Notes From the Underground: Rootstock Selection

Since the unfortunate importation of Phylloxera from the new world to Europe in the 1850s, and their subsequent spread to just about every other wine region worldwide, rootstocks have been an integral part of modern viticulture. While all current rootstocks are bred to be resistant to Phylloxera to some degree, other viticultural considerations have been driving modern rootstock breeding programs. Before delving into those modern challenges, let’s have a look at the Vitis species that are the backbone of all breeding programs, because their characteristics are passed to the rootstocks bred from them, allowing breeders to try and get the best from both rootstock parents to provide the most useful material for growers. 

Figure 1: Common rootstocks and their parentage (Shaffer et al. 2004)

Vitis riparia - Shallow rooting, low vigour, earlier ripening

Vitis rupestris - deep rooted, drought tolerant, vigourous, later ripening

Vitis berlandieri - Lime tolerance, drought tolerant, vigourous

Vitis champinii - Salt tolerance, nematode resistance

Choosing Rootstocks 

In order to choose the best rootstock suited for your particular site, there are 5 key components to consider. These are the soil’s physical properties, chemical composition, presence of soil borne pests, the vineyard’s susceptibility to drought, and traits the rootstock may impart on your scion.

Physical Properties and Chemical Composition

For example, if the soil has high lime content, as do the soils of many famous wine regions, a rootstock must be chosen that has tolerance for such soils, otherwise the vines will not be able to access sufficient iron and won’t grow well. The drainage of the site is also important. No rootstocks like wet feet, but some are more tolerant of it than others. Similarly, no rootstocks like saline soil or irrigation water, but some are able to grow better under these circumstances than others. 

Soil Borne Pests

The presence of soil borne pests, such as plant parasitic nematodes, is also very important to know. Dagger namotode (Xiphinema index) can vector fanleaf virus, which makes vines unproductive. In some regions, populations of other nematodes are so high that they threaten the establishment, or economic viability of vineyards. Rootstocks vary in their susceptibility to these pests, and breeders around the world have been developing rootstocks that are resistant to them, such as the GRN rootstocks out of the US.

Drought Tolerance

Rootstocks also vary in their tolerance for drought, and so water availability and rainfall patterns for the vineyard is another consideration. Drought tolerance is conferred by rootstocks that are deep rooted and can explore a large volume of soil for water. Rootstocks with Vitis rupestris and Vitis berlandieri backgrounds tend to have this trait. There is also evidence from some work we have done that rootstocks may affect the stomatal response of the scion grafted to them, another trait that can influence drought tolerance.

Transferable Rootstock Traits

As important as knowing your soils is having specific aims for the vineyard. The most successful vineyards are those that are planted with a clear target in mind, and all the choices made during establishment/development (including rootstock choice) should be made to achieve those goals. Are you planning on making a high-end red wine where you might want to control yield, or are you wanting to more or less maximise yields from your block? Are you willing to hang fruit out longer, or would you prefer to minimise risk of late harvest? How much fruit exposure is wanted, and how do you plan to manage the canopy? All of these, and more, will impact which rootstocks are the best choice for your block.

Figure 2: Rootstock characteristics (Lambert et al. 2008)

An example of a transferable rootstock trait is the differences in vigour conferred by different rootstocks. Figure 3 shows canopy assessments of gap percentage from a Chardonnay rootstock trial in Marlborough NZ we were studying this past season. As you can see, there are huge differences in canopy size, which will in turn influence potential productivity, bunch exposure, evapotranspiration, and fruit chemical composition.

Figure 3: Percent canopy gaps from a rootstock trial in Marlborough NZ in 2023-24. Examples of canopies with high, medium, and low vigour are shown on the right. Scion for all rootstocks is UCD6 Chardonnay.

Inspecting Grafted Vines and Planting

Once you have made your choice of rootstock and have received your vines they should be inspected. Here in NZ there is a grafted grapevine standard that all vines must pass to be sold. There are similar standards in other countries. However, these standards specify minimum requirements, and a grower obviously wants to receive and recognise the best quality plants. Make sure you’re checking the following:

• The graft union should be strong, and should not be able to be moved by pushing the scion sideways.

