Critical Weed-Free Period
One important factor to consider regarding agricultural weed management is the critical weed-free period of your crop(s) of interest. The critical weed-free period refers to the length of time after planting that weed proliferation must be kept to an absolute minimum. If they aren't controlled the weeds could hamper crop yield by depriving the crop plants of resources during their early growth stage, or by proliferating to such an extent that they prevent the use of harvesting equipment at the end of the growing season. The critical weed-free period differs from crop to crop so checking what that period is for your crop(s) before planting is ideal.
Conventional Weed Control
Conventional methods of weed control involve the use of specially formulated chemical compounds to interrupt specific biochemical pathways (called modes of action, or MOA) in order to eliminate weeds without affecting crops being grown. The herbicides used depend on the type of weed being targeted, the developmental stage of these weeds, and the crops being raised. For a breakdown of different types of weeds that can be encountered in the field see Weed ID and Biology.
Vegetable Weed Control
***HERBICIDES WITH THE ACTIVE INGREDIENTS GLYPHOSATE, CARFENTRAZONE, PARAQUAT, CAPRIC ACID & CAPRYLIC ACID SHOULD UNDER NO CIRCUMSTANCES BE APPLIED TO CROPS, INTENTIONALLY OR OTHERWISE, AS THEY ARE NON-SELECTIVE HERBICIDES. THE ONLY EXCEPTIONS ARE TRANSGENIC CROPS WITH SPECIFIC RESISTANCE TO ONE OR MORE OF THESE ACTIVE INGREDIENTS
Fruit Weed Control
Herbicide Formulation Acronyms
Herbicides usually have acronyms on the label which denotes the physical properties of the herbicide itself, information which is important to know before mixing and applying the herbicide.
EC- emulsifiable concentrate DG/WDG- water dispersible granule
EW- emulsion, oil in water SP- soluble powder
SE- suspo-emulsion WP- wettable powder
GR- granule SL- soluble concentrates
GG- macro granule CS- capsule suspension
SG- water soluble granule SC- suspension concentrate
DF- dry flowables XP- extruded paste
Adjuvants are substances added into a mixture tank to change the physical characteristics of a mixture. These changes serve to aid in improving the functionality of the herbicides, changes which depend on the type of adjuvant added into the mix. Generally, the types of adjuvants that can be added to an herbicide mixture will be noted on the label of the herbicide.
Surfacants improve the activity of herbicides by increasing the amount of surface area herbicides occupy when applied via foliar spray. Generally surfacants are added at about a 0.25% v/v to the herbicide mixture. The type of surfacant to be used depends on the herbicide it will be mixed into as well as weather conditions out in the field. Different surfacant types include:
- Anionic surfacant
- Nonionic surfacant
- Organo-silicone surfacant
Crop oil concentrates improve the activity of herbicides by acting as a surfacant as well as improving their ability to penetrate past trichomes and the epidermal tissue layer of leaf tissue while also having some phytotoxic effects. Generally crop oil concentrates are added at about a 1% v/v to the herbicide mixture.
Ammonium sulfate (AMS) can enhance herbicide activity by mitigating some of the interactions that herbicide molecules may have with dissolved minerals present in hard water sources, thereby making more of the herbicide available for its intended use rather than being locked up into complexes with said minerals. Ammonium sulfate is usually added at 2% to 2.5% v/v to the herbicide mixture.
Methylated seed oils can enhance herbicide activity by increasing absorption and movement of the herbicide into target weeds while also acting as a surfacant. Methylated seed oil is usually added at 0.25% to 2% depending on the herbicide mixture it is being added to.
Cover crops are most often used as a means of reinvigorating a field following a harvest. Cover crops serve to recycle nutrients into the soil, prevent soil erosion following the removal of crop plants, increase organic matter in soil, and maintain soil moisture. Cover crops can also be used to control weeds by outcompeting invasive weeds in the field. Ideally, this cover crop is capable of taking up nutrients and space at a faster rate than weeds while also being able to be controlled such that it won't affect new crops that will be planted. While this is a difficult balance to achieve, some promising candidates have been tested at CREC.
Suppress is a non-selective herbicide composed of capric acid and caprylic acid. This herbicide works by essentially burning the leaves of any plants it is applied to, thereby taking away their ability to adequately photosynthesize. Weed control efficacy trials carried out at CREC suggest that Suppress is most effective when used while weeds are still fairly young and small. The label for Suppress can be found here: Suppress Label.
