Mulches can be helpful in desert climate

     By Bob Morris
     View columnist
      The following are typical issues our Master Gardeners deal with 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/.
     
      Some of the newer residents to Las Vegas don't realize the value of applying mulch to the base of plants.
      If you haven't used mulch before, you should really consider it. It helps cool the soil, keep moisture in the ground, keeps weeds under control and helps keep salts. Mulches can be made from anything that can shade the soil and slow down evaporation.
      This could be 2 to 3 inch layers of wood chips, shredded newspaper, grass clippings and the like. Even our yard waste, if it's chopped finely enough, can be used.
      Another option is organic mulches, which are derived from plants.
      Organic mulches usually have one disadvantage. These mulches, when in contact with wet soils that are inevitably underneath it, begin to decompose. As they decompose, the soil microorganisms require nitrogen.
      The decomposing mulch is much better at finding and using nitrogen than plants. The organisms will take this nitrogen from any source it can and rob any plants growing in the area of the nitrogen they need for growth. Plants that aren't getting enough nitrogen usually turn yellow, with the oldest leaves (the ones toward the inside) becoming more yellow than the newer ones.
      So for that reason, it's imperative to fertilize plants with a little extra nitrogen through the growing season to supplement the nitrogen used by microorganisms in the decomposition of the mulch. A very light application of nitrogen through the growing season and watered into the organic mulch will suffice. ...
      n We have had reports of damage to petunias, geraniums and other plants, resulting from tobacco budworm.
      To protect these plants, use sprays of bacteria called Bt. Please be aware this spray is not totally specific to only tobacco budworm but also many other larvae of insects that turn into butterflies and moths.
      Direct your sprays only on the plants you are trying to protect. ...
      n We are seeing increasing numbers of grasshoppers coming in from the desert as desert plants are starting to die.
      Expect to see problems with these insects in neighborhoods close to desert areas or open lots. Other insects that may surface are leafhoppers that attack lawn grasses. For the most part, damage will be confined to lawns during June and July. The grass may appear scorched or browning, with tiny insects that jump when you walk over the grass or brush your hand across the top. ...
      n Watch for tomato hornworm damage, which results in large tomato plant leaves disappearing overnight.
      Hand pick these critters from the plant. That's much easier and safer than using sprays. ...
      n Consider fertilizing tall fescue lawns if the weather isn't too hot. Pick a cool day to do it and make sure to water thoroughly when finished. This would be the last application of fertilizer on tall fescue before the summer heat hits.
      A good fertilizer would be one that is high in potassium and nitrogen but lower in phosphorus. Potassium does seem to help in protection from disease and heat.
      The lawn should have been aerated by now, but if you haven't done it yet, there is still time. Watch out for irrigation pipes and heads.
      Also, we are now into monthly applications of fertilizer to Bermuda grass. They are much heavier feeders of fertilizer than tall fescue.
      Q: My tomato plant has new leaves on it that are beginning to turn brown, shrivel up and die. I'm not sure what's going on. Can you give me some advice?
      A: There are several things that can be causing problems like this.
      What we do know is there's too much water being lost through those tender new leaves, and the root system and stems can't keep up with the demand for water, so the leaves are drying up. We also know it has gotten hot and windy lately.
      First, let's look at the roots. If the roots are damaged or if there just isn't enough water surrounding the roots then not enough water will reach the leaves and the leaves will get brown, crusty and die. Make sure the soil is moist and mulched. If there is damage to the stems that are transporting the water to the leaves, then the leaves can't get enough water and the leaves scorch.
      Check the stems for damage. There are two diseases that can plug up the stems on tomatoes and block water from moving up the stems to the leaves. They are called verticillium and fusarium. They are both very destructive on tomatoes and plants in the tomato family, such as peppers and potatoes. These diseases will kill the plant in a matter of a few days.
      Make sure you purchase plants that are resistant to these diseases. They will be marked VF on the nursery tag right next to their name if they possess this resistance.
      You will have to destroy infected plants immediately, as there's no cure except removal and disposal. They can cause the spread of this disease to other tomato plants if left in the garden. It's too bad, but you will have to start all over again with new plants if they are infected.
      You can recognize this disease because the disease organisms will cause the inside of the stem below the scorched leaves to darken or become sunken.
      Cut off a stem below the leaves and see if you see this discoloration. If you do, then destroy the plants and don't use the knife again on other plants unless you sterilize it with alcohol or wash it.
      The last thing that can be wrong is there just isn't enough water reaching the leaves because the plant just can't support all it's new growth with its current root system. This would be because it just got too hot and windy too soon -- which was the case recently. Mulching will help.
     
     Bob Morris is a horticulture specialist with Nevada Cooperative Extension.
      Question: Why is my Indian hawthorn shrub getting yellow leaves with brown spots all over it?
      Answer: This is classic nutrient deficiency, probably caused by insufficient available iron in the soil. This is usually much more of a problem with certain plants grown in unamended desert soils in desert landscapes. It will be even more marked if the soils are kept continuously wet and located in a southern- or west-facing exposure.
      Plants like Indian hawthorn grow best in relatively rich soils with plenty of organic matter. They don't do well in desert soils without organic matter. You can try using fertilizer sprays of iron to see if you make it green again. Soil-applied iron fertilizers will not perform well this time of year. It's best to use soil-applied iron fertilizers in late winter or early spring when the plant is first starting to grow. Iron doesn't move well once inside the plant.
      Indian hawthorn is notorious for not responding well to iron sprays because of the leaf surface. For this reason I would also purchase a spray additive, called an adjuvant or spreader, to mix with the iron spray.