Mowing: A Casually Thought of Integrated Weed Management Tool

Mowing defoliates plants and because leaves collect carbon dioxide and sunlight, defoliation alters their competitive ability. Mowing can reduce weed vigor, growth, survival and reduce or prevent seed production. Mowing kills existing shoot growth. However, mowed plants can produce additional shoot material and there is also potential of new stem development from previously dormant lateral buds. Still, this may be desirable as new stems grow at the expense of below-ground stored food. As such, repeated cutting hastens food depletion and death of some plants. Under frequent mowing, a plant must generate enough photosynthates under limited leaf area to fuel normal plant function while not depleting carbohydrates stored within the roots. Mowing can also delay flowering of some weed species and impact weeds indirectly by changing their environment. For example, light, temperature and soil moisture among other abiotic factors may change in a mowed plant community. This can occur because mowing creates vegetative gaps by removing plant parts that form a canopy and shades the soil surface. As a result, light intensity and quality changes at the soil surface. Further, the average daily soil temperature and diurnal temperature range at the soil surface increases. These abiotic changes on the ground can favor the germination or emergence of one or more weed species that would have otherwise remained dormant or suppressed. Plant residue that remains after mowing may also change abiotic conditions at the soil surface and subsequently influence the weed community. Mowing also changes the competitive relationships between neighboring weeds and other plants because different plants are impacted varyingly by mowing, some may die or regrow at different rates. As such, mowing can change the flora in an area. Thus, understanding how mowing impacts the biology of different weeds is important as it can be used to manipulate a plant community so that it favors native or other desirable plants.

Properly timed mowing can suppress unwanted vegetation while favoring desired plant flora. Integrated weed management should target the susceptible stages in a weed’s life cycle and if mowing is being conducted to prevent seed production, it should always be done prior to flower formation. Mowing weeds during this stage can weaken them as they have invested a lot of energy into producing reproductive structures. Mowing to limit weed seed production is usually initiated well after mowing designated to minimized weed-crop competition and yield reductions. Oftentimes, a single mowing will not prevent seed production. New stems below the initial cut can flower and produce seeds. Thus, two or more mowing may be required to inhibit seed formation. However, some weeds such as common ragweed are able to survive repeated mowing. As a general rule of thumb, if only one mowing per growing season is allowed, it should be timed to match weed flowering. Certainly, mowing can be challenging when several weeds with different flowering phenologies co-infest the same site. For instance, mowing timed to prevent viable seed production of one weed species may fortuitously be timed to spread viable seeds of other species. This suggests that tradeoffs may persist when multiple weed species are present in a habitat and mowing is the option. Further, developing seed heads should be mowed before viable seeds are formed. Thus, timing is critical and should precede anthesis, pollination and fertilization. Some viable seeds can form less than 7 days after anthesis. For example, mowing musk thistle within 2 days after anthesis prevented viable seed production. However, mowing was ineffectual if it was conducted 4 or more days following anthesis. The production of viable seeds can occur so instantaneously that if weeds are not mowed before flowering, the benefits may only be cosmetic, especially if the delay results in more weed seeds being deposited into the soil seed bank. Still, variation exists among weed species relative to the best phenological stage to mow. For example, a six-year study in MD found that mowing plumeless thistle (Carduus acanthoides; Fig. 2) at the full bloom stage reduced plant densities compared to mowing it at the full bud or post bloom stage. In the same study, musk thistle (Carduus nutans; Fig. 3) declined only when mowed after the bloom stage. Some weeds have the ability to compensate for mowing effects. They may deploy several strategies to survive mowing such as increasing their photosynthetic rate and tillering, and obtaining greater nutrients.

Properly timed mowing helps minimize weed reinfestation, population increase and seed dispersal of new weed species within fields or from field borders into neighboring crop fields. If mowing is poorly timed, it can spread viable weed seed including herbicide-resistant weeds beyond the current field of infestation. Weed seed dispersal by mowing has been reported to be greatest when mowing and seed set coincide. A study showed that early mowing of chervil (Chaerophylum aureum L.), a weed that infests pasture, reduced seed production by decreasing shoot density and seed set. However, mowing it later resulted in seeds spreading outside the study site. Relative to this, some studies have shown that mowing practices can enhance seed dispersal of some weed species especially those that are favored by disturbances. Mowing may spread weed seeds by blowing them from the mowed area or transporting them on different mower parts. Thus, cleaning mowers between sites may help prevent weed spread. The regularity of mowing weeds is partially contingent on their tolerance to mowing which is a function of their growth rate, foliage replacement ability and its potential to increase photosynthesis to compensate for leaf loss following mowing. Further, multiple mowing will be required to mitigate seed production if weeds being targeted set seed or emerge in flushes over an extended time period. However, for weeds such as common ragweed, time of mowing and stage of growth may be more important than mowing frequency. Common ragweed can tenaciously regrow after most of its above ground tissues has been removed (Fig. 4). A study demonstrated that de-spite a substantial loss in aboveground plant tissue, surviving ragweed plants were able to reach the flowering stage after four clippings during the growing season. Similarly, spotted knotweed (Centaurea stoebe) produced tillers and flowers following 3 clippings in a single summer.

