The root of the matter

This excavated root system is from a 40-year-old Concord own-rooted vine.

PHOTOS COURTESY OF ALAN LAKSO

Grapes, with their flowing vines and lack of inherent structure, are free spirits compared with tree fruits, which have a fairly rigid form and structure. Not surprisingly, grape roots are free spirits as well.

So why should anyone care about grape roots and what they do? Since they take up the water and nutrients the vines need, provide carbohydrate and nitrogen reserves for the spring, and help regulate vine growth, they are a critical part of the plant.

“Studying them is the hard part,” says Cornell University grape physiologist Dr. Alan Lakso. He was concerned that juice grape growers, in their quest for higher yields, might be putting too much stress on the vines as they went to minimal pruning with more shoots per vine and higher yields. Generally, higher yields inhibit root growth. They wanted to know how they could assure there was sufficient root growth to support the vine sustainably from year to year, and they wanted to know the best time to water and fertilize for best root performance.

Lakso teamed up with Dr. David Eissenstat, a root biologist at Pennsylvania State University, and together they worked in a Concord vineyard with the staff at Cornell’s Lake Erie Fredonia Laboratory. They’ve studied Concord roots for 12 years and wrote a report on their work late last year.

Minirhizotrons

As well as digging up roots, they used an unusual method, minirhizotrons—clear two-inch-diameter plastic observation tubes inserted into the root zone. With a special video camera, they could see the new roots as they grew during the season, observing how many grew, when they grew, and how long they lived, without having to dig up the roots.

“Grape roots don’t grow in any particular pattern over the season,” Lakso said. That was a somewhat disappointing discovery.

Root growth was similar to shoot growth in that roots grew between bud break and veraison, but the pattern each year was not nearly as consistent as shoots, he said. After veraison, very few new roots were produced, especially with heavier crops. In warmer regions, root growth may persist into the postharvest season and even occur in winter if soils are warm. But in Concord grapes in New York, new roots are rarely produced after harvest.

Production of new fine roots was reduced by drought periods, and irrigation ­supported up to three times more root production in a very dry year.

“We don’t really understand why, though it seems that each year the weather, soil moisture, and crop level interact uniquely to affect the amount and timing of root growth,” Lakso said.

Using minimal pruning, which leads to earlier canopy development than with ­balanced pruning, root growth did start somewhat earlier in the season and peaked in early June, about a month earlier than it did under the normal pruning regimen. Even with heavier crops, minimally pruned vines did not seem to produce fewer new roots. This may be because most roots grow before the major competition from the crop.

Grapes don’t expand their root system much once they are mature.

“Young vines can produce a great many roots to get established, but once the vines mature and begin cropping, only 10 percent or even less of the vine’s annual growth goes into root production,” the researchers reported. One of the implications is that vine roots are not very competitive with weeds. “Mature vines have very few roots compared to cover crops or weeds on the vineyard floor, so vine roots don’t compete well against them for water or nutrients,” Lakso said.

On the other hand, grape roots don’t have an inherent geometry, so they grow where conditions are best or they redistribute over time to avoid competition from weeds or covers. Their distribution is affected by what the soil has to offer in water, oxygen, nutrients, organic matter, depth, and soil texture.  In deep soils, grape roots might simply grow deep to avoid cover crop competition, Lakso said.

Root life

So how long do new roots live?

“Although a small number of roots become major roots and live as long as the vine, the overwhelming majority have relatively short lives,” they said in their report. “Most fine roots die in the same season they are produced, just as leaves do. Concord grape roots age quickly—most only live 50 to 100 days at most and are really functional for only about a month when they are still white. In our studies, root lifespan surprisingly was not clearly affected by pruning intensity, crop level, or water availability.”

They conclude from their studies that in juice grapes or bulk wine grapes, where high yields are needed, it might be wise to limit external stresses like drought as much as possible to avoid overtaxing the root systems. But high yields, in their studies, did not lead to decreased fine-root production, although there was evidence of gradual declines in the medium-size storage roots with very high yields.

On premium wine grapes, excessively low yields may stimulate large root systems that support excessive vine vigor, they said. More research would be needed to address that.

Although much was learned about the roots and how they behave, Lakso said that the results were “bit disappointing on a practical basis. We’d hoped roots would be quite predictable, so growers could really optimize management of the roots. But variability is just the nature of grape roots.”