Weed Management in Organic Farming

1. About organic weed management

2. Cultural method

3. Mechanical method

4. Thermal method

5. Biological method

1. About organic weed management?

Farmers have struggled with the presence of weeds in their fields since the beginning of agriculture. Weeds can be considered a significant problem because they tend to decrease crop yields by increasing competition for water, sunlight and nutrients while serving as host plants for pests and diseases. Since the invention of herbicides, farmers have used these chemicals to eradicate weeds from their fields. Using herbicides not only increased crop yields but also reduced the labor required to remove weeds. Today, some farmers have a renewed interest in organic methods of managing weeds since the widespread use of agro-chemicals has resulted in purported environment and health problems. It has also been found that in some cases herbicides use can cause some weed species to dominate fields because the weeds develop resistance to herbicides. In addition, some herbicides are capable of destroying weeds that are harmless to crops, resulting in a potential decrease in biodiversity on farmers. It is important to understand that under an organic system of seed control, weeds will never be eliminated but only managed.

Critical period of weed control

This period has been defined as an interval in the life cycle of the crop when a must be kept weed – free to prevent yield loss. If weeds have been controlled throughout the critical period, the weeds that emerge later will not affect yield and can be controlled prior to harvest with a harvest and to burn down the weeds and desiccate the crop. Horticulture crops are very sensitive to weed competition, and need to kept weed-free, from planting, emergence or budbreak, until the end of their critical weed –free period. If the crop is kept weed-free for the critical period, generally no yield reduction would result. Again, weeds emerging after the critical weed-free period will not affect yield, but control efforts after this time may make harvest more efficient, or reduce weed problems in subsequent years in perennial crops.

Critical Weed-free Period for Horticultural Crops

2. Cultural Method

Crop rotation

Crop rotation involves alternating different crops in a systematic sequence on the same land. It is an important strategy for developing a sound long term weed control program. Weeds tend to thrive with crops of similar growth requirements as their own and cultural practices designed to contribute to the crop may also benefit the growth and development of weeds. Monoculture, that is growing the same crop in the same field yea after year, results in a build-up of weed species that are adapted to the growing conditions of the crop. When diverse crops are used in a rotation, weed germination and growth cycles are disrupted by variations in cultural practices associated with each crop (tillage, planting dates, crop competition, etc).

Within a rotation, crop choice will determine both the current and the potential future weed problems that a grower will face. Traditionally, potato (Solanum tuberosum L.)  was included  in the rotation to reduce weed problems before a less competitive crop was grown. For an organic grower, crop choice is complicated further by the need to consider soil fertility levels within the cropping sequence and to include fertility building  periods in the rotation. Variations in crop and weed responses to soil nutrient levels can also play an important part in weed management. The inclusion of a fallow period in the rotation in known to reduce perennial weeds. It is best to alternate legumes with grasses,  spring planted crops with fall planted crops, row crops with close planted crops and heavy feeders with light feeders.

Cover crops

Rapid development and dense ground covering by the crop will suppress weeds. The inclusion of cover crops such as rye, red, clover, buckwheat and oilseed radish or over wintering crops like winter wheat or forages in the cropping system can suppress weed growth. Highly competitive crops may be grown as short duration 'smother' crops within the rotation. Additionally, cover crop residues on the soil surface will suppress weeds by shading and cooling the soil. When choosing a cover crop, consideration should always be given to how the cover crop will affect the succeeding crop. In addition, decomposing cover crop residues may release allelo chemicals that inhibit the germination and development of weed seeds.

Intercropping

Intercropping involves growing a smother crop between rows of the main crop. Intercrops are able to suppress weeds. However, the use of intercropping as a strategy for seed control should be approached carefully. The intercrops can greatly reduce the yields of the main crop if competition for water or nutrients occurs.

Field Scouting

It involves the systematic collection of weed and crop data from the field (weed distribution, growth stage, population, crop stage etc). The information is used, in the short term, to make immediate weed management decisions to reduce or avoid economic crop loss. In the long term, field scouting is important in evaluating the success or failure of weed management programs and for making sound decisions in the future.

Mulching

Mulching or covering the soil surface can prevent weed seed germination by blocking light transmission preventing seed germination. Allelopathic chemicals in the mulch also can physically suppress seedling emergence. There are many forms of mulches available. Listed are three common ones.