Mowing height is also critical as the blades must be low enough to cut off developing seed heads. However, if plants are initially mowed too low, later forming seeds may develop so close to the ground that a second mowing misses them. In addition, conditions may favor the regrowth of mowed weeds. Some annuals such as horseweed will sprout new stems below the cut. This growth may be managed by cutting high at the initial mowing and markedly lower at the next mowing so as to cut off any stems that have sprouted. This strategy is most effective if by the second mowing, the stem is hard and woody, and incapable of developing new sprouts beneath the cut.

Perennial weed control can be costly as well as time consuming. Perennial weeds are typically managed with herbicides and/or aggressive tillage. However, being dependent on herbicides to suppress perennial weeds encourages the development and spread of herbicide-resistant weeds; and an intensive tillage program increases the risk of soil erosion and on-farm energy use. A primary reason that perennial weeds are so resilient is their ability to store reserves in their underground storage organs. This allows above ground regrowth to occur after disturbances. Additionally, the resources in these storage organs are passed on year after year. As such, control measures should target and destroy the underground storage network or disrupt them by reducing or eliminating their ability to translocate resources formed during photosynthesis to other plant parts. Tillage can be used to weaken and destroy the underground storage organ. If it weakens it, the plant becomes more susceptible to other management tactics such as growing a competitive crop and mowing. As such, management tactics that are timed to deplete the food reserves of perennials are most likely to prevent regrowth and spread. For example, the lowest root carbohydrate reserves in Canada thistle occurs just before flowering, when the plant is in the “bud to bloom” stage. Repeated mowing coupled with a competitive crop can deplete carbohydrates reserves from Canada thistle roots and frequent mowing can kill young shoots before they replenish their reserves.

In addition to preventing seed production of perennial weeds, repeated mowing may starve their underground parts. Cutting the leaves and other above ground plant parts reduces bio-mass accumulation and eliminates the food producing organs as photosynthesis occurs within the leaves, although in some instances, photosynthesis occurs within the plant’s stem. Regrowth that occurs following a cutting drains sustenance from the stored food supply of the weed. As such, repeated mowing can reduce reserves housed in the storage organs. Still it is important to note that using this repeated mowing protocol may not result in the quick death of a perennial weed patch. Effective mowing of large infestations may be a long-term commitment. It may require two or more years of repeated mowing to fully kill a perennial weed stand. The best time to initiate mowing is generally when the underground root reserves of weeds are at a reduced level. This generally occurs when weeds are between full leaf development and flower occurrence. Interestingly enough, these tips related to mowing time can be applied to the timing of herbicide applications for managing perennial weeds as these weeds may also be most vulnerable to herbicides during this stage of their development.

In perennial crops such as forages, weeds and crops are mowed concurrently. The aim is usually to suppress weed competition and seed production while harvesting and managing crop biomass and maintaining pleasing aesthetics. In non-cropland habitats such as field borders typically perennial vegetation is mowed to maintain ground cover and prevent erosion. When crops are planted in rows such as orchards and fruit tree systems, weeds and other vegetation between crop rows are mowed to limit competition with the crop. However, it is important to maintain a stand of “beneficial” vegetation in the inter-row areas as this helps prevent erosion.

Crops are sometimes planted without tillage into terminated cover crops. Relative to weed control, the main purpose of the “leftover” cover crop residue or mulch is to suppress weeds within the crop row. To this point, weed control in the between row area may be inadequate in a mulch system especially if the crop is not planted at narrow row spacing and/or the biomass of the residue is inadequate. This is more of a problem in the between row area because the crop’s canopy may not contribute to shading the soil in this area. This suggests that overall weed suppression in a mulch system may be more satisfactory if weeds in the between row area can be subjected to an additional management tool. Fortunately, several implements such as high-residue cultivators exist and can be readily used in established crop fields with cover crop residue without jamming (Fig. 5). Mowing can also contribute to weed suppression in a mulch system. Relative to this, a study examining cultivation as a weed suppression tool in a herbicide-killed rye mulch system found that two inter-row cultivations provided adequate weed suppression in dry beans when field margins were mowed to prevent seed production by dandelion, Taraxacum officinale.

