Grow oleanders outdoors in U.S. Department of Agriculture zones 8 and higher, where winters are mild.
A heat- and drought-tolerant shrub, the oleander (Nerium oleander) displays long evergreen leaves on pale-colored, smooth branches. During the warm months of the year, clusters of attractive blossoms don the branch tips. A Mediterranean native plant, oleander is not overly cold-hardy, sustaining leaf scalding and branch dieback once temperatures drop below 15 degrees Fahrenheit. A light frost won't harm the plant, unless newly emerging shoots have already appeared in spring. These tender tissues will desiccate and blacken as a result.
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Wait to prune the frost-damaged oleander until the spring when frosts are no longer expected in your area. Prematurely trimming oleander shrubs leads to tender new sprouts that cannot survive more frosty nights. Contact your cooperative extension office to learn of the expected last spring frost date. Schedule any trimming for just after that date.
Examine the oleander shrub thoroughly by walking around it to know exactly where the frost damage occurred and how severe it is. If only leaf tips are scalded, trimming maintenance may not be necessary. Dead, curled leaves after the winter often suggest branch dieback, and swelling buds and green leaves farther down the branch reveal where the plant is alive and will sprout replacement shoots.
Trim off dead branch tips with bypass or hand pruners. Make the pruning cut 1/4 to 1/2 inch above a lower branch junction or a cluster of leaves. Trim branches evenly across the oleander to shape the shrub into a more symmetrical, rounded plant. Do not shear the plant or prune so tightly to create perfectly geometrical shapes, as that destroys the natural billowy silhouette of an oleander.
Cut back severely cold-damaged branches — those with no leaves or dry, brittle bark — to a lower branch junction or a main trunk. If you cut back into moist, living branch tissues, new sprouts will grow from just below the cut. This creates multiple new branches to rejuvenate that area of the shrub.
Discard pruning debris into the garbage or its own rubbish pile. Do not compost oleander foliage or wood, nor burn it. Oleander contains toxins. Chopped up debris may be scattered on the ground underneath the shrub to act as a mulch and organic fertilizer, however.
Even if you witness frost damage on the oleander in autumn or early winter, do not conduct any pruning maintenance. More frost and cold is likely the rest of the winter. Dead leaves and branch tips help to insulate and protect lower leaves and branch tissues from later cold snaps during the winter. If you feel you did a poor, uneven trimming job and the oleander looks awful, you can rejuvenate the shrub in spring. Cut back all branches at varying heights from 6 to 18 inches tall with a pruning saw or loppers. New leafy branches arise by early summer, creating a dense, lush-looking plant.
Wash your hands and clean the pruning tool blades after completing the trimming maintenance on oleanders. This removes the toxins from the sap that lingers afterward.