Homeplate is another non-selective herbicide composed of capric acid and caprylic acid. Like Suppress, Homeplate is applied via foliar spray and destroys the leaf tissue of target weeds, rendering them unable to carry out photosynthesis which results in their death. This herbicide is capable of controlling a wide variety of broadleaf weeds including common chickweed, Canada thistle, shepherd's purse, and common lambsquarters. The label for Homeplate can be found here: Homeplate Label.
Gluten 8 is a pre-emergent organic herbicide comprised mostly of corn gluten meal that has been shown to effectively control annual grass weeds as well as some broadleaf weeds like dandelion, clover, and plantains. Gluten 8's mode of action is to release dipeptides into the seedbed where it's applied which decreases the ability of weed seedlings to germinate & set roots in the soil. The Gluten 8 label suggests seeding either 3 weeks before or 3 weeks after application. The label for this herbicide can be found here: Gluten 8 Label.
Stale Seed Bed Technique
Creating a stale seed bed is a weed management technique that removes non-dormant weeds from the germination zone of a seedbed prior to crop planting. A stale seed bed is established by cultivating/tilling a seed bed, allowing weed seeds that get kicked up to germinate and grow to a decent size, culling these weeds, and planting crops in the newly sterilized seed bed. Any weed control methods can be used to kill off weeds in the top layer of soil in the seed bed, though ideally the method used does not disturb soil beneath the stale seed beed, else nondormant seeds buried beneath this layer could inadvertently be exposed.
The timing of and the number of weed flushes that need to be carried out largely depends on the type of crop being planted. Multiple weed flushes weeks or months in advance of planting may be necessary for proper stale seed bed preparation of crops that grow slowly after seeding or transplanting. In addition, the effectiveness of this technique may be affected by factors out of one's control such as temperature, soil moisture levels, wind, etc. As such, growers should still be prepared to carry out normal routine measures of weed management throughout the growing season, just not as early or as frequently throughout the growing season.
Anaerobic Soil Disinfestation
Anaerobic soil disinfestation is a technique that has generally been used as a method of controlling microbial pests present in the soil of smaller scale agricultural operations. This method consists of introducing organic material into a soil area, saturating (but not flooding) this area with water, and covering this area with a plastic material impermeable to oxygen. In doing so, an anaerobic environment is created which sterilizes the soil of any aerobic soilborn pathogens. This technique may have potential as a means of weed control as well.
Robotic Weed Control
Robotic weed control is a still-developing method of weed management that uses remote-controlled or pre-programmed autonomous drones to eliminate weeds growing in between crop rows. Although this technology is still very much in the early stages of efficient implementation, it has the potential to create a means of significantly streamlining the weed management process.
Flame Weed Control
Flame weed control involves using a propane-fueled flame thrower to incinerate weeds growing alongside and in between crop rows. This is an effective means of destroying most any shoots emerging above ground but will have to be repeated periodically for weeds with rhizomes, corms, tubers, or bulbs. This method also presents the obvious risk of causing fire damage to row crops so careful application and technique are of the utmost importance.
Figure 1: Flame Weeding
Tractor equipped with flame weeder burning weeds along raised bed.
Plastic mulch is a commonly used method of weed control which prevents the germination and growth of weeds by smothering them. Even if seeds germinate beneath the plastic, the absence of light & flowing air often ensures they won't survive for long. While this method is effective for the majority of weeds, there are some persistent weeds that are unaffected by plasticulture due to their unique anatomical properties. One such weed is yellow nutsedge, which is capable of producing leaves and rhizomes from tubers far beneath the normal germination zone of 2.5 inches. Said leaves are fairly strong and tapered, enough to puncture through the plastic bed cover. Nutsedge tends to thrive in plasticulture due to the lack of competition from other weeds that are incapable of growing under the conditions created by plasticulture.
Preliminary trials testing the effectiveness of different colors of plastic on the growth of weeds capable of penetrating through the plastic have been carried out at CREC. These trials are testing whether or not the amount of light penetrating through the plastic cover could affect the growing habits of weeds germinating beneath the plastic. The results of these trials indicate the possibility that the amount of light penetrating through the plastic may affect the ability of weed shoots to penetrate through the plastic layer.
Figure 1: Black Plastic Mulch
Black plastic mulch being used to control weeds on rows in a plot of Fragraria x ananassa.
Figure 2: Red Plastic Mulch
Red plastic mulch being used to control weeds on rows in a plot of Fragraria x ananassa.