How Grape Growers Can Reduce Vine Size with Root Restriction

The Wolf Lab at Virginia Tech has investigated the use of root restriction as a tool to reduce vine vegetative growth and improve fruit quality in a humid environment for more than a decade. Grape growers don’t have a range of vine size-restricting rootstocks available as some other crops, like apple rootstocks, to control tree size. Therefore, we looked at root restriction as a means to limit vegetative growth of a grapevine. Our work with root restriction has been reported in a number of peer-reviewed articles. In short, root restriction is an effective tool to dwarf grapevines in a humid environment.

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Root restriction was achieved by permanently planting a grapevine in a root bag: a synthetic fabric cylinder with an open top and solid bottom. Developed for the nursery industry, the fabric is UV-stabilized, and the bags we used each hold approximately 4 gallons of soil (available from High Caliper Growing System). We utilized three different low-vigor grapevine rootstocks (101-14, 420-A, and Riparia Gloire), all grafted to ‘Cabernet Sauvignon’.

Sixty vines per rootstock were planted in root bags and 60 per rootstock were planted conventionally, without any root restriction, for this project. Root health of grapevines was not a direct research objective, but our experience with these root bags provides these indirect observations about grafted grapevine roots, including:

We saw differences in the vigor conferred by rootstocks, even when vines were grown in root bags.

Even with the volume of soil that vine roots could easily explore, kept constant between rootstocks, we observed differences in vine size. Particularly vines grafted to Riparia Gloire were smaller than those grafted to 101-14 or 420-A. This suggests that the biophysical (e.g., hydraulic resistance) or biochemical (e.g., hormone) contributions of the roots to the aerial growth of the vines are potentially as important as the morphological adaptations of the root system to soil conditions.

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We saw differences in the nutritional status of grapevines by rootstocks, even when vines were grown in root bags.

Consistent with work evaluating potassium (K) update across a range of rootstocks, plant tissue analysis results for K were lower in 420-A than the other two rootstocks. Similarly, Riparia was associated with reduced magnesium (Mg) uptake.

We saw a lower incidence of Bunch Stem Necrosis (BSN) on vines grown in root bags than on vines grown without root restriction.

BSN is a physiological disorder with grapes that leaves the distal portion of some clusters undesirable for wine production. The direct cause of BSN is not clear; a number of stressors are related to the disorder. In 2013, we noticed a fairly high incidence of BSN in the research vineyard. We surveyed the clusters throughout the vineyard and found that the root-restricted vines had lower incidence of BSN than the more vigorous vines without root restriction.

The fabric eliminates the passage of roots across the barrier.

We have removed a handful of vines to examine the root restriction. There are fine roots that have growth though the fabric; however, they restrict root development after passage through the fabric. Some roots had penetrated the bag fabric and enlarged at the bag interface to as much as 1 cm diam. These roots were easily broken off at the bag interface, revealing a slender (<1.0 mm) trace of apparently functional stele transcending the fabric. The major diameter of the swollen root comprised darkened, apparently nonfunctional, callus tissue.

We trialed an open mesh root bag, but the open mesh did not provide the same reductions in vigor and vine size as the synthetic fabric. We suspect vine roots grew easily through the mesh.

In some cases, large vine roots escaped the root bags.

Since this block was installed in 2006, approximately five out of 180 grapevines have grown dramatically larger than their peers. We suspected that the grapevines roots had escaped the root bag. We removed one of these suspected escapees after the 2018 growing season to reveal a large ~1-inch root that had grown out of the bottom seam of the bag.

Again, root observations were not the direct focus of this research. We did this work to inspect root restriction as a means to grow smaller vines in a humid environment. Root restriction was successful in a research vineyard to dwarf grapevines. We are now collaborating with growers in the East to trial the root bags in commercial vineyards.

Growers do have a choice for degree of restriction via the size of root bag they utilize. We recommend growers have irrigation in place to alleviate water stress with root restriction in these trials. Root restriction proved to be both a viable tool to affect vine vegetative growth and shed some light on root characteristics of grafted grapevines.

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