For more information on this and other gardening topics, please visit the CTAHR electronic publication website at http://www.ctahr.hawaii.edu/Site/Info.aspx or visit any of the local Cooperative Extension Service offices around the Island. I can be reached at russelln@hawaii.edu.
By Russell T. Nagata
University of Hawaii at Manoa
College of Tropical Agriculture and Human Resources
Komohana Research and Extension Center
Whether you have a good attitude about weeds and weed control, weeds can be an ongoing challenge in many of our Hawaiian gardens. The tropical climate along with year-round growing season allows many of our landscape plants to thrive, and also allows weeds to flourish. Therefore, the successful management of weeds in your garden will depend on good planning and execution. Here are some of the points that your plan should consider and address.
First is to identify the problem. Do you have a weed problem? How large is the problem? It is very important to identify the weeds you are trying to control since every method of control will not work for every weed. The University of Hawaii at Manoa, College of Tropical Agriculture and Human Resources, Master Gardeners can assist in identifying weeds should assistance be needed. With proper weed identification, knowledge of plant biology and reproductive cycle can be found and proper culture and management control strategies can be formulated. In addition, proper weed identification will lead to the correct and efficacious herbicide use.
The control of weeds through culture and management practices includes pulling or removal of weeds through mechanical or physical means. To be effective, weeds should be removed prior to seed set or maturation of vegetative propagules such as corms, stolons and tubers. It is also easier to remove and kill younger weed plants when their root system isn’t well developed. Once seed set has occurred, the potential for literally hundreds to hundreds of thousands of new weeds plants has occurred. Preventing weeds from developing seeds will effectively reduce the number of seeds in the soil, reducing future weed population. In many weed species, only a portion of seeds germinate at any one time, mainly those that are lying at or very close to the soil surface. The remainder lay dormant in the soil waiting to germinate after being brought up to the surface. Young weed seedlings can be controlled using a hoe or mechanical cultivator. In areas with abundant sunshine, solarization is an option. This process involves using a clear plastic sheet to tent freshly tilled soil and allow the trapped heat to slowly pasteurize the surface of the covered plot. It will take several weeks to kill weed seeds and at best will be effective to a depth of a few inches.
Mulching is a good method to block light needed for seed germination. Organic mulches such as wood chips, ground vegetative matter or straw can make good mulch for certain plants. When applying organic mulches, you will need to check that no weeds are protruding above the mulch as this defeats the purpose of the mulch. Use 2 to 4 inches of mulch to effectively prevent weed seed germination. There are also many types of weed mat, weed blocks, plastic mulches that prevent weeds from germinating and growing. These are usually made of plastic that are either woven or extruded sheets. Paper mulches are available, however, these are less durable especially in wet climates. Plastic mulches are nearly always black in color to prevent light transmission through the mulch. Laminated white on black mulch is also manufactured where the bottom black layer blocks light and the top layer is white to reflect light and heat away, keeping the soil cooler than with black mulch.
Herbicides are available in many different formulations, time of use in the cropping cycle, plants they will control or kill, and mode of action. In compliance with federal laws, always read and follow label directions for all herbicides and pesticides that you use. When selecting and purchasing herbicides, there are several term you will need to be aware of. The first two are selective and nonselective herbicides. Nonselective herbicides will injury your crop plant, while selective herbicides will not affect your crop when used properly. Next, you have preemergence and post emergence herbicides. Pre-emergence herbicides prevent seeds from germinating or kill germinating seedlings. Postemergence herbicides are used after the weed have emerged from the ground and have begun to grow. Finally, we have contact or systemic herbicides. As the name implies, contact herbicides kill plant tissue they contact. Systemic herbicides are those that are taken up by the plant and moves internally disrupting plant function such as cell growth and development. Whenever applying herbicides you need to be careful to control unintended contact to nontarget plants. Some of these unintended contacts occur through wind cause drift of sprays or movement of herbicides on the ground due to water and mechanical means.
For more information on this and other gardening topics, please visit the CTAHR electronic publication website at http://www.ctahr.hawaii.edu/Site/Info.aspx or visit any of the local Cooperative Extension Service offices around the Island. I can be reached at russelln@hawaii.edu.