Pest Management Guidelines

Organic Weed Control

(Reviewed 12/09, updated 6/12)

In this Guideline: More about weeds in cucurbits:

Controlling weeds in organic cucurbits requires the use of many techniques and strategies in order to achieve economically acceptable weed control and yields. Weeds can always be pulled or cut out, but the question is simply how much can a grower spend in terms of time and money to reduce weed pressure. The more a grower is able to reduce weed pressure (seed and perennial propagules), the more economical it is to produce crops.


Monitor the fields and keep records of the weed species that occur in each field during the period of the year when the crop will be grown, especially at planting time but before hand weeding or cultivation to determine what weeds have escaped treatment. Not only are these records valuable in planning weed management strategies, but they also help track the occurrence of hard-to-control weeds. At harvest, keep records of specific locations in the field where weeds are producing seed. Weed patches are relatively stable within a field and management of these weed clusters may improve weed control in future crops. Avoid fields that have high populations of certain weeds such as common purslane, field bindweed, or nutsedge, as these weeds are not adequately controlled by currently available weed management methods.

Water management
Water management is a key tool for controlling weeds in cucurbits. There are a number of ways that careful use of irrigation management can assist growers in reducing weed pressure. A process called "pregermination" involves irrigating (or rainfall) before planting to germinate weed seeds, which can then be killed by light cultivation or flaming. Pregermination should be done as close as possible to the date of planting to assure that the weed spectrum does not change with the changes in the season or weather.

Dust mulches are also used in cucurbits. This technique is compatible with the process of pregermination. After weeds are killed by cultivation, the top 2 to 3 inches of soil are allowed to dry and form a dust mulch. At planting the dust mulch is pushed away and large seeded cucurbits can be planted into the zone of soil moisture. The cucurbit seed can germinate and grow with no supplemental irrigations that would otherwise germinate another flush of weeds.

Drip tape buried below the surface of the bed can provide moisture to the crop and minimize the amount of moisture that is available to weeds on the surface. If properly managed, this technique can provide significant weed control during the non-rainy periods of the year.

Cultivation is probably the most widely used method of weed control in organic vegetable systems. Mechanical cultivation uproots or buries weeds. Weed burial works best on small weeds, while larger weeds are better controlled by destroying the root-shoot connection or by slicing, cutting, or turning the soil to separate the root system from contact with the soil. Cultivation is effective against almost all weeds. Effective cultivation requires good land preparation for precision and accuracy. Shallow cultivation usually is best, because it brings fewer weed seeds to the surface. Level beds allow greater precision in depth of tillage. Cultivation requires relatively dry soil conditions; delay subsequent irrigations long enough to prevent re-rooting of weeds.

The goal of cultivation is to remove the weeds as close to the seed row as possible without disturbing the crop. In most cases precision cultivation can take care of weeds on over 90 percent of the bed. The remaining weeds must be removed from the seed row by hand or other mechanical means. Cultivation implements are often mounted on sleds for accurate, close cultivation in row crops. Guide wheels, cone wheels, and other devices are also used, but are generally less precise than sleds. Various implements are attached to these guidance setups to remove weeds.

Even the best cultivators will not eliminate all weeds, thus hand weeding is often needed. It is easier to remove weeds while they are small. The proper timing between cultivations depends on the speed of weed growth: in spring a 2- to 3-week period is about right, in the fall or winter, longer periods between cultivations may suffice.

Early occurrence of persistent weeds may be more damaging to crop yield than are weeds that establish late. Late-season weeding often disturbs crop root system or knocks off flowers or fruit and consistently results in reduced yields. Obviously, late season cultivations to reduce weed seed production must be weighed against the potential for yield loss.

Flamers can be used for weed control with propane-fueled models being most common. Fire causes the cell sap of plants to expand, rupturing the cell walls. Flaming can be used before the crop emerges. Weeds must have less than two true leaves for greatest efficiency of the burner. Grasses are harder to kill by flaming because the growing point is below the ground. After flaming, weeds that have been killed change from a glossy to a matte finish. This occurs very rapidly in most cases. Typically, flaming can be done at 3 to 5 mph through fields, although this depends on the heat output of the unit being used. Best results are obtained under windless conditions, as winds can prevent the heat from reaching the target. Early morning or evening are the best times to observe the flame for adjustment.

Mulches blocks light, preventing weed germination or growth. Many materials can be used as mulches including plastics or organic materials such as municipal yard waste, wood chips, straw, hay, sawdust, and newspaper. To be effective, a mulch needs to block all light to the weeds therefore different mulch materials vary in the depth necessary to accomplish this.

Plastic mulches vary in thickness from 1.5 mil to about 4 mil. The most common plastic color for weed control is black, as it completely blocks light. Plastic mulches are generally placed on the beds and their edges covered with dirt to prevent their blowing away. Drip irrigation is needed under the plastic mulches to provide the crop with moisture. Certain weeds such as nutsedge are not completely controlled by plastic mulches, because they are able to penetrate the plastic. Other weeds are also able to grow in the openings provided for crops. Further problems with plastic mulches include maintaining them in place under windy conditions, disposal after crop harvest, and the cost (including the need for drip irrigation).

Organic mulches such as municipal yard waste, straw, hay, wood chips, etc., must be maintained in a layer 4 or more inches thick. Coarse green waste works better as a mulch. Organic mulches can also be grown in place. Plants used in this way include subterranean clovers, ryegrass, fava beans, oats, barley, rye, etc. These mulches (or living mulches as they are sometimes referred) must die or be killed before planting the crop to avoid excessive competition with the crop.

Organic herbicides
Herbicides are chemicals that kill or suppress plants by affecting plant physiological processes. The number of herbicides that are organically acceptable are limited and include contact materials such as citric acid, clove oil, cinnamon oil, lemongrass oil, and various soaps. Herbicides can be used to selectively control weeds by application ahead of crop emergence or by use of shielded or directed applications. These herbicides kill plants that have emerged, but have no residual activity on those subsequently emerging.


[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Cucurbits
UC ANR Publication 3445

C. S. Stoddard, UC Cooperative Extension, Merced & Madera counties
Acknowledgment for contributions to Weeds:
C. E. Bell, UC Cooperative Extension, San Diego County
W. T. Lanini, Plant Sciences and Weed Science, UC Davis

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