Rootstocks

This is a subject that deserves more attention than I can possibly give it. Rootstocks was certainly a topic about which I had no understanding whatsoever until I tackled the WSET Diploma program, at which point I began to be rather too fascinated by the subject. I found the particular qualities and tolerances of each basic rootstock type - and that their crosses did not always bring only the best of both contributors - compulsively interesting. This is real agriculture. 

One hears often of vines being pulled up because they gave in some way less than optimal fruit.  But they can also be pulled up because the rootstocks were not suitable - most particularly if too prone to chlorosis (the lime in limestone turning the leaves yellow and inhibiting the uptake of nutrients) or phylloxera.  

Vinifera is the original native European rootstock - not therefore naturally resistant to phylloxera which was not present in Europe until imported from the US in the 1860s.  The US vines by contrast are all to some degree resistant to phylloxera since the insect is native to the US. So you graft a Vinifera scion (the part of the vine that you see above the ground) on one of the three American rootstocks or more likely some cross of these so as to generate the optimal results for the soil type and climate of the vineyard.  If you cross a Vinifera rootstock with a US rootstock you run the risk of the result not being sufficiently resistant to pylloxera since one of the parents is the non resistant European rootstock.  This was the problem with AXR1 in Napa. But that cross is still used in Champagne where the sandier soils are less hospitable to the insect, since AXR1 has the desirable quality of being particularly resistant to the high lime content of those soils. And so it goes….

Oversimplifying considerably the basic US rootstocks are - 

    Riparia           Gives low vigor but has low resistance to drought - so good in damp soil

    Rupestris       Gives high vigor and has good resistance to drought - so good in dry soil

    Berlandieri     This has strong resistance to lime - so good in soils with a lot of calcium

These can also be crossed. For example 

    AXR1                      Cross of Vinifera and Rupestris - still to some degree susceptible to phylloxera

    3309C, 101-14        Crosses of Riparia and Rupestris - does well in high acid soil

    161 / 49C /SO4      Crosses of Riparia and Berlandieri - moderate vigor and resistant to lime

    99 (Richter)           Cross of Rupestris and Berlandieri - Good resistance to chlorosis / deep rooting                   

And there are dozens of other combinations. 

The choice of rootstock when planting a vineyard is of course a huge decision. Errors are costly in the extreme. One has only to look at California’s experience with XR1 to witness what a catastrophe can result from a poor choice. As well as picking a rootstock with sufficient resistance to disease and that is suitable to the particular soil type in which it will grow, decisions have to be made about plant density and predictions about the future climate. And although the science in this space is hugely improved, there are still many unknowns. For example, it is not yet understood why certain rootstocks in Burgundy are dying earlier than expected.