Why Leaves Turn Yellow

 

When the leaves of a plant begin to turn yellow, determining the cause will take some investigation. There are several possibilities:

         The first consideration is a lack of nitrogen, especially if a number of plants are affected and there has been no recent application of fertilizer.  Apply a high nitrogen fertilizer, and if after a week or so the plants begin to turn green, the problem is solved.

          If the plants don’t turn green, the problem could be a disease.  Plants can become infected with a fungus, especially when over watered, causing a root or crown rot.  Inspect the roots and the crown of the trunk.  Scratch into the bark of the trunk at the soil line and also into a piece of root.  If the tissue inside is brown or black, rather than light colored, a root or crown rot is evident.     

         Another common reason for a general yellowing is a lack of oxygen in the root zone.  This is due to too much water – over irrigation or too much rain.  As long as the soil drains well the problem is minimized.  But in clay soils, water does not drain well, resulting in a waterlogged condition.  Without adequate oxygen in the soil, roots cannot function properly, and the plant turns yellow.  If the condition continues, root rot will develop and the plant will die.  That is why so many planting instructions suggest to plant in a well-drained soil. 

         If plants are yellow and lack vigor, a soil analysis can identify nutrient deficiencies and/or a pH imbalance.  The pH reveals the acidity or alkalinity of the soil. Most plants have a definite acid/alkaline range in which they will properly grow.

         Although not common, there is always the possibility of persistent chemicals in the soil.  A soil sterilant or preemergent herbicide may have been applied to the soil.  These compounds can persist for many years.  Preemergent herbicides are those that are sprayed on the bare ground and become incorporated into the top few inches of soil.  At label dosages they affect only the target weeds and last from several months to a year.  But if an overdose is applied, the chemical will be there for many years and adversely affect many plants. 

         An infestation of microscopic round worms called nematodes (example, root knot nematode) can also cause plants to yellow and be stunted.

Composting

Composting is a good way to reuse our natural resources, recycle nutrients and add good organic matter back into the soil.  

The popular practice of composting is defined as the process by which organic materials biologically decompose under controlled conditions. Perhaps the most notable point about composting is not to make it more complicated than it is.  By merely throwing a pile of twigs and leaves in the back corner of the garden, you are composting; of course, it may take a year or so to breakdown.  But by following a few simple rules you can speed up the process and produce good compost in about a month or so. The following are key principles:

 a. Proper moisture and air (oxygen) content - Compost works best if the moisture content of the materials is about 50%.  That’s not easy to measure, but it has been estimated to be approximately the moisture content of a wrung-out sponge.  If the material is too dry, decomposition will stop; if too wet, oxygen is excluded, and decomposition will slow and may smell bad. 

b. Proper carbon/nitrogen ratio - For effective composting, the raw materials must have a proper carbon/nitrogen ratio – set at about 30:1.  Since this too cannot be easily measured, mixing equal volumes of green plant material with equal amounts of brown plant material will give this ratio. The greens are fresh moist materials like grass clippings, weeds, manures and kitchen scraps. The browns are dry materials such as twigs, wood chips, straw, saw dust and paper.  If a pile of twigs are thrown to the side, they will eventually decompose. When leaves (greens) are combined with the twigs (browns) in the proper ratios, decomposition will occur more rapidly.    

Mixing grass clippings with twigs or chips is not only good for obtaining the proper ratio but also helps to maintain a good oxygen level.  Grass clippings or shredded papers alone tend to mat and exclude oxygen.  Adding twigs helps to open the pile allowing a better movement of air.   

c. Proper size of material - Soft, succulent plant tissue doesn’t need to be chopped into small pieces because it will decompose rapidly. Woody materials, however, will decompose better if pieces are ½ to 1 ½ inches in size: the smaller the pieces the quicker the decomposition.

d. Proper pile size - The size of the compost pile is important.  The minimal size is 3 cubic feet (3x3x3). Maximum size would be around 5x5 and as long as desired.

e. Proper turning- Turning the pile is not required but will certainly speed up the process if turned every day to every ten days. Turning helps ensure proper air circulation, moisture and heat distribution. 

What should NOT be put into the compost pile? Meat, fat, manure from meat-eating animals as well as human waste. Manure from herbivores such as goats, cows, horses, rabbits and even elephants can be used.  Don't throw diseased plants into the compost pile, because the pile may not reach the temperatures that are required to kill plant diseases and weed seeds. 

Finally, here’s how to know when the composting is ready: the majority of the pile has become dark, loose, crumbly and sweet smelling.  Also, the original materials will not be recognizable with the exception of a few pieces of tough woody material. 

Miconia "Sound the Alarm!"

                                                  Clearing Miconia stand in Tahiti

Miconia calvescens, is a tree from 35 to 50 feet tall with large leaves up to 3 feet in length. Although attractive with its beautiful leaves, green on top and purple on the bottom foliage, it is perhaps the most invasive and damaging, alien plant species to the wet forest lands of the Pacific Islands. These trees will form a large, thick canopy which can produce 100% shade, killing out or inhibiting the growth of all native species below. Miconia plants are a threat to completely take over moist and wet forests.

Miconia was introduced to Hawaii as an ornamental in the 1960s and now poses a threat to completely take over forests receiving 75-80 inches or more of annual rainfall. If left uncontrolled, it is estimated that miconia could invade up to 121,000 acres on Oahu. On Maui 37,000 acres could potentially contain miconia.  Presently, the Big Island has large infestations on the windward side, particularly on the Hamakua Coast and smaller populations on the leeward side.

A single mature tree has the potential of producing 3 million seeds, two or three times a year. These seeds can remain viable in the soil for 10 years or more. The seeds are spread by man, through the mud on his boots and other equipment including bulldozers. Seed is also dispersal by birds; in Hawaii, dispersal is presumably by the Japanese white-eye, the common mynah and perhaps the northern cardinal. In trials in Tahiti, a square yard of the top 2 cm. of soil from a dense Miconia stand, produced over 17,000 miconia seedlings in six months.

Miconia was introduced to Tahiti in 1937 and has since destroyed nearly 70% of the native forests and is directly responsible for threatening 25% of their native forest species with extinction. Miconia also causes serious landslides due to its shallow root system.

Hawaii residents: For many years the Big Island Invasive Species Committee (BIISC) has worked to maintain a miconia-free buffer zone and to stop miconia from spreading to upper-elevation, pristine watershed forests. Funding is now inadequate to continue this focus. For more information, you may contact them at (808) 430-3090 or email at pageeles@hawaii.edu.

Today, the strategy for control is to implement a serious of biological control measures: to introduce natural enemies from Miconia’s origin in Brazil. The approach is to use a variety of agents including weevils which bore out the stems, caterpillars which feed on the leaves, and other larvae which feed on flowers and seeds. The list of biological control agents also includes a fungus which attacks the leaves and causing premature leaf drop.  

See the invasive species website at http://www.hawaiiinvasivespecies.org/pests/miconia.html for more information.