Growers agree that weeds must be controlled to maintain a productive vineyard; however, the success achieved varies considerably. Whether you are establishing a new vineyard or attempting to reduce weed problems in one that already exists, choices and actions made along the way will determine the outcome. Weed control fails most often because growers do not understand the biology of weeds nor the tools they are using to gain control.
Weeds can cause tremendous problems in the vineyard. Besides competing with the grapevine for nutrients, light, and moisture, weeds provide shelter and food for other pest problems. Dense weed growth impairs air circulation in the vineyard, creating an environment where grape diseases can flourish. Weed growth may interfere with fungicide and insecticide applications.
Because each weed species may require a slightly different or a vastly different control method, the successful grower must be able to identify weeds. Once the weeds in the vineyard have been identified, a control program that considers biology, tools, economics, and the environment can be planned and implemented. Therefore, weed identification is the first skill set that should be learned.
To get started with weed identification, obtain an identification manual, preferably one with good photographs and line drawings of weeds at both seedling and mature stages. One book we have found particularly useful is Weeds of the Northeast. Once you are ready, remember that separating different weeds into sensible categories will help you identify one from the other and also help decide on the appropriate control. Categories most useful are grasses vs. broadleafs and annuals vs. perennials (grasses and broadleaf weeds may be either annual or perennial).
Most weeds can be categorized as annuals or perennials. Annual weeds complete their life cycle, from emergence to setting seed and dying, in less than 12 months (Figure 100). Summer annuals (common lambsquarters, pigweed, green foxtail, for example) emerge in the spring or early summer and complete the cycle by fall. Winter annuals (such as shepherd’s purse, red dead nettle, annual bluegrass) may germinate anywhere from mid-summer until freeze-up (depending on the species), overwinter as a rosette, and resume growth in spring, typically setting seed and dying by May or June. With annuals, destroying the top growth usually kills the entire plant and prevents further seed production.
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| FIGURE 100. Annual weeds complete their life cycle in less than 12 months./td> |
Perennials (e.g., quackgrass, Canada thistle, poison ivy) may live for several years. Most perennials (Figure 101) survive the winter as vegetative propagules, including below-ground rhizomes (e.g., quackgrass), tubers (e.g., Jerusalem artichoke), budding rootstocks (e.g., Canada thistle), budding taproots (e.g., dandelion), or above-ground stolons (e.g., ground ivy). Although most produce seed, new growth in spring usually is from the under- or above-ground vegetative propagules. Perennials are more difficult to control than annuals, because the propagules are protected by soil, foliage, and debris.
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| FIGURE 101. Most perennials survive the winter as vegetative propagules. L. Canada thistle; R. Quackgrass. | ||
Annual weeds are best controlled around the time of seedling germination and emergence. Perennial weeds, other than those emerging from seed, are best controlled either after a few weeks of growth in the spring, or after fall rains when growth has resumed. Some perennials are most sensitive to herbicides at or just before blooming. These critical differences in strategy for annuals and perennials determine the success or failure of control and can only be fully appreciated by understanding the biology of weeds.
Annuals are most sensitive at, or shortly after, germination because food reserves in the seed have been depleted, and the young seedling is not yet producing its own food. Also, at emergence, seedlings are most sensitive to the damage caused by herbicides or by cultivation, and the desiccating effects of wind and sun.
Beyond the 2- to 4-leaf stage, annual weeds become much more difficult to control; leaves develop cuticles that resist herbicide absorption, and root systems penetrate deeply into the soil where they are protected from herbicides applied to the soil. Because annuals persist only by producing seeds, anything you do to prevent seed production will reduce future costs of weed control.
In contrast to annual weeds, killing young foliage and shoots of perennials in early spring has little effect on the vegetative propagules. Cultivation or application of herbicides at this stage of growth merely causes the propagules to resprout, and the problem quickly returns. Wait until the correct growth stage of each species before applying systemic herbicides.
When a perennial reaches the correct growth stage for herbicide application, food reserves in the rhizome are depleted, photosynthesis is in full-swing, and the plant is starting to rebuild root reserves by translocating sugars downward into the rhizome or root-system. Systemic herbicides applied at this stage translocate, along with sugars, from the foliage to the vegetative propagules, and complete control can usually be achieved (Figure 102).
