When Can I Trim a Frosted Bougainvillea Bush?

Frost Effects

• Temperatures at or below freezing for more than four hours in duration can fully kill a bougainvillea. Frost causes leaf and twig desiccation as plant cells rupture when water molecules expand as they freeze. Leaf and flower drop is a common response to frost, but the full extent of twig and branch damage resulting from a cold night does not become apparent for several days. In some cases, the entire above-ground branching system is killed, and only the roots and a small portion of the lowermost trunk remains alive.

When to Prune

• Do not prune back a frost-touched bougainvillea for at least two weeks. Monitor the plant in the interim, maintaining the same light and watering regimen already used before the frosty night. After two weeks, gently bend branches to determine if flexible, living tissues remain. If the chance for winter frost remains, withhold any pruning of the bougainvillea until the last spring frost date, which may not occur until February or March. When new green sprouts appear, it's an indication that enough time has passed since the frost and a pruning is more timely.

Reasons to Delay

• Pruning will contribute to tissue desiccation on branches as well as promote new growth. You must avoid pruning bougainvillea branches prematurely so that new growth doesn't occur when weather remains chilly and more frosty nights can happen. Waiting to prune also prevents accidentally removal of living branches that would have later sprouted leaves. Keeping dead branches on the frosted bougainvillea can provide additional protection from cold. The remaining dead twigs take the brunt of any subsequent frosts, thereby preventing lower living tissues from being exposed to the cold. This can make the difference between all above-ground tissue being dead and the largest, lowest branches and trunks remaining alive to sprout more leaves come spring.

Regional Advice

• Bougainvilleas are mainly grown outdoors only in U.S. Department of Agriculture plant hardiness zones 9 and warmer. This correlates to Hawaii, Puerto Rico, coastal California, the low deserts of the Southwest, southern Texas and peninsular Florida. Elevation and regional winter weather patterns vary among these regions. Contact your local cooperative extension office or regional botanical garden to ask horticulturists about the best procedure and timing of pruning frost-damaged bougainvilleas. Time of the year, latitude, elevation and proximity to water can all affect whether frosts can still occur.