Weed Control in Vineyards

By: Rob Crassweller

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First Step is Identify

Vineyard weed control can be a troublesome prospect for new and experienced viticulturists. In most of my experience the problems lie in several areas. The first is a failure to know what their weed problems are. It is critical to identify what weeds you are trying to control and to know when the best time of the growing season to control them. You should map your vineyards as to where you have weed problems and then identify the weed. A good book to help identify weeds is “Weeds of the Northeast” by Uva, Neal and DiTomaso, the book can be ordered from Amazon.com and costs around $25 – $30. This valuable reference tool provides color images of the various stages of growth, a description of identifying characteristics, type of weed it is, how it spreads and species that appear similar. Be sure to pay special attention to the “Shortcut Identification Tables” on page five through eight. There is also a dichotomous key to help identify the weeds with an explanation on how to use the key.

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Types of Weeds

There are two broad classifications of weeds annual and perennial weeds. Annuals can be further classified as either summer or winter annuals. The summer annual germinates in the spring or summer flowers, sets seeds and dies in one year. The winter annual germinates in late summer to early spring flowers and dies. Winter annuals if the germinate in late summer can survive the winter months to flower in the early spring. Invariably when I talk to growers about their weed control problems they usually involve perennial weeds. Perennial weeds are those that survive multiple years. They have some form of storage organs such as extensive underground roots or stolons (horizontal stem close to soil surface) rhizome (creeping underground stem) or tubers. These types of weeds can be damaged with post emergent herbicides but due to their storage organs can quickly sprout back.

Weed control should begin before the vineyard is planted. Weeds like Canada thistle, quackgrass, yellow nutsedge, bindweed, horse nettle should be controlled before the vineyard is established. Generally perennial weeds are not controlled by pre-emergent herbicides. The herbicide of choice in vineyard preplant situations that are near existing vineyards is glyphosate. Be advised however, that if the perennial weeds have been established for a few years they will not be controlled with a single application of a glyphosate product; multiple applications will be needed. The best timing and rate of application will depend upon the weed species present and should be listed on the label of the product. Most perennials are best controlled with a late summer to early fall application of glyphosate. Early spring applications are usually not effective because the plants have not made enough growth to be readily translocated downward to attack the storage organs. Spring applications may serve to burn the plants down temporarily but if they are well established they will regrow.

As with any pesticide application be sure to read and follow the herbicide label. Labels for herbicides can be seen by going to www.cdms.net. The labels contain detailed product information and precautions. Pay particular attention to information pertaining to minimum age of the vineyard that the products can be applied. For example, Alion (indaziflam) cannot be applied to vineyards less than 5 years old; simazine to vineyards less than 3 years and Chateau to vineyards less than 2 years. Also be mindful of the days to harvest limitations.

 

Why Do Herbicides Fail?

          There are many reasons for herbicide programs not to work. Many growers assume that if the weeds show up after they have applied an herbicide it must mean the weeds are resistant to the material. Unfortunately, it is more often due to applicator error than the resistance. Below are some common points that can impact the effectiveness of an herbicide application

  1. A common reason for failure is that the material is not intended to control a particular weed. There are materials like Solicam, Prowl and Surflan that are primarily designed to control grasses as there are also materials like simazine, Goal and Karmex that are better at controlling broadleaf weeds. Make sure the material you applied was meant to control the weed that you are seeing.
  1. Another reason is that the weed present is not actually the one you thought you were treating for. Be sure you have correctly identified the weed. The previously mentioned book will help reduce that mistake.
  1. Conditions were unfavorable during, or immediately after application. Materials like Chateau and Matirx work best when the soil is moist. Glyphosate products should not be applied to weeds when they are under stress. Devrinol and Prowl need at least ½ inch of rainfall after application to move them into the weed germinating zone.
  1. The time of year or season was incorrect. Herbicides that inhibit seed germination often times are applied too late. A good rule of thumb is to make your early spring pre-emergent herbicide applications around the time forsythia blooms. Many growers miss this early timing because they are too busy worrying about disease and insect control. Remember weeds are pests too.
  1. Perennial weeds are rarely controlled by pre-emergent herbicides. Many times when I visit farms having weed problems an examination of the weeds show they are primarily perennials.
  1. Too little material, improper adjuvant or too low water volume. Trying to save money by applying less than labeled rates, can only results in poor control and an increased potential for resistance. Most herbicides should be applied in 20 to 30 gallons of water per treated acre. When applying post emergent materials too much water carrier can wash herbicides off the plants.
  1. When applying glyphosate products make sure the water you use is clean. Water from ponds, and hard water can result in less effective control. We know that glyphosate is quickly bound to organic matter in the soil and the same is true if your water source has organic matter in it.

Final Thoughts

Remember just as you rotate the mode of action for insecticides and fungicides to help prevent the development of resistance, you should also rotate the modes of action of your herbicides. Do not use the same herbicide program year after year but rather switch materials at least every two years. To learn about which grape labeled herbicides have similar modes of action go to Herbicide Mode of Action located within the Herbicide Resistance Action Committee website.

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