Q. How do I protect my bougainvillea in the winter here in Las Vegas? I've read posts that say to water a lot and others that say don't water at all. Some say to drape frost cloth or burlap directly over the plant and others say to put stakes in the ground and don't let the cloth/burlap touch the plant. Last winter 3 of our 4 plants (small ones) survived, but it took a long time for them to come back. Right now they are beautifully pink and very healthy.
A. This is a question I get asked frequently so I will post it on my blog very soon. So you are the lucky winner and I will soon post your question and the answer I provide below! Bougainvillea will not tolerate temperatures below freezing without some damage. The degree of damage will depend on how much below freezing the temperature dropped and for how many hours and if there was wind to go along with those low temperatures.
In order to answer this I want to explain a few things about winter tender plants and how to manage them in general first before I speak directly about bougainvillea. Winter tender plants should be "hardened off" prior to freezing weather. This is by not fertilizing this plant with nitrogen fertilizers a couple of months prior to freezing temperatures and reducing the frequency of watering. In the Las Vegas valley freezing could be as soon as December 15 and, rarely, even soon than this. So generally speaking do not fertilize ANY cold sensitive, aka winter tender, freeze sensitive, frost sensitive, etc. plants after mid September. Secondly, begin giving these plants less water to improve their winter hardiness.
Cover the plant with something if you hear it might freeze that night and remove the covering in the morning a couple hours after the sun comes up. If you use plastic, this plastic must also cover something that releases heat back under the plastic. This can be a warm, south facing wall, the soil beneath the plant, big jugs of water that was warmed during the day, rabbits with their long warmth emitting ears, etc. Do not just wrap the plant in plastic or it will freeze AND it may possibly get even colder inside the plastic than the outside. Blame it on physics.
As the plant gets older and larger it will withstand a few extra degrees. Its root system will also be larger and capable of storing more food. When freezing weather kills the top to the ground, the roots and collar region will regenerate the top very quickly. Just give it lots of water (bougainvillea likes deep watering) and fertilizer so it recovers quickly. If temperatures drop to the teens it is possible you could lose the entire plant so make sure the soil is mulched with several inches of wood or rock mulch to insulate the roots and collar as much as possible and give it some extra protection.