Thursday | June 30, 2016
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Organic weed control

Can you give me the names of some organic products I can use to control weeds?

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. They will burn the tops of perennial weeds but the weed 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, they do a moderately good job. In trying to control weeds in large areas (acres) however, the economics of using these products is poor.

Many of these organic herbicides work better when temperatures are above 75 Fahrenheit and 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 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 chemical sprays, mechanical, physical and biological methods may be undertaken to control weeds. Mechanical weed control includes hoeing, hand pulling, and mowing. Burning is another mechanical method — a 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 involves 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 3-6 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 I would 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.

I have a fair amount of drywall that I would like to dispose of but not dump it into the landfill. Is there any gardening use for drywall?

Yes, it is a good source of calcium. Most drywall is composed of gypsum which is calcium sulfate. For calcium deficient soils, gypsum would be one of the recommended amendments. It would be best to break the drywall into pieces, as small as possible, and incorporate into the soil. The only caution I have is what else is in the drywall besides gypsum. Companies do add foaming agents, plasticizers, starch, EDTA and other compounds. These products should breakdown in the soil and not pose a threat to the garden. I cannot, however, assurance you that everything in drywall is completely safe for the garden. If you are serious about using the drywall and worried about it, contact the manufacturer for further information.

Class Announcement: Growing Citrus in Hawaii

I will be teaching a citrus class Nov. 3, from 9:30 a.m.-12 p.m. at the University of Hawaii at Hilo Campus UCB 118. Topics to be discussed are preparing the soil, planting, pruning, fertilizing, diseases, insect problems and troubleshooting; learn basic information for the backyard gardener. Call 974-7664 to register; or go online at

Hilo resident Nick Sakovich is a professor emeritus of the University of California. He has worked in the field of agriculture for 30 years. Email your questions to Sakovich at


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