Physical weed management includes hand weeding and cultivation. Hand weeding is expensive but effective. The availability and cost of labor are key considerations, although it is common to perform some hand weeding in almost all vegetable crops. When pulling weeds or using hand tools, minimize soil disturbance to avoid moving more weed seeds close to the soil surface where they can more easily germinate.
Cultivation is an important component of weed control in vegetables, particularly when the use of herbicides and/or mulches is to be minimized or avoided. In general, weeds are most effectively cultivated shortly after they germinate, and crops are most sensitive to weed pressure during their early stages of growth. Thus, cultivation is most critical early in the growing season. Also, weeds will die more easily if the sun is out and the soil is not wet. Mechanical methods should be combined with cultural practices such as crop rotation, composting of manure, and mowing of headlands to keep nearby weeds from going to seed. In addition, it is imperative that cultivation tools be properly adjusted. Here is a summary of some commonly used weed control equipment and practices:
Blind cultivation is performed after crops have been planted, over the top of them, so that both in-row and between-row areas are cultivated. To minimize crop damage, this should be done before the crop has emerged and/or once it is well rooted. Slightly deeper planting depths and slightly higher plant populations are recommended to compensate for some crop loss that may occur. Very tender crops such as leafy greens are not amenable to this technique, but a surprising array of crops are suitable for blind cultivation, including corn, cucurbits, beets, etc. depending on stage of growth and equipment used.
Between-row cultivation can be performed with varying degrees of aggressiveness, so that soil may or may not be pushed into the row to obtain some between-row weed control. Commonly used cultivation setups consist of a shank (either straight, C-, or S-shape) attached to a toolbar, with a cultivating tool (shovel, sweep, knife, hilling disc, etc.) attached to the bottom. The more curve to the shank, the more it will vibrate and flex in the soil. Trip-shanks have a release mechanism that allows the shank to pop up when it hits a rock. Shanks can be arranged on multiple toolbars to offer complete coverage between multiple rows in beds. Planting crops using the same row spacing will make it easier to cultivate with a single set up by minimizing equipment adjustments for different crops.
There are many kinds of tools that can be attached to the shanks, and these should be selected to disturb only as much soil as is needed to kill the weeds present. In general, one starts the season with smaller, shallower tools to kill small weeds when the crop is small; as the season goes on, if larger weeds are present, then more aggressive tools are needed, and as the crop grows it is possible to more aggressively push soil into the row without causing damage. For example, shovels might be followed by sweeps and then hilling discs. Another strategy is to adjust the angle of cultivating tools such as hilling discs or rolling cultivators as the season goes on. Start by pulling soil away from the row when cultivating a young crop; that creates a small hill that can be pushed back into the row when the crop is a bit older to bury weeds.
In small and/or tender crops like leafy greens, shallow tools are needed that will not move soil into the row and cause damage. A side-knife (or beet-hoe) may be used to cultivate horizontally next to the crop, just below the surface.
Tine weeders (Einbock, Kovar, Lely) have multiple rows of flexible metal tines that cover the entire soil surface, wiggling slightly as they are pulled along, uprooting or dislodging very small weeds. They work best at relatively high speeds, and the wide span of the tool makes cultivation quick. They are rear-mounted and available in many widths. The tension on the tines can be adjusted on some units or by the pressure on the 3-point hitch on other units. Gauge wheels can be used to maintain uniform depth. Work best in friable soils free of rocks, and when weather allows weeds to dry out on soil surface after uprooting.
Rotary Hoes (Yetter, Deere) have many narrowly-spaced metal wheels each with about 16 curved teeth that work the surface of the soil. Used within the first few weeks of planting less tender crops like corn or beans, they destroy weeds that have just germinated. Ground-driven, spring-loaded wheels do little damage to the crop and work well at high speeds on dry, rock-free soils with little residue. They are rear mounted, in many widths. The teeth, or spoons, lose their effectiveness if not sharp.
