Category: Weed Management

Russell R. Hahn, Section of Soil and Crop Sciences, Cornell University

All perennial weeds can be troublesome, however “deep-rooted”, creeping perennial broadleaf weeds such as field bindweed, hemp dogbane, horsenettle, and common milkweed are among the most difficult to control.  Like annual and biennial weeds, these perennials reproduce by forming seed.  In addition, they spread by rhizomes (underground stems).  Buds or growing points are found all along these underground stems.  Effective control programs must control newly germinated seedlings and minimize the ability of these underground buds to produce new above ground shoots.  Between-cropping applications of translocated herbicides during late summer or early fall have proven more effective than other programs for control or suppression of these perennial broadleaf weeds.

Rhizomes are Key to Survival
Rhizomes are the key to the survival of these perennial broadleaf weeds since they serve as a storehouse for food reserves (carbohydrates).  It is these food reserves that allow these plants to survive winter.  In the spring these creeping perennials draw on these reserves to make new growth.  During this period of vegetative growth, carbohydrate movement is mainly upward in the plants.  The depletion of food reserves continues until the plants reach full leaf development and flower bud formation in mid- to late summer as shown in the accompanying figure.  At this time, these plants have the maximum leaf area and the lowest level of carbohydrate reserves that they will have all season.  After flowering, they start moving carbohydrates from the leaves into the rhizomes in preparation for winter.  Effective chemical control of established patches of these perennial weeds takes advantage of this food storage period to move translocated herbicides down to the underground buds or growing points.

Translocated Herbicides
Translocated herbicides are the key to chemical control of “deep-rooted” perennial broadleaf weeds.  Translocation refers to the movement of substances from one place to another, such as the movement of herbicides in plants.  Herbicide movement in plants may follow the pathway of sugars formed during photosynthesis and/or the pathway of water that us absorbed by plant roots.  Perennial weed control is most dependent on herbicide movement with the manufactured sugars.  These sugars move out of the leaves to areas of rapid growth (growing points).  Herbicide translocation to the growing points on the underground stems is most rapid and most effective when large amounts of sugars are being moved to the rhizomes.  This usually occurs after full bloom in late summer and fall.  Since 2,4-D, dicamba (Banvel, Clarity, etc.) and glyphosate (Roundup, etc.) are readily translocated from leaves into underground structures of perennial weeds, these herbicides can be effective in controlling or suppressing these weeds.

Between-Cropping Applications
Between-cropping herbicide applications are simply those that are made: 1) after harvesting one crop, 2) before killing frost, and 3) before planting the next crop.  Situations that meet these requirements include fields where small grains (not seeded to legumes) or certain vegetable crops (peas, early sweet corn, etc.) have recently been harvested, and where the next crop won’t be planted until fall (small grains) or until the next spring. These between-cropping situations provide the opportunity to use non-selective herbicides such as glyphosate or to use high rates of 2,4-D or dicamba that cannot be used safely when crops are present.  These herbicides should be applied when the weeds are actively growing.  It may be necessary to allow the weeds to recover from damage done during crop harvest. Herbicide labels should be consulted to determine application rates for the targeted perennial broadleaf weeds.  In all cases, tillage and other operations should be delayed for 7 or more days following application to allow time for herbicide translocation to the underground buds.

Rotational Crops
Glyphosate is inactivated upon contact with the soil so a variety of crops can be planted following the 7-day waiting period.  Since dicamba, the active ingredient in Banvel, Clarity, and numerous other products, has residual soil activity, rotational guidelines must be followed to avoid injuring subsequent crops.  Corn, soybeans, and all other crops grown in areas with 30 inches or more of annual rainfall may be planted 120 days after application of up to 4 pints/acre of dicamba products like Banvel and Clarity.  Small grains may be planted if the interval between dicamba application and planting is 20 to 30 days (depends on which product is applied) per 1 pint/acre east of the Mississippi River.  These waiting periods should exclude days when the ground is frozen.  The waiting period for planting winter wheat or barley following late summer dicamba applications can be shortened by applying reduced dicamba rates in tank mixes with glyphosate or 2,4-D.

Between-cropping applications of translocated herbicides provide the best opportunity to suppress or control “deep-rooted” perennial broadleaf weeds, however, growers must act now to take advantage of existing situations or to plan a rotation that will allow such applications next year.  Unfortunately, the typical dairy rotation of corn and perennial forages doesn’t provide good opportunities for these between-cropping herbicide applications.

