Careful weed management during the season is important, but it must be followed up with off-season weed control as well. Short-season crops such as lettuce can provide opportunities for frequent cultivations and a rapid turnover of crops on the land, thus reducing some weeds’ ability to mature and set seed. Highly competitive cover crops can also smother weeds. If you carry weeds with seed out of the field for disposal, you can also significantly reduce the seed bank. Each of these techniques can help growers minimize weed problems, and that translates to lower hoeing bills.Cultivation is probably the most widely used weed control method in organic vegetable operations. Mechanical cultivation uproots or buries weeds. Burial works best on small weeds, while larger weeds are better controlled by destruction of the root-shoot connection or by slicing, cutting, or turning the soil to eliminate the root system’s contact with the soil. Cultivation is effective against almost all weeds, with the exception of certain parasitic forms such as dodder. Effective cultivation must precisely and accurately target weed growth areas, and so requires good land preparation and bed shaping. Shallow cultivation usually is best, since it brings fewer weed seeds to the surface. Level beds allow more precise depth of tillage. Cultivation requires relatively dry soil; subsequent irrigations should be delayed long enough to prevent the weeds from re-rooting. In addition, vertical farming supplies cultivations should be carried out early enough in the growth cycle to kill weeds such as burning nettle and purslane that set seed early in the growth cycle. The goal of cultivation is to cut out weeds as close to the seed row as possible without disturbing the crop. In most cases, precision cultivation can take care of the weeds on over 80 percent of the bed.
The remaining weeds must be removed from the seed row by hand or using other mechanical means. Here are some common cultivation implements: Various knives, L-shaped and crescent-shaped beet hoes, and sweeps can be used to cut and uproot weeds on bed tops within 1 to 3 inches of the crop row. These can sometimes be combined with reversed-disc hillers that cut vining weeds such as field bindweed and move soil away from the crop row. Disc hillers are often reversed as crops get larger so they will throw soil around the base of the crop plant to bury weeds. Rolling cultivators have become common cultivating implements for a number of crops. A rolling cultivator’s primary purpose is to uproot weeds, but it can also be adjusted to throw soil and bury weeds in the crop row. A new generation of cultivators has been developed to remove weeds from between the seed rows, and in some situations from the seed row itself. Spring-tine cultivators, torsion Bezzerides cultivators, Budding in-row weeders, and brush hoes all can be adjusted to take out weeds between seed rows or close to the seed row. Some of these cultivators can remove weeds from the seed row itself in fields planted to tough-stemmed crops like cotton. Computer-guided cultivators that can distinguish the crop from weeds are under development and may soon be able to remove weeds selectively from within the seed row. 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 in general these are less precise than sleds. Various implements can be attached to these guidance setups to remove weeds. Even the best cultivators will not eliminate all weeds, so some hand weeding is often necessary. It is easier to remove weeds by hand while they are small.
The proper timing of cultivations depends on the speed of weed growth: in spring a two- to three-week periodis about right; in the fall or winter, longer periods between cultivations may be appropriate. The practice and experience of the grower are important factors in effective cultivation. Weeds that compete with the crop early in the crop cycle may be more damaging to crop yield than weeds that establish later in the season. Late-season weeding may disturb the crop’s root system or knock off flowers or fruit, which may reduce yields. Obviously, late season cultivations to reduce weed seed production must be weighed against the potential for yield loss.Flamers are useful for weed control. Propane-fueled models are the most common. Flaming does not burn weeds to ashes; rather, the flame rapidly raises the temperature of the weeds to more than 130°F; The sudden increase in temperature causes the plants’ cell sap to expand, rupturing the cell walls. For greatest flaming efficiency, weeds must have fewer than two true leaves. Grasses are difficult to impossible to kill by flaming because the growing point is protected underground. After flaming, weeds that have been killed rapidly change from a glossy appearance to a duller appearance. Flaming can be used prior to crop emergence in slow-germinating vegetables such as peppers, carrots, onions, and parsley. In addition, flaming can be used postemergence on crops such as young onion and garlic or as a directed treatment to the base of tougher crops when they are 12 or more inches tall. Postemergence flaming does adversely impact the yield of the crop, so its use must be weighed against the potential damage the weeds might cause. Typically, flaming can be applied at a speed of 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 weeds.
