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.  

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. 

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.