Professor Antonio DiTommaso, Cornell Department of Crop and Soil Sciences, in cooperation with the Cornell Cooperative Extension Invasive Species Statewide Program, held a one-day hands-on workshop on weed seedling identification for PRISM partners. Scott Morris, Kathy Howard, and Courtney Stokes, Department of Crop and Soil Sciences, assisted with the workshop. The workshop was held on the Cornell campus on Thursday, May 8 from 9 am – 4 pm. Live seedlings, seeds, mature plants, and photos were used in this very interactive event that included some lecture and presentation, a before and after weed ID “quiz”, hands on seeds, seedlings, and mature plants in the Muenscher greenhouses to view, and a walk in the new weed garden. Weeds, or invasive species, included both agricultural and natural area weeds. General weed management approaches were discussed during the presentations.

Nineteen participants came from around NYS, representing 5 of the 8 PRISMs (Partnerships for Regional Invasive Species Management). PRISMs, established by the NYS Department of Environmental Conservation, coordinate invasive species management functions in the 8 PRISM regions throughout the state.

The CCE ISP is a statewide program funded by the NYS Environmental Protection Fund through the NYS Department of Environmental Conservation.  The program, established in 2010, provides assistance to the PRISMs, including education for PRISM members and their audiences, on-line resources, and connections to Cornell faculty and CCE Associations. The NY Invasive Species Clearinghouse (www.nyis.info ) is a partnering CCE and NY Sea Grant program that provides on-line resources. The NYS Invasive Species Research Institute, housed in the Cornell Department of Natural Resources, is the third partner in the CCE system supporting PRISMs with the most effective and up to date control of invasive species in NYS.

An invasive species is a species that is non-native to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm or harm to human health (National Invasive Species Management Plan – 2001 (http://www.invasivespecies.gov/main_nav/mn_NISC_ManagementPlan.html )).

 

Russel R. Hahn,
Department of Crop and Soil Sciences, Cornell University

Two thistles common to New York State are bull thistle (Cirsium vulgare) and Canada thistle (Cirsium arvense). Both were introduced from Eurasia and became naturalized in Canada and the United States. Although closely related and somewhat similar in appearance, theses thistles exhibit some differences in form and have very different life cycles. These life cycle differences play an important role in timing of control measures.

Bull Thistle
Bull thistle is a biennial weed that reproduces by seed only. All biennials require two growing seasons to complete their life cycle. In the first year, bull thistle germinates from seed and forms a rosette or basal cluster of leaves (see photo) with a large fleshy taproot. After overwintering in this stage, the plants complete their life cycle by forming a flowering stalk and setting seed during the second growing season. The stems of bull thistle may be 3 to 6 feet tall, are often branched, and are more or less hairy. The leaves are deeply cut, spiny, and run down the stem (Figure 1). Deep purple or rose flower heads are formed during the second growing season. These heads are 1 to 2 inches in diameter and are surrounded by numerous spiny tipped bracts. Bull thistle is found in pastures, meadows, and waste areas. Although it is an aggressive weed in these situations, it does not survive in tilled fields.

Canada Thistle
Canada thistle is a perennial weed that reproduces by seeds and horizontal roots. These roots extend several
feet deep, some distance horizontally (Figure 2), and allow individual plants to live for more than two years. Canada thistle stems are grooved and are 2 to 5 feet tall with branching only at the top. The stems are somewhat hairy when mature. The leaves are smooth, somewhat lobed, and usually have crinkled edges and spiny margins (see photo). The flower heads are numerous, compact, and are borne in clusters. The lavender heads are ¾ inch or less in diameter. Male and female flowers are usually in separate heads and on different plants. As a result, some patches of this weed never produce seed. Canada thistle is found throughout the northern half of the United States. Like bull thistle, it can be problematic in pastures, meadows, and waste areas. In addition, its’ perennial nature allows it to thrive in cropland as well.

Control Recommendations
Both of these thistles are somewhat sensitive to growth regulator herbicides (synthetic auxin/Group 4 herbicides) such as 2,4-D and Banvel/Clarity. These readily translocated herbicides are recommended for control or suppression of both species in grass pastures, however application rates and timing differ.

