Monday, September 30, 2013

Organic Weed Control


Cinnamon, clove and thyme oils, acetic acid (vinegar), citric acid, d-limonene (a component of the oil extracted from citrus rind) and lemongrass have all been used as an organic approach in controlling weeds. They are strictly contact herbicides with no residual activity; good coverage is essential. These products are most effective on small annual seedlings; weeds in the first true leaf stage are easier to control. They are also more effective on broadleaf weeds than grasses.  These compounds will burn the tops of perennial weeds, but they will quickly recover.  Repeat applications are needed for perennial weeds.

This type of herbicide will damage any green vegetation they contact but are safe against woody stems and trunks.  In general, the organics listed above do a moderately good job. In trying to control weeds in large areas like acres, however, the economics of using these products is poor. It is expensive.

Many of the organic herbicides work better when temperatures are above 75 Fahrenheit; sunlight may also improve the effectiveness of some.
Adding a spray adjuvant, a product added to the herbicide mix to enhance the effectiveness of the herbicide, will result in improved control. 

As stated, the age of the weed is important, the younger the better. A group of organic herbicides were tested and found to control broadleaf weeds between 80 to 100 percent when the weeds were 12 days old. But after 26 days of age, control was between 17 and 38 percent. For grasses, control was between 25 – 42 percent for 12 day old seedlings; for 26 day old seedlings control dropped to 0-8 percent control.

In addition to the chemical sprays, other approaches in controlling weeds include mechanical, physical and biological.  Mechanical weed control involves hoeing, hand pulling, and mowing. Burning is another mechanical method – the use of a propane tank with attached hose and nozzle. This application is best when weeds are less than three inches tall.

Physical weed control consists of placing a layer of opaque material over the soil surface.  This may include inorganic (synthetic) material like plastic sheeting, or organic material like bark, wood chips, straw and compost. Place 4-6 inches of an organic mulch for best results.  Biological weed control is where sheep, goats and geese are used to chew down the weeds.

The main point to stress is: do not allow weeds to go to seed!  Get rid of existing weeds before they flower and produce seed. The average number of weed seeds in soil is 30,000 – 350,000 seeds/square meter, or 120 million to 1.4 billion per acre.  

Monday, September 23, 2013

Citrus Fruit Drop



In most cases, less than 5% of citrus blossoms will actually develop into mature fruit.  An overwhelming number of the blooms naturally drop from the tree during bloom and shortly thereafter. This is a natural thinning process.  Once the tiny fruit has reached about 1 inch in diameter they tend to stay on the tree.  After this point, if fruit continue to drop, it is due to some type of environmental stress. Here are several possibilities: nitrogen deficiency or excess, sudden high temperatures, a lack of water or too much, a heavy insect/mite infestation, hot dry winds, air pollution and severe pruning. Constant rain during the bloom period can also stress the tree. Any one of these factors that occurs around the time of bloom can cause the flowers and young fruit to drop.

The question may arise, “Why does one citrus variety drop its fruit, but other varieties in the same garden do not?”  Here are some of the more common answers.

 1. Different citrus varieties obviously have a different genetic composition.  Consequently, the various varieties will react differently to the numerous stresses. 

 2.  In the same manner, citrus trees are normally budded or grafted onto different rootstocks. These various rootstocks will react differently to the many diverse environmental conditions.  

3. And lastly, although difficult to observe, some locations in the garden may be more conducive to plant growth: better soil drainage, greater soil fertility or more protected from dry wind and high temperatures. 

