Jim Wolpert, University of California, Davis
When to Use Rootstocks
Rootstocks are used in vineyards when site conditions necessitate. They are rarely used in the absence of a compelling need because ungrafted vines are less expensive and are easier to farm. There are four characteristics of rootstocks that should be considered when making a selection:
- pest resistance,
- abiotic (non-pest) factors,
- propagation issues, and
- scion growth control.
The first step in selecting a rootstock is to determine which pests are present. This is especially true if the site was previously used as a vineyard, because pest problems are more likely in replant situations. When European grapes (Vitis vinifera) are grown, rootstocks are virtually mandatory throughout the U.S. due to the wide prevalence of the grape root louse, phylloxera (Daktulospaira vitifolia). Some American varieties and French-American hybrids direct producers are resistant enough to phylloxera that they may be grown without the use of rootstocks. State-by-state recommendations for varieties will usually indicate whether grafting is necessary for resistance to phylloxera.
The second-most observed pests are nematodes, microscopically small parasitic worms. However, it is critical to note there is no such thing as general “nematode resistance”. Virtually all resistance of rootstocks is specific to the species of nematode, so it is crucial to get a professional evaluation of the soil. Research references are available to help determine rootstock resistance to specific nematodes. Soil samples should be sent to a laboratory specializing in vineyard nematode analyses. Sampling methods and time of year are critical to obtaining an accurate assessment of nematode risk.
Every vineyard site brings a unique combination of conditions as a result of soil chemical and physical features, as well as climatic factors such as rainfall patterns. Rootstocks differ in their tolerance of saturated soils (“wet feet”), drought, salinity, acidity, and mineral deficiencies. Growers are advised to correct as many site deficiencies as possible prior to planting such as installing drainage and/or irrigations systems or by pre-plant soil amendments (e.g., lime). Ratings of tolerance or susceptibility of rootstocks to abiotic conditions is far from complete. In many cases ratings appear to have been simply observations and not based on research. Therefore, a grower’s best advice is to gather as much information from as many sources as possible, especially from state university Extension specialists, prior to making a choice.
Rootstocks differ greatly in their ability for successful propagation. Cultivars such as O39-16 and 420A, for example, are notoriously difficult to benchgraft and may cost more as a result. Also, the quality of planting material may suffer somewhat from normal standards if a nursery struggles to find enough material to complete an order. Difficulty of propagating rootstocks by one method does not necessarily extend to all methods. For example, even though 420A is difficult to benchgraft, rootings can be very successfully propagated by field budding.
Scion growth control
Rootstocks exert tremendous influence on growth of scions. Even in the absence of site limitations, scions may be 2- to 3-fold larger on growth-inducing rootstocks than on growth limiting rootstocks. Thus, rootstocks are a critical component in an effort to achieve a balanced vine. In the absence of pests or other limitations, rootstocks are most often chosen as a counterbalance to a site and/or a scion growth capacity. That is, when site and scion are conducive to high amounts of growth, the rootstock should be chosen for low growth and, conversely, when both site and scion are conducive to low growth, the rootstock should be chosen to promote higher growth. Obviously, there are many gradations between the extremes, and this is where rootstock choice becomes difficult. In replant scenarios, the growth of the previous vineyard can be a guide to the performance of the new one.
In some situations, pest conditions may necessitate using a rootstock with high growth potential in a situation where a rootstock with lower growth potential would be advised. In this case, a grower should consider using a divided trellis system to accommodate the anticipated growth and/or be prepared to incorporate a cover crop  to provide competition for water and nutrients in the rooting zone.
In conclusion, choosing the right rootstock for a vineyard is not an easy decision but is critical to long-term success. Growers are advised to obtain as much information as possible prior to making the decision.
Considerations & Resources for Vineyard Establishment in the Inland Pacific Northwest, Washington State University
Reviewed by Bruce Bordelon, Purdue University and Eric Stafne, Mississippi State University