1. Living mulch

Living mulch is usually a pant species that grows densely and low to the ground such as clover. Living mulches can be planted before or after a crop is established. It is important to kill ad till in, or manage living mulch so that it does not compete with the actual crop. A living mulch of Portulaca oleracea from broadcast before transplanting broccoli suppressed weeds without affecting crop yield. Often, the primary purpose of living mulch is to improve soil structure, aid fertility or reduce pest problems and weed suppression may be merely an added benefit.

2. Organic mulches

Such materials as straw, bark, and composted material can provide effective weed control. Producing the material on the farm is recommended since the cost of purchased mulches can be prohibitive, depending on the amount needed to suppress weed emergence. An effective but labor-intensive system uses newspaper and straw. Two layers of newspaper are placed on the ground, followed by a layer of hay. it is important to make sure the hay does not contain any weeds seeds. Organic mulches have the advantage of being biodegradable. Cut rye grass mulch spread between planted rows of tomatoes and peppers was more economic than cultivation.

Fresh bark of conifers and oak as well as rapeseed straw gave good control of weeds when they were laid as mulches under the trees in apples orchards. Materials such as black polyethylene have been used for weed control in a range of crops in organic production systems. Plastic mulches have been developed that filter out photosynthetically active radiation, but let through infrared light to warm the soil. These infrared transmitting mulches have been shown to be effective at controlling weeds.

Planting patterns

Crop population, spatial arrangement, and the choice of cultivar (variety) can affect weed growth. Fr example, studies have shown that narrow row widths and a higher seeding density will reduce the biomass of later-emerging weeds by reducing the amount of light available for weeds located below the crop canopy. Similarly, fast growing cultivars can have a competitive edge over the weeds.

Variety selection

Careful selection of crop varieties is essential to limit weeds and pathogen problems and to satisfy market needs. Any crop variety that is able to quickly shade the soil between the rows and is able to grow more rapidly than the weeds will have an advantage.

Tillage system

Tillage systems alter the soil seed bank dynamics and depth of burial of weed seeds. Studies have found that almost 75% of the seedbank was concentrated in the upper 5 cm of soil in no-till fields. In the moldboard plough system however, the seedbank is more uniformly distributed over depth. Other conservation tillage systems are intermediate to these two systems.

Weed seedling emergence is often more uniform shallow buried weed seeds and may result in better weed control. Weed seeds closer to the soil are more likely to be eaten or damaged by insects, animals, other predators and disease causing organisms.  

Sanitation

It is possible to prevent many new weeds from being introduced onto the farm and to prevent existing weeds from producing large quantities of seed. The use of clean seed, mowing  weeds around the edges of fields or after harvest to prevent weeds from going to seed, and thoroughly composting manure before application can greatly reduce the introduction of weed seeds and difficult weed species. It is even possible to selectively hand-eradicate isolated outbreaks of new weeds, effectively avoiding future infestations. Planting clean, high-quality seed is essential to crop success. Other sanitation factors to consider would include thorough cleaning of any machinery which might have been used in weedy fields, and the establishment of hedgerows to limit windblown seeds.

Nitrogen fertility

Nitrogen fertilizer can affect the competition between crops and weeds and in the subsequent crops. For example, nitrate is known to promote seed germination and seed production in some weed species. Nitrogen fertilization may result in increased weed growth instead of increased crop yield. Selective placement of nitrogen in a band can favour the crop over the weed. Use of legume residues are opposed to chemical nitrogen  fertilizer to supplement nitrogen needs of the crop can enhance weed suppression. Legume resules release nitrogen slowly with less stimulation of unwanted weed growth.

Feed the crop, not the weeds

Avoiding pre-plant broadcasting of soluble nutrients  that may be more readily utilized by fast-growing weeds than slow-growing crops, and may even stimulate weed germination.

• Applying fertilizer near the rows where it is more likely to be captured by the crop.
• Expensive bagged organic fertilizer, may be applied low rates at planting or sidedress, relying on mid-season release of nutrients from compost and / or green manures for primary fertility.

Water management

Effective water management is key to controlling weeds in a vegetable operation. There are a number of ways that careful irrigation management can help you reduce weed pressure on your crops:

Pre-germination of weeds

In pre-germination irrigation or rainfall germinates weed seeds just before the cash crop is planted. The newly germinated weeds can be killed by light cultivation or flaming. Pre-germination should occur as close a possible to the date of planting to ensure that changes in weather conditions do not have an opportunity to change the spectrum of weeds (cool vs. warm season) in the field.