The efficiency of flaming is greatly reduced if moisture from dew or rain is present on the plants. Early morning and early evening are the best times to observe the flame patterns and adjust the equipment.Soil sterilization in organic agriculture involves the use of heat or naturally generated biocides to kill weeds. Heat is applied as steam or by soil solarization. In steam sterilization, the steam is injected into the soil to kill weed seeds. The large quantities of fuel and water required by this technique make it an expensive choice, so its use is limited to small acreages of high-value horticultural crops or landscaping. Ozone is a naturally occurring biocide that is being researched for use as a soil sterilant. The ozone is generated mechanically and then injected into the soil. Ozone injection shows promise as a weed-reduction tool, but it is unclear at this time whether this technique will be considered an organically acceptable practice. Soil solarization involves placing a clear plastic mulch over a tilled, moist soil to allow the solar energy to heat the soil and kill germinating weed seeds. To be most effective, solarization should be performed during summer and fall periods of maximum solar radiation exposure. Mulching is another weed control method. A mulch blocks light, preventing weed germination and growth. The materials that can be used as mulches are varied, and include plastics and 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, and some mulch materials require a thicker application layer that others to accomplish this. Plastic mulches vary in thickness from 1.5 mil to about 4 mils. The most common color for weed-control plastic is black, since it completely blocks light. More recently, a clear, infrared-transmitting plastic has been introduced. The IRT plastic blocks certain wavelengths of light but allows others to pass, and that heats the soil better for early-season crop growth. Plastic mulches are generally placed on the beds and their edges covered with dirt to keep them from blowing away. Drip irrigation is needed to get moisture to the crop under the plastic mulch. Certain weeds, including nutsedge, vertical weed grow are able to penetrate the plastic and so are not completely controlled by plastic mulches. Other weeds can grow in the openings provided for crops. Further problems with plastic mulches include difficulties keeping them in place under windy conditions, disposal after the crop is harvested , and their cost . Organic mulches such as municipal yard waste, straw, hay, and wood chips must be maintained in a layer 4 or more inches thick in order to block out light.
Organic mulches break down over time, and the original thickness typically reduces by 60 percent after one year. Coarse green waste works better as a mulch. Organic mulches are mostly used for permanent crops, landscaping, and non-crop areas, although they are also very effective for transplanted vegetables. Organic mulches can be grown in place. Plants used to produce organic mulches include cereals, clovers, vetches, and fava beans. These mulches must die or be killed before or shortly after crop planting in order to avoid excessive competition with the crop. Living mulches were developed in the eastern United States, but are currently being tested on various fruiting vegetables in California .Although winter squash can be planted “on the flat” , a bedded system improves moisture retention and weed management. Perform standard tillage practices to incorporate crop or cover crop residue , break compaction, and adequately loosen soil. Then, form the planting beds using bedding shovels or a rolling cultivator. If there is no rainfall following bed formation in the spring, preirrigate with overhead irrigation to wet the root zone and germinate weeds prior to planting. This pre-irrigation further improves soil conditions and tilth by breaking down soil clods or clumps of cover crop residue, leaving the soil loose, moist, and friable. Following the pre-irrigation , eliminate newly germinated weeds with a rolling cultivator or other suitable cultivation technique. If timing is good and the moisture is uniform, such a run can work wonders. This initial cultivation breaks surface crusting and provides a “soil mulch” to slow evaporative loss of deeper soil moisture. Once crop or cover crop residue is adequately decomposed and soil temperatures are above 60ºF, use a suitable planter to push aside the drier soil on the bed tops and plant the squash seeds into the deeper moisture in the bed.In general, winter squash can be planted from mid-May through June on California’s Central Coast. Shorter-maturing varieties can be planted in early July. Planting dates are based on timing of adequate seedbed preparation , soil moisture , and optimal soil temperature . Plant late enough in spring to allow for rapid plant growth; this will help limit cucumber beetle and other herbivore damage to seedlings. Planting dates must be early enough to allow the crop to mature and adequately field cure before fall rains, heavy dew, or frost.Winter squash seed can be planted to moisture by hand with a shovel or trowel. There are also “seed stick” planters that are very effective for planting winter squash. Push planters such as the Planet Jr. are effective for garden-scale production, but require a special “deep” opening shoe to get the seed far enough into moist soil. On larger field-scale blocks , use a tractor-mounted planter such as the John Deere 71 “flexi” planter or other similar plate-type planter . Adjust planting depth with a rotating cam on the side of the planter, which changes the angle of the press wheel in relation to the disc openers. For mixed blocks of winter squash on relatively small plots, the planter hopper can be removed and the seeds hand dropped into the drop tube. This circumvents the need for multiple seed plates to match each variety. Note that it is better to plant into soil on the drier side. On many soil types, if the soil is too wet at planting, soil surface crusting can impede successful crop emergence. Squash plants that struggle to break through crusted soil may remain stunted. In cases when the soil is either too wet or too dry, you can form a “cap” of soil over the seed line to either minimize crusting or to minimize further evaporative loss . Run soil cappers behind the planter to create a loose cap of soil right over the seed line behind the planter’s pack wheel . With optimum soil conditions and planting depths, plants should emerge in 7–10 days. Uniform emergence is the best sign of optimal planting conditions and potential for a successful crop. The most critical aspect of effectively “planting to moisture” is your ability to judge soil moisture and decide on seed depth.