• The diameter of the grafted scion and rootstock should ideally be 8-10 mm across. This ensures your vines have sufficient plumbing and carbohydrate reserves to support strong growth.
  

Figure 4: A graft union from citrus showing callus bridging the gap between the scion and rootstock.

• The callus tissue, which is what connects the rootstock’s vasculature to the scion’s, should be present all around the graft connection. This ensures the best flow of nutrients and water to produce a resilient grapevine. 

• The roots should be healthy and ideally coming out in all directions from the base. This ensures the vine starts out exploring as much soil as possible from the start, when it is most sensitive to water stress.
  

Proper planting is just as important as starting with quality grafted vines. A major issue I’ve seen around the world is J rooting. This comes about when the vine is planted in a hole that is just deep enough, resulting in the root tips pointing up at planting. This reduces the ability of the vine to most efficiently explore its soil, and puts the vine on the back foot from the start. Mechanically planted vineyards, which are the norm here in New Zealand, are particularly susceptible to J rooting. The most frustrating thing is that it is so easy to prevent, it just requires a little more effort. 

Figure 5: Roots of a struggling Cabernet Sauvignon vine clearly showing J rooting. From Stamp (2011).

• Machine Planting: The trench should be at least 15 cm deeper than the desired final planting depth. Vines are planted as normal, to the bottom of the trench, then crew should be sent behind the machine to gently lift the vines to the desired height and pack the soil around the base of the vines re-settle the soil around the roots, which are now pointing downwards. 

• Hand Planting: Holes should be dug at least 15cm deeper than is needed to have the graft unions where you want them above the ground. Vines should then be placed in the hole, ensuring root tips are pointing down, and soil should be added and packed around the roots before filling the rest of the hole. 

In both circumstances it is advisable to check random vines to ensure roots are positioned appropriately. Given both the short and long term negative effects J rooting causes, it’s more than worth the extra effort to avoid it and you only get one chance to do so.

Figure 6: Rooting profile of j-rooted vine with minimal access to subsurface irrigation.

Parting Thoughts

You’re stuck with the rootstock choice you make for the life of the vineyard, so have a clear plan. Scions can be changed by top working the vines, but there’s no going back on rootstock after planting. Management can make up for some of the challenges of a poor rootstock choice, but why make your job harder than it needs to be? Choosing a middle of the road rootstock allows for flexibility of wine style but ensures that you’re always working against your vines to some degree.  Don’t forget to keep an eye trained on the future climate your vineyard will be challenged with. In many regions, climate change means longer droughts and more heat spikes, so choices should be made with those in mind.  

If you plant with a wine in mind, you can work with the vines rather than against them. Please reach out if you’d like to discuss rootstocks for your site. 

References:

Lambert, J. J., Anderson, M. M., & Wolpert, J. A. (2008). Vineyard nutrient needs vary with rootstocks and soils. California agriculture, 62(4). <https://calag.ucanr.edu/archive/?type=pdf&article=ca.v062n04p202> [Accessed 1 May 2024]

Stamp, J. A. (2011). Vineyard Development: Principles, Problems and Perspectives. Wine Bus. Mon. December, 52-59. <https://smallvines.com/wp-content/uploads/2014/08/Wine-Business-Monthly-Vineyard-Development.pdf> [Accessed 1 May 2024]

Shaffer, R. & Sampaio, Tiago & Pinkerton, J.N. & Vasconcelos, Maria Carmo. Grapevine Rootstocks for Oregon Vineyards. Oregon State University, December 2004. <https://ir.library.oregonstate.edu/downloads/4q77fr64m> [Accessed 5 May 2024]

Helpful Resources to Check Out:

Christensen, L Peter, et al. Wine Grape Varieties in California. UCANR 2003. Rootstock Selection <https://iv.ucdavis.edu/files/24347.pdf> [Accessed 1 May 2024]

Cox, Catherine. Water & Vine Managing the Challenge Module 14: Rootstocks as a management strategy for adverse vineyard conditions. Wine Australia. <https://www.wineaustralia.com/getmedia/b6f9712e-d735-432f-95a1-bc82e808771c/Rootstocks-as-a-management-strategy-for-adverse-conditions.pdf> [Accessed 5 May 2024]