Planting to moisture

Another technique similar to pre-germination is planting to moisture. 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 vegetables such as corn or beans can be planted into the zone of soil moisture. These seeds can germinate, grow, and provide partial shading of the soil surface without supplemental irrigations that would otherwise provide for an early flush of weeds.

Buried drip irrigation

Drip tape buried below the surface of the planting bed can provide moisture to the crop and minimize the amount of moisture that is available to weeds closer to the surface. If properly managed, this technique can provide significant weed control during dry period.

3. Mechanical Weed Control

Mechanical removal of weeds is both time consuming and labor-intensive but is the most effective method for managing weeds. The choice of implementation, timing, and frequency will depend on the structure and form of the crop and the type and number of weeds. Cultivation involves killing emerging weeds or burying freshly shed weed seeds below the depth from which they germinate. It is important to remember that any ecological approach to weed management begins and ends in the soil seed bank. The sol seedbank is the reserve of weed seeds present in the soil. Observing the composition of the  seedbank can help a farmer make practical weed management decisions. Burial to 1 cm depth and cutting at the soil surface are the most effective ways to control weed seedlings mechanically.

Mechanical weeders include cultivating tools such as hoes, harrows, tines and brush weeders, cutting tools like mowers and stimmers, and dual-purpose implements like thistle-bars. The choice of implement and the timing and frequency of its use depends on the morphology of the crop and the weeds. Implements such as fixed harrows are more suitable for arable crops, whereas inter-row brush weeders are considered to be more effective  for horticultural use. The brush weeder is mainly used for vegetables such as carrots, beetroot, onions, garlic, cerely and leeks. The optimum timing for mechanical weed control is influenced by the competitive ability of the crop and the growth stage of the weeds.

Hand hoes, push hoes and hand-weeding are still used when rouging of an individual plant or patch of weed is the most effective way of preventing the weed from spreading. Hand-weeding may also be used after mechanical inter-row weeding to deal with weeds left in the crop row.

Blind, 'over-the top' cultivation controls very small weeds, just germinated  or emerged, before and sometimes after planting. The entire surface of the fields is worked very shallow using flex-tine cultivators (e.g. Lely weeder or rotary hoes, Inter-row cultivations with a rotary hoe in pinto beans (Phaseolus vulgaris L.) gave adequate weed control without reducing plant stand or injuring the crop.

The hoe-ridger is specifically designed to achieve intra-row control in sugar beet, Thistle-bars are simple blades used to undercut perennial weeds with minimal soil disturbance. The brush weeder, or brush hoe, is used primarily for inter-row weeding of vegetable crop.

Shallow between-row cultivators such as basket-weeders, beet-hoes, or small sharp sweeps are used to cut off and uprrt small weeds after the crop is up. These can get very close to the crop when it's small, without moving much soil into the row, and may  be the only tools used on delicate crops like leafy greens, As vigorous crops grown, soil can be thrown into the row to bury in – row weeds using rolling cultivates (e.g. Lilliston), spyder wheels  (e.g. Bezzerides), large sweeps or hilling disks. Some of these tools can be  angled to pull soil away from the row when plants are small and later turned around to throw soil back on the row during subsequent cultivators.

4. Thermal Weed Control

Flamers

Flamers are useful for weed control. Thermal weed control involves the use of flaming  equipment to crate direct contact between the flame and the plant. This technique works by rupturing plant cells when the sap rapidly expands in the cells. Sometimes thermal control involves the outright burning down of the weeds. Flaming can be used either before crop emergence to give he crop a competitive advantage or after the crop has emerged. However, flaming at this point in the crop production cycle may damage the crop. Although the initial equipment cost may be high, flaming for weed control may prove cheaper than hand weeding.

Propane – fuelled models of flamers are the most commonly used. Flaming dose not burn weeds to ashes; rather the flame rapidly raises the temperature of the weeds to more then 130 °F. The sudden increase in temperature causes the plants cell sap to expand, rupturing the cells 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. Flame weeders can be used when the soil is too moist for mechanical weeding and there is no soil disturbance to stimulate further weed emergence.