Monday, September 16, 2013

Perils of Growing Zucchini


For some gardeners, growing zucchini  is an effortless task, the end result being zucchini bread, zucchini cake, zucchini pancakes, zucchini salad,  baked zucchini, broiled zucchini, fried zucchini, etc., etc. Having lived in California, I know this to be true.  Living in the tropics, on the other hand, is a different story. Here is mine:

My first attempt at growing zucchini squash failed because a bird or rat stole the seed. Then I planted the seeds in small containers and placed them in a netted area waiting to be transplanted. On my second attempt, the plants grew well but finally died, succumbing to the powdery mildew (PM) fungus. Next, I planted PM resistant varieties. This helped, but I still needed to use fungicidal sprays such as horticultural oil, Serenade and/or baking soda. See this website for more information on powdery mildew. This points out that the word ‘resistant’ means just that; it doesn't mean immune! It could also mean that in tests, the resistant variety simply did ‘better’ than nonresistant varieties.

Well, the bird or rat problem seemed resolved; the PM problem was addressed, more or less, and a new crop was growing well. The plants were blooming, and small fruit developing on the plant, but then they dropped off. This was an attack of the pickleworm! So I threw some ¾ inch bird netting over the plants. This will keep most of the pickleworm moths out but allowed bees to come in and pollinate.

When the zucchini was nearly ripe, I observed that the whole plant was dying with what appeared to be a virus, one of several that attack squash: squash mosaic virus SMV, cucumber mosaic virus CMV and watermelon mosaic virus WMV. These different viruses are transmitted by insects which feed on virus-infected squash plants or some nearby weeds. Once the plants are infected, there is nothing that can be done. The virus  will eventually kill the plant. Symptoms include distortion of the leaves, chlorotic (yellow) mottling and a dark green mosaic pattern. Infected fruit coming from such plants show a strong mottled pattern. The best preventive measures include insect control along with host weed management.

Note: Squash, along with melons and cucumbers (cucurbits) are unique vegetable plants that bear two kinds of flowers on the same plant, male and female.  In order to produce fruit, the pollen from the male flower must be transferred to the female flower.  Insects do the major pollination work.  Female flowers can easily be distinguished from the male by the presence of a miniature fruit at the base of the flower. 

Sometimes gardeners are concerned because none of the first blooms produce any fruit.  This is because the first flowers produced on the plants will be male and cannot give forth fruit. In time female flowers, and subsequent fruit, will be produced.

Unfortunately,  zucchini is one vegetable I will not be planting in the future. 

Monday, September 9, 2013

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.  There is a BIISC Hotline to report new infestations, (808) 961-3299.


Monday, September 2, 2013

Too Much Rain & More about Corn


 Some areas of the world receive plenty of rainfall.  When there’s too much rain, the following ill-effects on plants may be observed.

1.  A general yellowing.   a.) This can be due to the soil remaining wet for a long period of time thus excluding soil oxygen. In this anaerobic condition, plants will yellow. b) Likewise, the profusion of rain is washing the soil nitrogen below the root zone, and plants become nitrogen deficient, causing yellowing.

2. More fungal leafspot diseases (most notably tomatoes ). Disease causing spores land on the surface of a leaf, germinate and infect the leaf. In a short time thousands of spores are produced. When a rain droplet or water from irrigation lands on the leaf, these newly produced spores will be splattered to other areas of the leaf and to nearby leaves. They in turn germinate and infect these new areas. Thus, more rain, more splashing, more spreading of the spores and more disease.

3. Problems with corn. 
 If the abundance of rain occurs during corn pollination, the ears of corn may lack a full complement of kernels. This is due to a lack of pollination because  the wind-born pollen is washed away. Low pollination is also caused by improper planting densities. Do not plant one or a few individual seedlings, or even a single row. Plant a minimum of 4 rows, 8ft long.  For more on growing corn:  search corn in the blog search

Tips for Picking Corn
·        Mark the calendar the day the corn was planted. In Hawaii, sweet corn is harvested 7O-85 days after planting.
·        Corn is ready when the silks begin drying and turning brown.
·        Record the date at which half of the plants show the silks emerging from the new ears. Under warm summer conditions, the corn will be ready about 18 days later.

·        Perhaps the easiest clue is to pull the green husk back from the cob and check the appearance of the corn kernels. If the corn looks ready, it’s time to pick.