Harden winter-tender plants before winter freeze

Q: How do I protect my bougainvillea in the winter ? 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.

A: This is a question I get asked frequently . Bougainvillea will not tolerate temperatures below freezing without some damage. The degree of damage will depend on how much below freezing the temperature dropped, for how long 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 before I speak directly about bougainvillea.

Winter-tender plants should be "hardened off" prior to freezing weather. This is by not fertilizing plants with nitrogen fertilizers a couple of months prior to freezing temperatures and slowly withdrawing water.

This could be as soon as Dec. 15 in the Las Vegas Valley, but rarely sooner than this.

Generally speaking, do not fertilize any cold sensitive (aka winter tender, freeze sensitive, frost sensitive, etc.) plants after mid-September.

Secondly, begin watering these plants less often to improve their winter hardiness.

Those are the textbook answers. Actually holding back on water can be very difficult to do with automatic irrigation systems. You should cut back on watering on all your landscape plants this time of year anyway.

The easiest and most realistic thing to do is managing the fertilizer. When you fertilize these plants and encourage growth late in the year they could push new succulent growth, which more likely will be damaged in freezes.

If you hear it might freeze that night, cover the plant with something and remove it in the morning a couple of hours after the sun comes up. If you use plastic, you must include something that releases heat inside and underneath the plastic. This can be a warm, south-facing wall, the soil beneath the plant, big jugs of water that were warmed during the day, rabbits with their long warmth-emitting ears, etc. Do not just wrap the plant in plastic without some form of radiant heat at night, or it is possible that it might get even colder inside the plastic than it is outside. Blame it on physics.

The other problem is wind. Cold and wind together are more damaging than either alone. Try to keep the winter wind from the plant, so an enclosed courtyard, patio, windscreen or other protected area of the yard is a good location for planting.

As the plant gets older and larger it will withstand a few extra degrees. Its root system also will be larger and capable of storing more food. When freezing weather kills the top to the ground, the roots and collar will regenerate the top very quickly. Just give it lots of water (bougainvillea likes deep watering) and fertilizer so it recovers quickly after the danger of freezing has passed.

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.

Q: Because of an overwhelming whitefly infestation in the summer, I cut down my eggplant in the heat when it no longer had blossoms. I cut it flat to the ground. I left it to rot so removal would be easier in a week or so. Well, what do you think? It came back like all get out and blossomed in late September and set eggplants. Some whiteflies remained, but with the cool nights they are giving up. This is similar to advice about keeping tomatoes until fall and getting a late crop in December.

It never worked for me - until this year. In late summer, I let the tomato plant just be. It was an Early Girl, indeterminate in growth. I figured I would remove it with the eggplant corpse. Well, it started to blossom in late September and is setting tomatoes. It has some whitefly boarders but not too bad.

My question: Is the success of trying to prolong summer tomato plants for a second crop in fall dependent on whether or not the tomato growth is determinate or indeterminate? Thanks for all your help.

A: Those are some great observations that you made in your garden. That was pretty radical to cut the eggplant down to the ground and still have it come back. I realize that was not your intent, but I am sure you could see, with an already established root system, how quickly that eggplant grew back to the point where it could begin flowering again. This is one advantage of cutting plants back and letting them regrow.

Once it started to flower and set fruit, the energy was focused into reproductive growth thus slowing its vegetative growth.

So, ideally if someone were to do something like you did and wanted a fall crop, they would cut it back about 30 days before temperatures dropped back into the mid-90s .

Also, your thoughts of determinate vs. indeterminate was something I really had not mentioned. Both will work but certainly indeterminate types will give you more growth to work with.

For early flowering and fruit set you usually want determinate types. They typically will set fruit earlier than indeterminate types.

The other option is to replant in mid-July for a fall harvest. But transplants are hard to come by at that time of year, so you have to grow them yourself or grow plants from seed directly in the garden which is easy to do at that time of year.

In your case though I could see why you would want to cut them back to eliminate some pest problems. Remove the food and you remove the pest problem.

Bob Morris is a horticulture expert living in Las Vegas; he is on special assignment in the Balkh Province, Afghanistan, for the University of California, Davis. Visit his blog at xtremehorticulture.blogspot.com.