Mowing typically contributes to short term weed suppression. As such, it may only be effective in a fast-growing crop particularly one in which a cover crop mulch has restricted weed establishment. Notwithstanding, in some situations, mowing can reduce the effectiveness of a mulch by speeding up its decomposition. Mowing clips cover crops into smaller pieces which break down more rapidly than thick mats. Differences in mowed residue fragment size and de-composition rate influence the duration of residual weed control from cover crop residue. For example, mowing residue (e.g., barley, crimson clover, hairy vetch, rye, subterranean clover) with a sicklebar at or after the mid- to late-bloom stages suppressed yellow foxtail, common lambsquarters, and redroot pigweed better than flail mowing (Fig. 6). It is believed that this occurred because flail mowing left smaller fragments of residue on the soil surface. Though mowed cover crop residue can provide some weed suppression, the duration and level of weed control by mowed residue is inconsistent and often will not provide good suppression the entire cropping cycle.

Mowing may also be done in concert with a living mulch (e.g., cover crop that lives the entire duration of the crop life cycle). A study found that mowing buckwheat living mulch between tomato rows after the critical weed control period suppressed weed seed production. The critical period for weed control is the period in the crop growth cycle during which weeds must be controlled to prevent unacceptable yield losses.

A field experiment was conducted in Illinois to study the combined effect of mowing and growing a summer annual cover crop on Canada thistle growth. The study showed that a sudan-grass or sudangrass-cowpea mixture alone or combined with mowing suppressed Canada thistle shoot density and mass to less than 20% of initial shoot and mass compared to buck-wheat or fallow treatments. However, intensive management must be continued for several years to eliminate patches.

Several investigations have been conducted to examine impacts of combining mowing with other weed management tactics. Relative to this, a between row mowing tactic in combination with herbicide applications and crop canopy shading was evaluated in soybean and field corn. The investigation showed that if properly timed, mowing weeds located in the between row area close to the surface < 3.8 cm (1.5 inch) two or more times can kill or suppress annual grass and broadleaf weeds, such as giant foxtail, common ragweed and waterhemp species. During the investigation, weeds within the soybean rows were managed with herbicides and those that “escaped” treatment were suppressed by early crop shading and competition. It was suggested that this management strategy which consists of a) planting a competitive crop, b) banding herbicide(s) over the crop row and c) mowing weeds between crop rows close to the soil surface before crop canopy closure can be successfully used in competitive crops such as corn, soybean and grain sorghum which closes their canopy and shade weeds early in their cropping cycle. Thus, the implementation of husbandry practices that enhances crop competitiveness with weeds is critical to the success of this management plan.

Unfortunately, between row mowing is not adapt-able to most cropping systems especially those grown in very narrow row spacing (e.g., drilled grain) and are weakly competitive and lack the ability to form a closed canopy. In addition, many producers will lack the equipment needed for mowing between crop rows. However, if it can be used in combination with herbicides, it is expected that the amount sprayed can be reduced by 50 percent or more as sprays are only banded within the crop row. Mowing has also been investigated for its ability to supplement herbicides and cultivation in peanuts. After weeds had grown 20 cm (7.9 inch) above the peanut canopy, a tractor pulled rotary mower was used to cut off seed heads just above the canopy 8 and 13 weeks after planting. The mowing prevented peanut shading by bristly starbur, sicklepod and Florida beggarweed. Mowing and applying herbicides in the within row areas of citrus trees are the most widely used weed control practices in Brazil. However, a three-year study in Brazil indicated using ruzi grass, Urochloa ruziziensis as a cover crop, combined with within row glyphosate and an ecological mower is a more sustainable IWM option for citrus trees. An ecological mower cuts the cover crop in the between row areas and the resulting residue is launched within the crop rows. Eco-mowing, which involves placing the cuttings from cover crops under the canopies of citrus trees rather than leaving them in the middle rows, is also being researched in Florida.

