. Water is lost faster when the relative humidity is low, when dry winds are blowing and when warm, sunny days occur. Affected needles, turn red or brown from the tip down and, often have dark bands or a mottled appearance. In late winter, the needle discoloration intensifies and becomes more noticeable. Buds are usually not killed. Normally, snow cover prevents winter injury of young conifers by providing shelter from drying winds and from the glare of the sun. In some years it is common to see young conifers with a strong line of demarcation separating the brown, desiccated tops from the healthy, green branches that were covered by snow.
Undoubtedly the most evident damage occurred on white pines growing along highways and was caused by the application of de-icing salts. Earlier in the winter, each passing car sent up clouds of water with a little salt dissolved in it. This salty water settled on nearby objects, including the pines. Salt was absorbed into individual needles, accumulated to toxic levels in the needle tips and killed the needles back, starting at the tips. Trees within 150 feet of a highway can be easily reached by salt spray. De-icing salt damage usually occurs on the side of the tree closest to the road. Spring rains will rinse off accumulated salts, new shoots will develop in May and June and the dead needles will be shed so that by summer these trees will look reasonably healthy. Trees with thick wax layers on needles, large resinous buds and/ or with thick, robust needles are more resistant to salt spray damage. White and red pines are very susceptible to salt spray damage; Scots pine, Norway spruce, juniper and eastern red cedar are moderately susceptible and jack pine, Austrian pine, larch and black spruce are tolerant to salt spray damage
Some trees or groups of trees seem to get winter injury every year. In these situations, it is likely that the trees are under stress and do not have the resources to tolerate any internal desiccation and therefore suffer winter injury every year. For example, some clumps of roadside red pines show winter injury symptoms every year because they are growing offsite, either in soils that are too wet or in soils that restrict rooting depth. This stress predisposes the trees to needle desiccation and ultimately to repeated winter injury.
Winter injury is the most important factor limiting the northern range of conifer species. Temperate species, such as red and white pines, are much more vulnerable to injury than are the boreal species, such as black spruce and jack pine. Native tree populations are adapted to their locality. Moving them ( seeds or seedlings) 100 miles north or south of their site of origin can result in damage due to winter injury. Exotic species, like Austrian or Scots pine, should be planted in climatic zones similar to their site of origin in Europe.
Conifers growing in Minnesota have long winters with plenty of opportunities for winter injury. But in spite of their appearance, chances are good that your trees are live and healthy beneath their mask of red needles.