The evergreen oleander (Nerium oleander) grows quickly in the warmth of spring and summer. The leafy branch tips bear clusters of faintly fragrant flowers from spring to fall. Depending on the cultivar, oleanders may remain shrubs only 4 to 6 feet tall and wide, or mature much larger, almost as small trees, up to 20 feet tall and 12 feet wide. Oleanders can tolerate subfreezing temperatures in winter, but extreme cold can cause leaf desiccation and branch dieback, and even kill tissues all the way back to the trunk base.
Expect most oleanders to survive winter unharmed as long as temperatures don't drop below 15 to 20 degrees Fahrenheit. Temperatures below 15 F result in progressively more leaf and branch tissue death from too much cold. Dwarf oleanders sustain freeze damage earlier -- at 20 F -- than standard, larger-growing selections. The longer the duration of subfreezing temperatures over the winter, the more dieback you should expect. Temperatures below 10 F will kill an oleander to the ground, and progress even to kill the roots so there's no chance of rejuvenation the following spring. Grow oleanders outdoors only in U.S. Department of Agriculture plant hardiness zones 8 and warmer.
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Once temperatures drop below freezing, even into the teens, don't assume you can identify any freeze damage on oleanders immediately. It may take days or weeks for any cold damage to finally reveal itself on leaves or branches. Therefore, don't prune the oleander. Premature pruning exposes living lower tissues on branches to the full brunt of any recurring cold the rest of winter. Although the oleander may begin to look tan and sickly, it likely is still alive in its lower branches and roots.
Pruning Time Frame
Wait until late winter or early spring -- after the danger of frost passes -- before scheduling any pruning of cold-damaged oleander shrubs. Across the southern United States, the first buds begin to sprout from oleanders as early as late February and into March. This growth helps reveal which branches remain alive. Trim back dead branches to 1/4 to 1/2 inch above these living shoots. Alternatively, you may cut back oleanders very hard, to branch stubs 12 inches tall, and allow the shrub to sprout new shoots from the roots and lower trunk.
After an oleander endures a very cold winter with lots of branch dieback, it may be more susceptible to fungal disease, according to Clemson University. When pruning out dead branches in early spring, make the pruning cuts into lower, living, healthy branch tissues. Once the pruning cut is made, the center of the wound should look white. If any discolored black tissue is revealed, make the cut even lower on the branch until the branch wound reveals no more dark-colored tissues. This physically removes any advancing traces of fungal infection, allowing the plant to regrow without the fungus remaining to attack the new tender branch shoots and leaves.