A study was conducted to test whether integrating early season mowing with a systemic herbicide application would improve the control of perennial pepperweed (Lepidium latifolium, Fig. 7). Mowing alone did not reduce weed biomass or its density the following year. However, mowing fol-lowed by application of an herbicide to re-sprouting plants reduced biomass in three different environments (high desert, roadside and floodplain habitats). The combination of mowing and the herbicide, chlorsulfuron reduced pepper-weed biomass > 99% at all three sites and glyphosate + mowing > 80% at two sites, one year after application. It was noted that an initial mowing increased the effectiveness of glyphosate to a level where it became an effective control option.

Mowing may be repeated over a longer period than some herbicide treatments or cultivation. Further, mowing can be used to suppress weeds that have become too large to be managed with herbicides or cultivation; and mowing has fewer off site environmental effects. Thus, it can be an option where ground cover is desired and herbicide use would be restricted or undesirable. Mowing may also be used in circumstances where concerns exist regarding herbicide contamination of water bodies. Mowing can also be used during situations where weather conditions such as wind speed causes herbicide drift or reduces its efficacy. To this point, mowing is advantageous in highly populated or suburban housing areas where the public is concerned about herbicide exposure. Mowing can be used as a substitute in field conditions where cultivation might damage root systems or lead to soil erosion. Mowing limits erosions caused by wind and water by allowing live vegetation and plant residue to remain on the soil surface. Moreover, mowing is compatible with other soil and plant conservation measures such as no-till practices and land conservation programs. Fields can also be mowed faster following a heavy rainfall event compared to cultivation which requires much drier soil conditions.

Weed species vary in their response to mowing height and frequency and some readily accommodate mowing. “Weeds adapted to mowing tend to grow short, in a rosette form, creeping above the soil surface or show high plasticity and softness of aerial parts and stems and be-come difficult to mow and also escape hand weeding.” Thus, if multiple weed species of varying height persist in a habitat, mowing may be-come more of a challenge. Mowing can favor weeds that develop and reproduce below the mowing height and repeated mowing of similar weeds can cause a shift: 1) in biotype from an upright growing form to a more prostrate form, and 2) to a community of weeds that are tolerant to mowing. For example, mowing was correlated with differences in plant size and degree of erectness caused by genetic differences be-tween mowed and un-mowed broadleaf plantain (Fig. 8). This suggests that an integrated or more holistic approach should always be the goal of any weed management program as weeds will adapt to a single management approach. Further, though mowing may reduce aboveground competition, if it fails to kill weeds, they may still compete with crop plants for resources such as space, nutrients and water below the soil. Moreover, weeds that form rosettes or mats and/or grow close to the ground are naturally adapted to mowing. This is why weeds such as dandelion, bermudagrass, crabgrass, goosegrass and buckhorn plantain, once established, are immune to mowing. Additionally, wheel traffic that occurs during mowing can compact some soils such as silty clay loam. Mowing can also be noisy and though vegetation remains on the surface, it may still raise dust.

Mowing is a relatively inexpensive form of mechanical weed control that can reduce the use of tillage, herbicide and manual weeding. It may serve as an alternative to herbicide and cultivation or part of an integrated approach. However, mowing to manage weeds has not been well studied compared to other IWM tools and is more popular in habitats with perennial stands of vegetation. Consequently, limited information is available on mowing use in crops. As such, it is not adaptable to numerous cropping systems; and partially for this reason, it is used mainly for aesthetic reasons and preventing seed production in perennial stands of vegetation neighboring cropland. Still, research has shown that mowing can be used jointly with other weed management tools such as applying herbicides, cover cropping and growing competitive crops. Mowing may also be used to successfully manage perennial weeds by removing the above-ground plant parts and consequently reducing food reserves in their storage organs. This, however, may take multiple years and the integration of other weed management tactics. Some research has found that combining mowing with herbicides enhances perennial weed control. Still, there are advantages and disadvantages of using mowing as a weed management tool. Mowing generally does not have any negative environmental effects. However, many weeds especially those that grow close to the ground such as buckhorn plantain are naturally tolerant of mowing (Fig. 9). As with any IWM program, it is important to “keep weeds guessing” by utilizing different management tactics; and mowing is no exception to this rule. For example, repeated use of mowing as a single weed management tactic may result in a selection pressure or shift to weed species or genotypes that can reproduce even if repeatedly mowed. These species may overtime become more difficult to manage. As such, in those situations where mowing is practical, one should consider making it part of an overall IWM program.

Financial support for the publication of this article is via USDA NIFA EIPM grant award numbers 2021-70006-35384 and NESARE - Research for Novel Approaches (LNE20-406R).