If it Survives the Winter, Don't Cut Bougainvillea Back

Q. Should bougainvillea be cut back in the winter?  I have two in large pots. I want to see them grow fuller this spring and summer. Cut back or leave them?

Bougainvillea is a witty perennial shrub in semi tropical and tropical parts of the world. We can grow it in winter freezing climates because it grows back from the roots if the top part freezes. If it gets very cold, the crown or top part of the plan may die each winter this happens. In our climate we grow it as an "herbaceous perennial" like lilies and Iris. If it freezes during the winter, we cut off the top debt part and let it regrow from the roots. Otherwise, leave it alone.

A. Wait until about March 1 and see if there is any winter damage from freezing. Then decide. If there is, cut it back close to the ground and let it regrow again.

Winter freezing damage to bougainvillea. When temperatures drop just one degree below freezing, damage begins to occur on bougainvillea. When this freezing temperature remains for any length of time, more and more freezing damage occurs. It will grow back from the crown (that part of the stem and roots that connect the two) if temperatures do not get too cold for a long time.

            If there is no winter freeze damage, make this plant fuller by cutting the stems back at various heights (so it doesn’t look like a butch haircut) anywhere from a foot to foot half from the ground. For every cut you make, three new shoots will grow and increase the density of the plant.

When to Cover Winter Tender Plants in the Mojave Desert

Q. When should we cover Pygmy Palm, Bougainvillea and other plants that will freeze here in Henderson?  I have burlap to cover them.

Bougainvillea freeze damage

A. Both of these plants can tolerate temperatures to near freezing and they don't seem to have problems at temperatures below 45° F that causes chilling damage to some plants. Some tropical plants such as tomatoes may show chilling damage to fruit at temperatures below 45° F. This is why it is best to not refrigerate tomato fruits.

            It seldom freezes in the Las Vegas Valley before Thanksgiving. After Thanksgiving you might expect freezing temperatures at the higher elevations such as in Summerlin or in very low spots in the Valley where cold air collects. In Henderson, this might be in the old Pittman area or along the wash.

            Cold air, being heavier than warm air, settles into low geographic areas. Cold air tends not to settle on gently sloping land or hills.

            Freezing temperatures are more frequent where plants are exposed to wind. Plants growing along major streets tend to freeze more often than those in protected backyards. Major streets are urban canyons that channel cold wind in the winter. Backyards are more nestled away from these exposed urban canyons.

Sehgal palm cold damage

            Plants growing close to brick or cement walls that face West or South are less likely to freeze than plants growing further away unless wind is involved. Brick and cement walls exposed to the sun store heat during the day and radiate this heat at night keeping the plants a few degrees warmer. If wind is involved, it removes this radiant warmth making these plants more susceptible to freezing temperatures.

            Bottom line, when the weather forecast is for freezing temperatures, cover the plants with an old sheet, blanket, or in your case burlap, before nightfall. Drape this covering over the soil or any surface that can radiate heat at night. Remove this covering the next morning after temperatures are above freezing.

            In open areas, expect freezing temperatures after Thanksgiving and up to March 1. In protected areas, expect these temperatures anytime between mid-December and mid-February but watch your local weather forecast closely and adapt this recommendation accordingly.

What to Do Before the Freeze Comes

We already had a taste of freezing weather back in November in some parts of the Valley. Unless this is a very unusual winter, more is coming. In Las Vegas we can expect nighttime temperatures to drop into the mid to high teens degrees F (-10C).

• Rarely does the soil freeze more than a couple of inches deep. 
• The coldest time is just before sunrise. 
• Clear, cloudless nights will be more likely to give us freezing weather. 
• If you think it might freeze, go outside at night and see if the night sky is clear (cloudless).
• If you cover plants at night, uncover them the next day as soon as the temperatures are no longer freezing.
• Water the day it may freeze or make sure the soil is wet going into freezing temperatures.

Check weather forecasts. Look for future minimum temperatures AND WIND!!!

I like Weather Underground for the wind reason and you can see it here and bookmark it.

Protect from the wind. Remember, cold is bad enough but when it is combined with wind it will be much more damaging. Protect anything of value or that can freeze and cause damage by first blocking the wind. This can be as simple as throwing a light blanket over a tender plant or a thick blanket around pipes that could freeze.

Wrap pipes totally. Remember that metal freezes quicker than plastic. Plastic pipes will freeze but any metal fittings exposed to the wind will freeze more quickly and be more expensive to replace. It is always best to wrap exposed pipes with insulation starting a couple of inches below ground. Wrap ALL exposed metal parts above ground. Leave nothing exposed to the wind.

Drain pipes. If the pipe and metal components don't contain any water, they are less likely to be damaged when there is a freeze. Drain pipes by opening irrigation valves after you shut off the water to your irrigation system. Shut off the water to the valves and loosen the bleed screw on top of the valve or loosen the solenoid 1 1/2  turns (not all the way!), enough so water squirts out from the solenoid or bleed screw.

Protect Tender Plants. A good resource on protecting plants from winter cold was published by the University of Arizona. You can find it here.

Plants that you should consider protecting at the first sign of a freeze include: limes, lemons (not Meyers), and citron, bougainvillea, many hibiscus, pygmy date palm, plants that are flowering. Don't cover plants with plastic. The cold plastic touching the leaves can damage them.

Mulch. Put a 3 to 4 inch layer on top of the soil surrounding the plants during the day when the soil is warm. Mulch helps trap soil heat and keep it from freezing. Plants that might freeze to the ground (Lantana) will have a couple of inches of stems protected from the cold and wind with a mulch.