Flaming can be used prior to crop emergence in slow-germinating vegetables such as peppers, carrots, onion, and parsley. Onions have some tolerance to flaming and flame weeding has eben successful in both pre and post-crop emergence conditions and after transplanting. Transplanted cabbage has some tolerance to heat, allowing band flaming to be used along the crop row. Damage can occur when the treatment is applied too early, but the crop usually recovers. In a young pear orchard, where treatments were started on a clean soil after cultivation, flaming kept weed growth in check. In an established apple orchard, there was insufficient control of perennial weeds. 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 solarization

During summer and fall, organic farmers sterilize their soil through solarization. In this process, a clear plastic film is placed over an area after it has been tilled and tighly sealed at the edges. Solarization works when the heat crated under the plastic film becomes intense enough to kill weed seeds.

Infrared weeders

Infrared weeders are a further development of flame weeding in which the burners heat ceramic or metal surfaces to generate the infrared radiation directed at the target weeds. Some weeders use a combination  of infrared and direct flaming to kill the weeds. In general, flame weeders are considered  to be more effective because they provide higher temperatures, but burner height and plant stage are important too. Infrared weeders cover a more closely defined area than those of the standards flame weeder, but may need time to heat up. '

Freezing

Freezing would be advantageous only where there is an obvious fire risk from flaming. Liquid nitrogen and solid carbondioxide (dry ice) can be used for freezing weeds.

Various test systems using electrocution, microwaves and irradiation have also been evaluated for weed control purposes, but high energy inputs, slow work rates and the safety implications for operators have hampered  developments. Lasers have been shown  to inhibit the growth the Eichornia crasispes (water hyacinth) but did not kill the weed completely. Weed control using ultraviolet light has been patented but remains at an experimental stage.

5. Biological Weed Control

Biological control would appear to be the natural solution for weed control in organic agriculture.

Allelopathy

Allelopathy is the direct or indirect chemical effect of one plant on the germination, growth or development of neighboring plants. I is now commonly regarded as  component of biological control. Species of both crops and weeds exhibit this ability. Allelopathic crops include barley, rye, annual ryegrass, buckwheat, oats, sorghum, sudan sorghum hybrids, alfalfa, wheat, red clover, and sunflower. Vegetables, such as horseradish, carrot and radish, release particularly powerful allelopathic chemicals from their roots. Suggestions have been  made that allelochemicals and other natural products or their derivatives could form the basis of bioherbicides. However, it is unclear whether the application of natural weed killing chemicals would be acceptable to the organic standard authorities.

The alleopathic effect can be used to an advantage when oats are sown with a new planting of alfalfa. Alleopathy from both the alfalfa and the oats will prevent the planting  from being choked with weeds in the first year. Buckwheat is also well known for its particularly strong weed suppressive ability. Planting buckwheat on weed problem, fields can be an effective cleanup technique. Some farmers allow the buckwheat to grow for only about six week before plowing under. This not only suppress and physically destroys, weeds; it also release phosphorus and conditions the soil.

Beneficial organisms

Little research has been conducted on using predatory r parasitic microorgniams or insects to manage weed populations. However, this may prove to be a useful management tool in the future. Natural enemies that have so far been successful include a weevil for the aquatic weed salvinia, a rust for skeleton weed and probably the most famous, a caterpillar (Cactoblastis sp.) to control prickly pear. There is also considerable research effort aimed at genetically engineering  fungi (myco-herbicides) and bacteria so that they are more effective at controlling specific weeds. Myco-herbicides are a preparation containing pathogenic spores applied as a spray with standard  herbicide application equipment.

Weeds are subject to disease and insect attacks just as crop are. Most biological control of weeds occurs in range or non crop areas. As a result, biological control has little relevance for vegetable growers. Geese have been used for weed control in trees, vine, and certain row crops. Most types of geese will graze weeds, but Chinese weeder geese are considered the best for row crops. Chinest weeder geese are smaller than other types and tend to walk around delicate crop plants rather than over them. Geese prefer grass species and rarely eat crops. If confined, geese will even dig up and eat Johnson grass and Bermuda grass rhizomes. Care must be taken to avoid placing geese near any grass crops such as corn, sorghum, or small grains, as this is their preferred food. Fruiting vegetables, such as tomatoes when they begin to color, might also be vulnerable, so geese would have to be removed from tomato fields at certain times. Geese require drinking water, shade during hot weather, and protection from dogs and other predators.

Use of biocontrol agents for weed control

Use of fish for weed control

Use of competitive plants for weed control

Commercial mycoherbicides