If most vines are grafted onto rootstocks, why do some grapevines thrive in soils where others struggle?
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Dear Dr. Vinny,
If some grapes do better in certain soils (Merlot on clay, for example) but most vines are grafted to non-vinifera rootstock, does grape variety actually matter? Wouldn’t the rootstock be more important than the grape vaiety?
—Mark, Sacramento, Calif.
You’re correct that most vines are grafted—that is, a vine cutting of one type of grapevine is attached to the root of another type of grapevine. The reason is that Vitis vinifera roots are susceptible to the devastating, root-sapping aphid phylloxera, but those vines can be grafted onto rootstocks that are phylloxera-resistant. (Some vinifera vines, in places where phylloxera isn’t present, can be “own-rooted.”)
You’re also right that rootstock choice has a big impact on how a grapevine reacts and adapts to the soil and climate where it is planted. I asked professor Andrew Walker, who works in the viticulture and enology department at U.C. Davis, to explain the relationship between rootstocks, grape varieties and soils.
Walker explained that, starting in the 1880s, hundreds of grapevine rootstocks were bred, but only about 20 are widely used today, and only about 10 are widely used in California. “When the vines are grafted, the rootstock adapts/compensates for the soil texture and chemistry,” said Walker. “It's the rootstocks that are better or more poorly adapted to clay or gravelly soils.“
So I asked Walker if it was possible to pick a site considered unsuitable for a grape variety and make it suddenly suitable with the correct rootstock choice. Turns out it’s not that simple. “Soil texture, rooting depth, rain fall and irrigation amounts, soil chemistry (particularly lime) all have a large impact on vine growth and fruit quality,” Walker says. “In terms of compensating for an unsuitable site, it would depend on how unsuitable, and why.”