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| FIGURE 102. Systemic herbicides translocate downward to vegetative propagules. |
For most perennial grasses, the sensitive stage is reached after a few weeks of growth, typically when there are 6 to 8 inches or more of new growth. For broadleaf perennials such as Canada thistle and field bindweed, the sensitive stage is later, just before or at the time of flowering. Nearly all grass and broadleaf perennials can be controlled with fall-sprays of systemic herbicides, as the weeds are actively building root-reserves at that time.
Understanding how the tools of weed control work is as important as understanding the biology of weeds. Cultivation kills by breaking and shearing individual plants and exposing them to the drying effects of sun and wind. When annual weeds get beyond a few inches high, cultivation becomes more difficult and much less effective, because weeds can tolerate some drying and can recover from the physical damage.
Cultivation also stimulates germination of more weed seeds. Growers can use this behavior to their advantage by planning ahead. Cultivate the surface to stimulate seed germination and then kill young seedlings soon after emergence with more cultivation. The second cultivation will, in turn, cause more seed germination, and the cycle can be repeated.
The cycle of cultivation, weed emergence followed by more cultivation, is called summer fallow. The cycle can be repeated many times in a single spring or growing season and is a very effective strategy to reduce the number of weed seeds in the soil and thus the need for weeding after planting grapevines.
Herbicides kill weeds by interfering with various physiological processes occurring within their tissues. Typical symptoms of herbicide use are malformed growth, discolored or dead tissue, and stunting. Young seedlings are more sensitive to these effects than older plants.
Terms used when discussing herbicides are listed here and are useful in understanding the important differences between the various herbicide products.
Herbicides are either selective or non-selective. A selective herbicide affects some plants but not others. A non-selective herbicide will kill any plant it contacts.
Contact herbicides affect only the tissue sprayed. They do not move within the plant. Only above-ground tissues that are directly sprayed will be affected. Contact herbicides (e.g., Gramoxone) are useful for controlling small annuals and for temporarily reducing growth of perennials.
Systemic, or translocated, herbicides, are those that move within the plant. Systemic herbicides applied to the soil (e.g., Karmex) are absorbed by roots and translocated to leaves where they inhibit physiological processes such as photosynthesis. Systemic herbicides applied to the foliage (e.g., Roundup) translocate to the root system, provided the herbicide is applied at the correct growth stage.
A residual herbicide or persistent herbicide (e.g., Princep) is one applied to the soil (may also be applied to leaves) where it slowly breaks down over time but continues to control germinating weeds for some time after application. Some residual herbicides may persist long enough to damage crops planted a year or more after application. Residual herbicides may or may not be selective, and they may or may not be systemic.
The more you understand about the biology of the weeds you need to control and the tools available, the better decisions you will be able to make about control; however, an extensive review of these topics is beyond the scope of this publication. Fortunately, many excellent sources of detailed information on weed biology, herbicides, cultivation implements, and other control tools are available on the Internet and in local libraries.
Now that you have mastered some basic concepts of weed biology and control techniques, you should plan a weed-control program. A weed-control program is simply a pre-planned schedule of activities based on the weeds you have identified, the age of your vineyard, and economic and environmental considerations.
Developing a new vineyard, nurturing one that is newly planted, or cleaning up an established weed-infested site all require different approaches. The best time to implement a weed-control program is before planting. This is when perennial weeds can be eradicated. Once a vineyard is established, perennials are much more difficult to control. While specific perennial weeds may require specific controls, the guidelines presented here will apply in most instances.
The practices recommended prior to planting are referred to as site preparation. Start site preparation the year before planting. Land that is in sod should be treated with a non-selective systemic herbicide after new growth reaches a height of about 8 inches. Plow and disk within one to two weeks of herbicide application, even if control is not obvious.
Systemic herbicides work slowly, and control symptoms take time to develop. Delaying tillage until after symptoms are obvious can result in reduced control. After tillage is a good time to take soil samples for nutrient and pH analysis (see Vineyard Establishment). Practice summer fallow until late May by allowing weeds to regrow until they are a maximum of 1-inch high and then cultivate.