Basket weeders (Buddingh) are relatively high-speed, between-row cultivators good for control of small weeds in narrow rows when crop is small. The wire baskets work the soil surface and do not move soil into the row, but they don't work well in crusted or rocky soil. The front set of baskets are ground driven, and they turn the rear set of baskets a bit faster via a chain and gearing, causing scuffing of the soil. These are available in 2- to 6-row units that can be rear or belly-mounted.
Finger weeders(Buddingh) Finger weeders consist of steel cone wheels that are ground-driven by spike tines on the bottom, with rubber fingers on the perimeter. The rubber fingers work the soil just below the surface, uprooting small weeds located very close to the crop. Finger weeders work best for single rows, to control small weeds in dry, friable soil with few rocks or residues. Clay soils may stick to fingers.
Spring-hoe, torsion weeders, and spyder weeders (Bezzerides) are flexible blades and square metal stock that disturb soil around base of plants, and ground-driven spyder wheels with staggered teeth in an uneven pattern that break clods and throws soil into row, or pull it away, depending on the angle. These can be toolbar-mounted, either under the belly or in the rear. They can be used together, separately, or in conjunction with other cultivators.
Row crop cultivators (many brands are available) have gangs of S-shanks with sweeps that break up the soil while destroying small to moderate-sized weeds. The gangs work behind the tractor between the rows at uniform intervals. Shields may be added to protect the crop from moving soil, and a fertilizer hopper can be used for sidedressing during cultivation.
Rolling cultivators (Lilliston, BHC) have gangs of soil-driven 'spider wheels' that mount independently on a toolbar. The angle that they work the soil, and thus their aggressiveness, is usually adjustable. The number of gangs grouped together determines cultivator width, and these are usually rear-mounted, but pairs of gangs may be belly mounted to work a row or two. Soil can be thrown into row to bury small weeds or to form hills, depending on angle of the gangs. This is a relatively heavy, aggressive tool.
Wiggle hoe (Friday) is an in-row cultivator that uses hand-steered side knives controlled by an operator who moves them in and out of the row. Knives can reach very close to the row and eliminate the weeds, but if the operator isn't careful, the crop can be damaged. Good for irregular plant spacing in the row or between rows.
Reigi weeders are a relatively new tool that also requires a rear operator who steers a pair of rotating horizontal wheels in and out of the crop row. The wheels are turned by a PTO-driven belt, and they have stiff tines on them that root out weeds. The wheels come in several sizes useful for various row spacings, and the units come in 1 or 2-row models. These are very effective for killing weeds in and next to the row in widely spaced crops like pumpkins, first-year strawberries, or sweet corn with a lot of skips.
Flame weeders can be used to kill weeds before planting without causing soil disturbance that brings up new weed seeds, after the crop has been planted (but before it emerges), inter-row weeding, and for stale seed bed preparation. Flame weeding, also know as thermal weeding or flame cultivation, exposes plants to brief periods of high temperature that causes the water in the plant tissue to expand rapidly, rupturing plant cells and leading to tissue damage. Plants are not burned or incinerated, but "blanched". They will not show symptoms of injury for several hours after exposure. Some weeds, such as purslane, can tolerate high temperature, and grasses with their growing points below ground are not controlled by flaming. When weeds are moist from rain or dew, more heat (a slower tractor or walking speed) will be necessary.
Like with contact herbicides, flaming kills weeds without soil disturbance, it is ideal for stale seedbeds. Once broadleaf weeds reach the three-leaf stage, they should be flamed to prevent them from growing too large. For longer lasting weed control, apply the final flaming as late as possible prior to crop emergence after seeding or just prior to transplanting.
Hand-held propane torches are commonly used to flame single rows at a time, but multi-row bed-flamers and tractor-mounted flamer kits are also available. Larger units require greater attention to safety during construction and operation. Safety is a big issue with flaming. Consult with a gas professional if constructing your own flaming unit. Do not mount propane tanks intended for stationary use onto tractors. Flame against the breeze and avoid areas with dry residues or dry hedgerows. Liability concerns may hinder the use of flaming.