Protect plants from insects, winter cold

The South Valley Rose Society will present its annual rose pruning demonstration Saturday in the healing garden at St. Rose Dominican Hospital, Siena Campus, 3001 St. Rose Parkway in Henderson. Demonstrations will be continuous from 10 a.m. to 2 p.m. It is open to the public and free of charge. Bring your friends and any rose questions you may have.

Don’t forget to apply dormant oil to your landscape trees and shrubs as well as fruit trees. Dormant oils are inexpensive sprays that are relatively safe for the environment and your landscape ecosystem. They are used to suffocate insects that make it through the winter and are hiding in plant cracks and crevices.

If you had those nasty little critters with the funny leaf-looking back legs (the leaf-footed plant bug) on pomegranate and almonds, then this is a must to reduce populations for next year. Don’t forget to apply this spray to other woody landscape plants since these critters will not necessarily just stay on the fruit trees I mentioned.

You must spray all of the branches that are half inch or bigger in diameter, as well as the trunk all the way to the ground. Will this spray eliminate the problem? Heavens, no. But it will reduce the number of insects that will have direct access to almonds and pomegranates this coming spring and summer. Sprays will be needed every two weeks to a month to keep their population under control.

Their numbers are reduced at first but, if left unchecked, they will multiply and rebuild their numbers quickly in late spring and early summer. Egg laying will probably occur just after flowering. One insect in your yard can lead to hundreds in just a couple of months so you will probably not see them until their numbers climb, about mid- to late June. But they were active long before that.

Another place they invade from is your neighbor’s yard. Insects don’t respect fences or block walls so these adults can fly easily from yard to yard, and start breeding their own young and colonies. Once the young are large enough, they begin feeding with their needlelike stylet and pierce plant tissue (leaves and fruit) sucking plant juices from their interiors. Feeding damage can be bad enough to cause nut or fruit drop or deformities.

Dormant oils can be found at nearly any home and garden outlet or nursery. Mix according to the label and spray the entire plant from top to bottom on a warm, nonwindy day.

Watering will need to be increased for landscape plants and fruit trees beginning the first week of February, a time when many plants begin their blooming period. When I talk about increasing the water I am not talking about increasing the number of minutes that you water but rather the number of times you water during the week.

It is important to make sure that fruit trees have adequate water when the fruit is increasing in size. If water is inadequate, fruit will never get to its appropriate size or, even worse, will drop from the tree. We see fruit drop because of a lack of water a lot in figs.

Inadequate water for landscape trees usually means diminished growth. Since growth in many of our landscape plants occurs during the spring and early summer, finishing around July or August, it is important that there is adequate water during these times. Inadequate water decreases the amount of new growth.

Pines that are not watered adequately for several years in a row have canopies that appear open and thin. This is because a lack of water also affects the number of needles a pine tree can support.

Beyond moving plants indoors or building a structure around them, there isn’t much that will protect sensitive plants if temperatures get too cold.

How cold is too cold? It depends on the cold hardiness of the plant. Some plants are more winter hardy than others. If plants are collected from the wild, those collected from colder parts of their natural range will exhibit more cold hardiness than the same plant collected from warmer parts of its range. So, for instance, a saguaro cactus collected from a higher elevation may show more hardiness than a saguaro cactus collected from a lower elevation.

Planting sensitive plants in warm spots in a community and in protected microclimates in yards is the best long-term protection that costs very little.

Cold hardiness can be manipulated to some degree through plant management. Anything that stimulates growth, decreases cold hardiness. We know, for instance, that decreasing fertilizers, particularly nitrogen, in the fall will add a few degrees of cold hardiness to most plants.

We also know that restricting water in the fall helps force the plant into earlier dormancy. This also may protect it better from cold temperatures.

One of the reasons fall pruning is avoided in tender plants is because of the potential decrease in cold hardiness. Horticultural techies know that there are chemical treatments that will increase plant tolerance to cold. Knowledge of these treatments is probably more important to a homeowner playing “Jeopardy” than they are practical in nature.

High winds during cold temperatures can be devastating to plants and contribute to winter damage. Simply moving plants in containers out of the wind and into a sheltered location until the brunt of the cold is over may be enough if temperatures don’t get overly cold. Wrapping the trunks of young trees that have not yet developed a thick, insulating bark may be helpful during short-term cold temperatures.

Tender perennials, shrubs and vines grown here, such as African bird-of-paradise, cape honeysuckle, red bird-of-paradise and bougainvillea, need the crown of the plant protected during cold weather. The crown can refer to the point at the soil surface where the trunk and roots meet or it can mean the leafy “head” of a tree. In this case it is the part of the plant just above the soil.

This can be protected by mounding 4 to 8 inches of mulch around the stem and keeping it in place with wire mesh. Or, you can use an upside-down, plastic container with the bottom cut out and then filled with mulch. The top of the plant may die from freezing but the regenerative crown will be protected.

In the past, cold frames were built to protect small, tender plants that could be moved easily into temporary shelters. Some recommend that plastic be used to cover tender plants. The plastic must be clear, not black. This can work provided there is some sort of heat source inside or under the plastic. The heat source can be passive or active.

Passive sources of heat include the soil under the plant. The soil warms during the day and releases heat at night under the plastic . Another passive source of heat can be large containers of water that are warmed by the sun and radiate heat at night .

Active sources of heat are compost or green manure. Ornamental lights wrapped around the trunk, another active source of heat, may offer some protection if there is no wind. The higher the wattage used, the more heat produced.

Clear plastic that is used to cover plants like a “bubble” is worthless against extreme cold for long periods and may actually cause the air inside the bubble to get colder than air outside of it.

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           

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