Covering certain plants in desert climate is necessary

     By Bob Morris
     View columnist
      The following are typical questions our Master Gardeners answer daily at the Nevada Cooperative Extension. If you have gardening questions, call the Master Gardeners' hot line at 257-5555 between 9 a.m. and 4 p.m. Monday through Friday. There is also a gardening Web site at http://www. intermind.net/mgarden/.
     
      Many of the phone calls coming into our office now are concerned with fall winterization of the landscape. Questions like what kind of fertilizers to use and whether to fertilize or not, pruning trees and shrubs and fruit trees, and covering cold sensitive plants to protect them are popular now.
      Generally speaking, delay all applications of fertilizers to plants in the yard except lawns and flowerbeds used for winter color. Fertilizing woody plants now can only increase the possibility of winter damage if temperatures drop dramatically.
      Lawns and flowerbeds need the new growth to increase plant densities and improve winter color.
      Covering plants in our climate might be a mistake on some and justified on others. In colder climates, roses are covered to protect the bud union from winter killing if there isn't enough snow cover. On evergreens, covering helps protect the plant from winter drying, called desiccation, due to strong, dry winter winds.
      This happens in colder climates when the ground freezes and the plant can't take up enough water to support its loss of water during the cold, winter months. Damage from winter desiccation comes in the form of branch dieback on deciduous trees and shrubs and needle or leaf browning or scorching on conifers like pines and broadleaf evergreens.
      The ground doesn't freeze in the winter in Southern Nevada, so winter desiccation is more rare. Here, we are more likely to experience loss of tender plants due to winter cold than desiccation.
      Shrubs like Natal plum, dwarf oleander, red bird-of-paradise or vines like bougainvillea, and trees such as shoestring acacia and queen palm are all subject to loss if we have winter temperatures in the teens. Shrubs that are subject to winter freezing will benefit from some protection of the part of the plant that connects the roots to the tops. With temperatures in the teens, tender trees will be heavily damaged at the least if not killed outright.
      Covering plants like Joshua and saguaro is a bit more controversial. Green plants rely on the pigment that gives them their color, chlorophyll, to provide sugars for energy. If this green pigment is covered, this reduces the amount of sunlight and energy the plant can produce. This can affect the plants overall health through the winter season.
      Also, covering a plant doesn't allow air to move across the plant's surface and could possibly increase the chances for disease.
      What is the answer?
      Ultimate survival of very cold sensitive shrubs or perennials may be improved by covering or mulching the base of the plant. The top may freeze back but the base of the stem or stems may be protected. Regrowth of the plant from the base would be rapid because of the established root system.
      Cold sensitive trees may be lost since the tree's shape may be lost due to freezing. This may not be the case with shrubs that don't require a central trunk. In the case of Chir pine, we may lose limbs but regrowth from the trunk may mimic the original tree and, like we saw in 1991, these trees may recover beautifully.
      Of course that is very dependent on temperature. In short, start with plants that you know will survive unless you are willing to participate in a costly and uncontrolled experiment.
      Question: I was reading information from your office and it is recommended that when planting a tree or shrub that a hole should be dug that is three times the size of the container. Why is such a large hole needed?
      Answer: That's the minimum size of the hole that should be dug. It is recommended that a hole should be dug at least three times the diameter of the container for the plant. Five times would be better. It is also recommended that your energy should be used in digging the width of the hole not in digging it deep unless the hole drains water poorly.
      Good drainage in a planting hole is essential. Roots, for the most part, grow more horizontally than vertically and plants grow more successfully in wider holes than deeper holes. Loosening the soil around the newly planted tree or shrub encourages good horizontal root growth.
      Q: I have an African sumac that has been staked for three years now and when I tried to unstake the tree it just flopped over. How long should a tree like that be staked?
      A: It would be nice if trees or shrubs being planted did not require staking at all. This is not always possible. Most of the problem comes from growing trees too close in the field, growing them too fast, staking them very early in their life to get them upright as soon as possible and pruning practices that remove the lower limbs too soon. Several things can be done to try and get the stakes off as early as possible.
      Leave lower limbs attached to the trunk until they reach pencil diameter. This encourages stronger and larger trunks. Water deeply but infrequently. Try to stretch the irrigations as long as possible particularly in the spring and winter months. Use nitrogen fertilizers sparingly. One light, nitrogen fertilization in the early spring is plenty.
     
     Bob Morris is a horticulture specialist with Nevada Cooperative Extension.