Weed Management
For bull thistle control, application of 3 pt/A of 2,4-D (3.8 lb/gal formulation) or 1 pt/A of Banvel or Clarity to the rosette stage in fall or early spring before the plants send up the flower stalk is recommended. For Canada thistle, the ideal timing would be during periods of active growth after weeds have reached the bud stage in mid- to late summer, but before killing frost. At this time, the plants have maximum leaf area to absorb herbicides and begin moving carbohydrates into the rootstocks. These stored carbohydrates allow the plants to survive winter and emerge again in the spring. Herbicide movement into these rootstocks is facilitated by this process. Application of 4 pt/A of 2,4-D (3.8 lb/gal formulation) or of 2 pt/A of Banvel or Clarity are recommended for Canada thistle suppression in grass pastures. Repeated applications of these herbicides would likely be needed to bring this tough weed under control.

Grazing Restrictions
With both 2,4-D and Banvel/Clarity, label instructions specify grazing and harvesting restrictions for pasture situations. Lactating dairy animals should not graze 2,4-D treated areas for 7 days following application and meat animals must be removed from 2,4-D treated areas for 3 days before slaughter if less than 14 days have elapsed since treatment. Lactating dairy animals should not graze for 7 days after treatment with up to 1 pt/A, and for 21 days after 2 pt/A of Banvel or Clarity. Meat animals should be removed from areas treated with Banvel or Clarity 30 days before slaughter. Applications made at the end of the grazing season in late summer or early fall can minimize concerns about these grazing restrictions.

Russell R. Hahn, Department of Crop and Soil Sciences, Cornell University

According to the International Survey of Herbicide Resistant Weeds (http://www.weedscience.org), 372 resistant weed biotypes representing 116 dicot (broadleaf) and 85 monocot (grass and sedge) species have been confirmed as herbicide resistant as of February 1, 2012. Historically, there was great concern about triazine resistant (Photosystem II inhibitors – Group 5 herbicides) weed biotypes, which now number 69 (Table 1). In recent years, concern has shifted to ALS (Acetolactate Synthase inhibitors – Group 2 herbicides) and to glyphosate (EPSP Synthase inhibitors – Group 9 herbicides) resistant weeds. In the case of ALS resistance, this heightened concern is due to the rapid increase in the number of weed biotypes (116) that are resistant to these Group 2 herbicides and due to the fact that there are numerous herbicides with this mechanism of action that are used on multiple crops. Finally, although there are only 21confirmed cases of glyphosate resistant weeds, the widespread adoption of glyphosate resistant crops and the increased used of glyphosate herbicide has placed a spotlight on weed populations resistant to this mechanism of action.

The continued increase in the number of herbicide resistant weeds along with the fact that no new herbicide mechanisms of action have been introduced in recent years prompted the Weed Science Society of America (WSSA) to develop a lesson module on herbicide resistant weeds. The intended audiences for these lessons are ag professionals, including Certified Crop Advisors, extension educators, dealers, custom applicators, and others who interact with or advise growers on weed management practices. The lessons can be accessed on the WSSA website at http://www.wssa.net under “WSSA News”. The modules are available for download as PowerPoint slides or as a Flash File. According to the Training Module Agreement, “WSSA grants you a limited license to use these materials for training and educational purposes. Slides may be used individually, and their order of use may be changed; however, the content of each slide and the associated narrative may not be altered.” A brief description of the five lessons in the WSSA Lesson Module on Herbicide Resistant Weeds follows:

Lesson 1 – Current Status of Herbicide Resistance in Weeds, helps understand the need for herbicide resistance management and provides information on the status of herbicide resistance by mechanism of action, on the increase in weeds with resistance to multiple mechanisms of action, and on the global distribution of herbicide resistance.
Lesson 2 – How Herbicides Work, provides information on terminology associated with herbicide use along with an understanding of how herbicides are grouped according to their mechanism of action.
Lesson 3 – What is Herbicide Resistance? A definition of herbicide resistance, an understanding of the different types of resistance, and information on how resistant weed populations develop are included in this lesson.
Lesson 4 – Scouting After a Herbicide Application and Confirming Herbicide Resistance, provides information on the importance of scouting for herbicide resistant weeds, on reasons why weeds can be present after herbicide application, on knowing how to identify herbicide resistance in the field, and on procedures for confirming resistance in the field or greenhouse.
Lesson 5 – Principles of Managing Herbicide Resistance, provides an understanding that diversity is an important concept in resistance management, identifies broad strategies and specific tactics for managing herbicide resistance, and compares the value of proactive and reactive management practices.

Although New York State has been fortunate to have limited problems with herbicide resistant weeds, these lessons provide an in-depth understanding of this growing problem and information on how farmers can avoid or deal with herbicide resistance.