Land that has been in annual crops, such as corn and soybeans, may be relatively free of perennials. Such land should be tilled in early spring, sufficiently to stimulate weed emergence, and then summer fallowed as recommended for sod ground. If perennials are obvious prior to tillage, then treat the field as if it is in sod. In June plant a cover crop of oats, buckwheat, or annual ryegrass to build organic matter and suppress weed growth.
Oats and ryegrass can be treated with a selective, systemic broadleaf weed herbicide to kill germinating annuals (see Weed Control Guide for Ohio Field Crops, Ohio State University Extension Bulletin 789). Plow the cover crop before it goes to seed. If significant annual weeds germinate in late summer, after plowing-under, consider additional summer fallow. Do not let late summer weeds go to seed.
When fall rains resume, surviving vegetative propagules of perennials will sprout and emerge. Late September through October is a good time to apply a final treatment of a non-selective systemic herbicide to kill remnants of perennial weeds. Apply the herbicide to actively growing weeds when the foliage is 6- to 8-inches high. Annuals co-emerging will also be killed. The site can be re-seeded in as little as four days after the last systemic herbicide application.
Cultivate (only if needed to prepare an adequate seedbed) and plant a cover crop of companion grass (see the section on Soil Management) if a permanent sod is required for the row middles. Otherwise, plant fall rye or winter wheat (rye can be established throughout November, depending on latitude and prevailing weather) if bare row-middles are needed. Remember too that winter wheat can be underseeded with a permanent sod species that will persist after the wheat dies. Cover in the row areas can be killed in the spring by spraying new growth (wait until it is 6- to 8-inches high) with a systemic herbicide.
Scout the vineyard for emerging weeds at the same time as scouting for insects and diseases, and map their location in the vineyard at the individual species level. Record the distribution of each species as:
GENERAL = found throughout the field.
LOCAL = found in a small portion of the field.
SPOTTY = found in just a few places.
Also record the density of each species as either:
1 = SCATTERED, just a few weeds.
2 = SLIGHT, 1 weed per 6 feet of row.
3 = MODERATE, 1 weed per 3 feet of row.
4 = SEVERE = more than 1 weed per 3 feet of row.
Take samples of unknown weeds for later identification. Hopefully, perennial weeds were eradicated before planting as outlined previously. If not, it is not too late to start but don’t expect control to be as good as the outcome of a correctly implemented site preparation program.
Perennial grasses in the row can be treated whenever they occur with selective, systemic herbicides such as Select, Fusilade, or Poast. These herbicides will not affect broadleaf weeds or grapevines, even if directly sprayed. Grass control will take time, but with persistence, perennial grasses will be largely eliminated.
In the meantime, mow the sod between grape rows to discourage growth of broadleaf weeds and to prevent any seed production. Install grow tubes on young vines. With grow tubes correctly installed, i.e., with the bottom of the tube buried about 1 inch in the soil, non-selective herbicides can be used under the trellis.
Contact herbicides such as Gramoxone and Rely (glufosinate) will control annuals up to 4-inches high. Systemic herbicides (Roundup) will control annuals and perennials. For persistent broadleaf perennials, such as Canada thistle, wait until the bud stage is reached and then spray with a systemic herbicide.
Perennials requiring herbicide treatment must not be allowed to grow higher than the grow tube or sprays cannot be used. Tall weeds can be gently bent to the ground and then thoroughly sprayed with a systemic, or alternatively, wiped with a topical application of a systemic herbicide. Weeds under the trellis should be controlled into late fall until winter-hilling takes place. Grow tubes should be removed prior to winter.
Remember that annual weeds under the trellis can be controlled with cultivation. However, trellis wires and location of vines and posts often limit the type of cultivation equipment that can be used. Nevertheless, several implements, generally operated hydraulically, have been used to work the area under the trellis. An example is the grape hoe.
Hand hoeing is sufficient for smaller plantings. Invariably, weeds are controlled only by repeated cultivations which can be costly and laborious. Missing just one timely cultivation may result in weed seeds being returned to the soil. With cultivation there is a significant risk of injuring trunks and root systems of vines.
