By Paul Domoto
Iowa State University Extension
Since 2000, Iowa grape acreage has increased from about 30 acres to more than 850 acres, and wineries increased from 11 to 70. With this rapid increase in the number of wineries, grape production is lagging behind demand. Before investing in such an enterprise, there are a number of factors you should consider to determine if a vineyard is right for you. These factors include available markets, site and soil characteristics, establishment and production costs, and available labor.
Site and soil considerations
Site suitability for growing grapes is related to minimum winter temperatures, frequency of spring and fall frosts, length of the growing season and annual precipitation. Growers in northern Iowa have to select hardier cultivars (cultivated varieties) with shorter growing season requirements than growers in southern regions.
Even within a localized area, the minimum winter temperature, frequency of spring and fall frosts, and length of the growing season are very site specific and are governed by the elevation of the site compared to the surrounding area. The coldest temperatures occur under radiation freeze conditions when there are no clouds and very little or no air movement. Under such conditions, cold air settles into low-lying areas. An elevated site will be warmer in the winter, less prone to late spring and early fall frosts and have a longer growing season better suited for grapes. A general rule would be to locate a vineyard at least 50 feet above a valley floor.
An ideal vineyard soil has good internal drainage yet sufficient moisture holding capacity, a pH in the 6.0 to 6.5 range, and organic matter content in the 2 to 3 percent range. Such soils are not commonly found in Iowa, but with some modification and adjusting of cultural practices, other soils can be suitable for grapes. The ISU Extension Iowa Soil & Land Use site, http://extension.agron.iastate.edu/soils/interpretations.html, has a Soil Interpretations resource that lists soils in each county suitable for vineyards based on slope, organic matter content, soil pH, internal drainage and permeability.
Soils with internal drainage classified as “well-drained” are ideally suited for growing grapes. These soils provide good aeration for root development and have adequate soil moisture holding capacity to sustain grapevines through intermittent periods of drought. Soils classified as “moderately well drained” may require some modification. Soils classified as “somewhat poorly drained” will require modification. If the soil is “somewhat excessively drained” consider an irrigation system.
Soils with a pH below 6.0 can be limed to bring up the pH. However, many Iowa soils are excessively high in magnesium (Mg) which can interfere with the uptake of potassium (K). Therefore, avoid using dolomitic lime to raise the soil pH. Soils with a pH above 6.8 can be acidified with elemental sulfur, but when the pH is above 7.4 or 7.5, it may not be economically justifiable.
For each percent organic matter, a typical Iowa soil has the capacity to release about 20 pounds of nitrogen (N) per acre per year. Grapevines do not have a high demand for N, but if N is available, they will continue to grow as long as temperatures are warm enough to promote growth. Because of this characteristic, they tend to be less winter hardy on high organic soils. When the organic matter content is in the 3 to 5 percent range, you can compensate for the N release by reducing fertilizer N applications and growing early maturing, very hardy cultivars. When the organic matter content is more than 5 percent, chances for establishing a successful vineyard greatly decline.
Before establishing a vineyard, it is important to test the soil to determine if the organic matter content is suitable, if the soil pH needs to be adjusted and if the soil needs to be amended to optimize the phosphorous (P) or K content. This is very important because calcium (Ca) in lime, P and K move slowly in the soil; correcting a deficiency of any of these nutrients after planting grapes is extremely difficult.
Test the soil for Mg to determine if it may interfere with uptake of K, and for zinc (Zn) because Zn is low in many Iowa soils. Because test results on the top six to eight inches often reflect past fertilizer practices, submit separate soil samples from 0-6 and 6-12 inch depths. Forms to submit soil samples can be downloaded at http://www.agron.iastate.edu/soiltesting/. Soil sample bags are available from your county Extension office.
If your site and soil conditions do not meet the criteria described, chances for success will be greatly reduced. For more information, go to the ISU viticulture site, http://viticulture.hort.iastate.edu/home.html.
This article is condensed from the June 2008 issue of Acreage Living, www.extension.iastate.edu/acreage/
Other articles in this month’s issue--
- Mower Maintenance Important on Larger Acreages
- Pesticide Applications in the Neighborhood