Container Plants. Container plants should be moved into the garage or the plant+container should be covered all the way to the ground. Remove the cover when temperatures are no longer freezing.

Lawns. If the lawn has frost or is frozen, DON'T WALK ON IT. Walking on frozen grass will damage the blades and leave footrpints due to the damage.

If damage does occur..... do not prune out the damage until all cold weather has passed.

Bamboo Can Be Cut Back to Repair Winter Damage

Q. I have bamboo that was damaged last winter as you can see from my pictures. New growth is emerging from them but they are still yellow and brown. Should I fertilize them or give them iron?

Lantana and Yellow Bells Should Not Be Dead from Winter Cold

Q. I have had Tecoma stans, Yellow bells, in the backyard as well as lantana for the past 3 or 4 summers. They do well but never winter over. When I dig them up to replant, the root structures seem to be healthy and alive. The tag on the plant says they are cold hardy. Is there anything I can do to help them winter over?

Sago Palms Damaged in December

Q. Do white fronds indicate my sago palm has frost damage? Can my sago be saved? Should sagos be wrapped and, if so, do I just cover them when the temperatures drop down to the 20's but then remove the wraps once the weather warms up?

You Will Not Protect Bougainville from Freezing by Feeding Them

Q. Before the first freeze here in Las Vegas, probably in 3-1/2 weeks, what may I feed them?  Might there be something simple, without figuring amounts and all kinds of mixing? I have three plants, all in the ground.  They are six years old this year, and I wrap their bases before the freeze. They always come back come March/April, and bloom beautifully in August/September through October – when the real heat hits.

A. I like your idea of wrapping the bases of the plants before any freeze. Unless you have a very warm microclimate in your yard, and there are some of those in Las Vegas, you will see damage to them at the first light freeze with temperatures below 32° F. The harder the freeze and the longer it lasts, the more severe the damage.

Bougainvillea after freezing weather in Las Vegas

Wood mulches will also protect the base of bougainvillea but wrapping it is more effective. This way when it freezes it will kill the top of the bougainvillea to the top of the mulch or wrap and not any further.

Of course this works to temperatures below freezing for short periods of time. If the temperatures are extremely cold, or if these temperatures last a long time, then it will freeze the plant to the ground. How well you protect the base of the plant will determine how strong it will come back, or not, next spring. Having bougainvillea in a spot in the yard which does not have a lot of wind and is very close to a warm, South or even better yet, West-facing wall will improve its chances for survival.

Freeze damage hits the youngest and thinnest plant parts first during a light freeze and the part of the plant most exposed to wind and less protected by warm south or west facing walls

Plants that freeze to the ground will come back like gangbusters next spring because of their established root system. Plants that are first planted have to grow both the roots and the tops and so growth is divided between the two. Plants that have an established root system and freeze to the ground only have to grow the top back. So all of the spring growth is funneled to top growth so we see huge amounts of top growth in the spring.

Spring recover of oleander from a winter freeze

You do not want to feed them anything this time of year. Just make sure they get watered. This goes for any winter tender plant. These plants should get no fertilizer applied to them that promotes growth any time after mid-August. Fertilizers which stimulate new growth will cause the plant to become more succulent and lower its chances of surviving winter freezes. This is not true of plants that have no problem surviving winter freezes. In fact, those plants can receive mid fall applications of fertilizer with no problems. A mid fall application of fertilizer can substitute for a Spring application but not winter tender plants.

Planting Zones for Las Vegas Nevada

USDA hardiness zones

Q. What do you say is our planting zone?  I got a Plant Smart Sensor for Christmas and it says we are a 7B which is way lower than I always read.  Just because a website says a tree or plant will be ok for a certain zone doesn't mean it is necessarily right?

A. I use both the USDA and Sunset’s planting zones for our area. Because we have various microclimates in landscapes I will normally include a couple of climate zones colder for very exposed microclimates and a couple of climate zones hotter for protected areas with lots of exposure to sun in the winter time and wind protection. Start with winter time lows reaching 10F and add zones that are just at freezing (32F) to cover our microclimates in backyards. Yes, they will vary that much.
USDA Hardiness Zones
http://www.usna.usda.gov/Hardzone/hzm-sw1.html

            I like Sunset’s zones but find them a little bit too narrow at times and too restrictive. That is the reason Sunset went ahead in developing their own which they did a fabulous job doing. On the other hand, the USDA zones are too broad. Very little in this world is perfect so I use these fudge factors I mentioned to “fudge in” the microclimates.

Mojave Desert Las Vegas and Arizona
Mojave Desert Las Vegas and California

            Frequently, the selection of a plant and whether it will work or not, depends on several things: the microclimate, where you plant it in the yard, how you modify the soil and irrigate it.

When Should I Cut My Grape Vines Back?

Q. When should I cut my grape vines back?  They're on a trellis, and are 1 year old.  During the summer they spread out very well, and produced several large groups of table grapes.

A. I would wait until late February or early March to prune them. We still have some potentially difficult times to go through this winter for grapes. Cutting them early may result in a loss of bud wood and fruit production.

            When pruning you will cut back this past years growth (it will be a different color) so that only one or two buds remain. I prefer two. Also I usually prune it back so that ten or twelve buds remain if I do it early. Then just before bud swelling in the spring (like about early March) I cut then spurs back to two buds. This way if there is dieback during the winter I won't lose the fruit producing spur.