Several soil-applied residual herbicides including Devrinol, Surflan, and Prowl can be used in the transplant year to prevent annual weeds. Herbicides typically lead to more vigorous vine growth and higher yields because they do not injure the crop’s root system. Plastic and organic mulches can effectively control weeds. Their use is described in the section Vineyard Establishment.
Additional herbicides are registered for use in older vineyards. In late March through April, apply a residual herbicide such as Karmex or Princep (three-year-old vines with these herbicides) to the soil under the trellis. Most residual herbicides only kill germinating weeds, thus they must be applied early before weed emergence.
Contemplate applying two or more micro-rate applications rather than a single application at the maximum rate. Micro-rates are simply a fraction of the maximum one-time application rate, applied to coincide with flushes of weed emergence whenever that occurs, provided the total used is no more than the annual maximum application rate. For instance, if the single maximum application rate is 3 lbs per acre, three micro-rate applications of 1 lb per acre could be applied.
The advantage of the micro-rate approach is generally better weed control with less herbicide. The disadvantages include a requirement for more careful management, more time and labor, and potentially more soil compaction because of repeated trips across the vineyard.
Watch for escapes (escapes are weeds in the treated area that have been missed or have survived the treatment) when scouting for insects and diseases. Typical escapes following application of a residual herbicide such as Karmex or Princep are old witchgrass and large crabgrass. Annual grasses tolerate these herbicides, and their populations quickly build up if the same residuals are used year after year. This buildup of tolerant species is known as a weed shift. To prevent weed shifts, do not depend on just one herbicide or on herbicides that work the same way (e.g., herbicides that inhibit photosynthesis) every year. Instead, rotate herbicides with different modes of action and selectivity on a yearly basis. When grasses occur in the vineyard, they can be treated with a systemic herbicide such as Poast. Poast and similar herbicides can be used several times during the growing season on an as-needed basis.
Be vigilant for the return of perennials and other invasive weeds and eradicate them, preventing seed production, whenever they occur. Invasion of the vineyard is an on-going problem as weed seeds enter in soil, wind, and water and on machinery and animals, etc. In mid- to late summer, an application of a residual soil-active herbicide will prevent establishment of winter annual weeds, such as shepherd’s purse and groundsel. The exception to this guideline would be vinifera, when there is an intention to establish a winter cover crop under the trellis after hilling.
In the recommendations presented in this section, application rates are provided for two of the most common formulations of glyphosate. The use of these brand names does not imply an endorsement of these over other glyphosate products, nor a criticism of those products that are not named.
For long-term control, use Roundup Ultra at 1 to 2 quarts per acre or Touchdown at 3.33 pints per acre. Use the 1-quart-per-acre rate of Roundup Ultra in 5 to 10 gallons of water per acre on land that has been in row crops. The 2-quart-per-acre rate will provide longer lasting control when spraying sod. Spray when the grass is about 8-inches high and wait at least three full days (72 hours) but not more than seven days before plowing.
Fall frosts before spraying will not affect control provided at least 60% of the foliage is still green when you spray. If planning a spring application, do not fall plow; simply wait until quackgrass reaches the right growth stage (four to five new leaves) and spray.
Apply Roundup Ultra at 1 quart per acre plus ammonium sulfate (17 pounds per 100 gallons of spray) in a spray volume of 5 to 10 gallons per acre, when Johnsongrass is in the boot to heading stage. In the fall, application can be made anytime prior to frost.
If using Touchdown, apply 2-1/3 pints per acre plus ammonium sulfate (17 pounds per 100 gallons of spray) and apply in 5 to 10 gallons per acre at the same stage of growth as for Roundup Ultra.
Yellow nutsedge is difficult to control. Nutsedge persists by producing nutlets which grow at the end of rhizomes. Nutlets break dormancy in spring, and emergence follows in late spring and early summer. Control with herbicides is rarely or never complete, because insufficient herbicide translocates into the nutlets.
Attack nutsedge one or two years before planting using an integrated approach. Apply preplant glyphosate on small nutsedge plants (control with Roundup Ultra is best when nutsedge is 6- to 12-inches high; earlier applications will provide some suppression—Monsanto Research) and/or tillage before planting.
