Successfully managing a weedy problem

BY RUSSELL T. NAGATA | SPECIAL TO WEST HAWAII TODAY

Whether you have a good attitude about weeds and weed control, weeds can be an ongoing challenge in many Hawaiian gardens. The tropical climate and year-round growing season allow many of our landscape plants to thrive, but also allows weeds to flourish. Therefore, the successful management of weeds in your garden will depend on good planning and execution. Here are some points that your plan should consider and address.

First, identify the problem. Do you have a weed problem? How large is the problem? It is 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 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 weed 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 lying at or 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 since 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 and plastic mulches that prevent weeds from germinating and growing. These are usually made of plastic and 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 terms of which you will need to be aware. The first two are selective and nonselective herbicides. Nonselective herbicides will injure your crop plant, while selective herbicides will not affect your crop when used properly. Next you have pre-emergence and post-emergence herbicides. Pre-emergence herbicides prevent seeds from germinating or kill germinating seedlings. Post-emergence herbicides are used after weeds 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 drift of sprays or movement of herbicides on the ground because of water and mechanical means.

For more information on this and other gardening topics, visit ctahr.hawaii.edu/Site/Info.aspx or visit any of the local Cooperative Extension Service offices around the island.

Russell Nagata is the Hawaii County administrator of the University of Hawaii at Manoa College of Tropical Agriculture and Human Resources. He can be reached at russelln@hawaii.edu.