If you do not already have nutsedge in a field, prevent its introduction. Wash all soil from recently purchased used equipment before allowing it on your farm. If you have some infested fields and others that are not, or if your equipment is used on fields of other farmers, be sure to wash all soil off the equipment before using it on land that is nutsedge-free. Make sure all transplant materials were produced under nutsedge-free conditions.
Canada thistle can be treated in the flower-bud to flowering stage in early summer or in late summer and fall during the rosette to flower-bud stage. In fallow fields, stop tillage in late July and allow thistles to regrow for at least five weeks.
Apply Roundup Ultra or Touchdown before a killing frost and when Canada thistle regrowth reaches the flower-bud stage or is at least 10- to 12-inches high. Apply Roundup Ultra at 2 to 3 quarts per acre in 5 to 10 gallons of water or Touchdown at 2 quarts per acre. Spot sprays of a 2% solution (0.5 pints in 6 gallons of water) of either herbicide will also be effective.
Field bindweed must be treated when it is actively growing and at or beyond bloom. Fall treatment is best, but apply herbicides before a killing frost. Apply Roundup Ultra at 3 to 4 quarts per acre or Touchdown at 5.33 pints per acre. Spot spray with a 2% solution of either product.
Hard water with more than 500 parts per million of calcium or magnesium will usually reduce glyphosate activity. If hard water must be used, keep the volume low (5 gallons per acre) or increase the rate of herbicide. Use clean water. Silt, clay, and organic debris in water will also reduce glyphosate activity. Always add ammonium sulfate (17 lb/100 gal water) to glyphosate for maximum effectiveness on perennial weeds.
| Table 16. Herbicides for Perennial Weed Control the Year Before Planting and for Spot Treatment. | ||
| Prevalent Weeds | Timing of Treatmenta | Herbicide/Ab |
|---|---|---|
| Canada Thistle | Late spring/early summer; early/late fall | Roundupc 1 qt. or 2% spot treatment |
| Field Bindweed | When plants are at or past full bloom/before killing frost | Roundup 2 qt + 2,4-D 1 pt or Banvel 8 oz + Roundup 1 qt |
| Horsenettle | Late bud to flowering stage | Roundup 4 qt or 2,4-D 2 qt |
| Dogbane | Late bud to flowering stage | Roundup 1 qt + 2,4-D 1 pt orRoundup 1 qt + Banvel 2 pt |
| Poison Ivy | 7/1 to 9/15 | Banvel 1 qt + Roundup 2 qt or Crossbow 2 qt |
| Quackgrass | Spring – 8” tall to heading orFall 8” tall regrowth | Roundup 2 qt |
| Wild Brambles | Bud to bloom stage | Banvel 1 qt + Roundup 1 qt or Crossbow 6 qt (1-1.5% solution) |
| Swamp Smartweed | 7/1 to 9/15 | Banvel 8 oz + Roundup 1 qt |
| a Apply at least three weeks before an anticipated frost (exceptions: C. thistle and quackgrass). | ||
| b Adding a surfactant to these herbicides will improve their effectiveness; Roundup already contains a surfactant. Rates are given in amounts of commercial product per acre. | ||
| c Apply Roundup with Banvel or 2,4-D where several weeds are present. Roundup alone is best applied on tall weeds, at 5 to 10 gpa with surfactant and ammonium sulfate. Follow label recommendations. Do not apply Banvel, 2,4-D, or Crossbow near grapes. Avoid drift. Apply spot treatments using low pressure or a wick applicator. | ||
Herbicide injury symptoms on occasional plants in the vineyard are not uncommon. However, yield and quality are rarely an issue unless injury is widespread and persistent. Sporadic injury in the vineyard can most often be traced to directed sprays of systemic herbicides contacting foliage or green bark, using too high a rate of residual herbicides on light soils, incorrectly calibrated sprayers, sensitive cultivars, and weak plants growing under unfavorable conditions. The grower has control over many of these factors.
Any factor that injures the crop—other pests, winter injury, exposure of crowns and root systems as a result of erosion, improper mineral nutrition, wet spots in the eld, etc.—will make the crop more susceptible to injury. Light, sandy soils require less herbicide than heavier soils for comparable levels of weed control. Grapevines growing on soils low in organic matter are more prone to herbicide injury from soil-applied herbicides than vines growing on soils high in organic matter. Accordingly, lower rates of herbicide should be used on elds low in organic matter (less than 2%).
Of much greater signicance to vineyards in the Corn Belt are risks associated with herbicide spray drift from nearby small grain, corn, and soybean elds. Spray drift that occurs while nearby fields are sprayed can severely damage grapevines. Grapes are extremely sensitive to 2,4-D and closely related growth-regulator herbicides used for weed control on pastures, wheat, oats, barley, and rye. Grapes may also be severely damaged by drift of glyphosate (Roundup and generics), currently the most commonly used herbicide on soybean. Grapes are sensitive to drift of many additional field crop herbicides including Atrazine, Sencor, and ALS-inhibitor herbicides such as Accent and Firstrate. Injury symptoms caused by some of these herbicides may easily be confused with disease and insect damage.
Symptoms of 2,4-D injury (Figure 103) to grapevines are easily distinguished. The youngest terminal growth is most severely affected. Vines and leaf petioles are severely twisted. Leaf blades may be stunted and misshapen with closely packed, thick veins. Terminal growth may cease for a time following the initial effects; if injury is severe, growth will be retarded for several weeks. Vines with these symptoms rarely produce new normal growth for the remainder of the season. If the injury is not too severe, normal growth will resume the following year. Severely injured vines may die or not recover for two years or more. Vines injured by 2,4-D also may have delayed fruit ripening. If vines are severely injured, fruits may never mature, regardless of the length of season. These delayed-maturity effects may persist for one to three years before normal ripening returns. Cultivars vary in their tolerance of 2,4-D. Concord is one of the most susceptible, while some vinifera cultivars are fairly tolerant to 2,4-D.
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| FIGURE 103. Symptoms of 2,4-D drift on Concord grape (L and R). | ||
Glyphosate injury (Figure 104) is similar to injury from growth-regulator herbicides. Glyphosate injury shows up rst on the newest growth, usually about two weeks after contact. New leaves are chlorotic, stunted, and misshapen. In particular, affected leaves may be elongated, and somewhat strap-shaped with a puckered surface. Terminal growth may die and slough off. In severe instances, vines may be killed or growth severely stunted. Recovery from severe injury may be slow, and symptoms can be expected to slowly decline over two or more years.
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| FIGURE 104. Symptoms of glyphosate injury (L and R). | ||
Atrazine and Sencor are photosynthesis inhibitors similar to Princep and Karmex, herbicides that are used on grapes. Symptoms are similar to that caused by Princep and Karmex—leaf chlorosis that may be a halo around the margin, interveinal, or veinal (Figures 105 and 106). In severe examples, leaves turn brown and drop off. Symptoms caused by drift are unlikely to persist beyond the year the incident occurred.
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| FFIGURE 105. Symptoms of Karmex injury. | FIGURE 106. Symptoms of Princep injury. |
Systemic postemergence herbicides, such as Accent (Figure 107) and Firstrate (Figure 108), used on corn and soybeans, respectively, can also damage vines.
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| FIGURE 107. Symptoms of Accent injury. | FIGURE 108. Symptoms of Firstrate injury. |
Grape growers should discuss the problem of drift and the risks of herbicide injury to the vineyard with surrounding growers of field crops. Stress the importance of not using high-volatile herbicides and avoiding windy conditions. Because of their volatility, many 2,4-D compounds can injure vines at great distances from the point of their application. Effects of volatile formulations of 2,4-D (ester formulations are generally more volatile than amine formulations) and related compounds have been observed in vineyards at least ve miles from the point of application. If a 2,4-D herbicide must be applied near grapes, use the least volatile forms. If possible, do not use 2,4-D products in the immediate vicinity of grapes. Additional information on herbicide injury is available in a recent publication Herbicide Drift and Injury to Grapes, Southern Illinois University, Bulletin C1382.