Managing irrigation and nutrients for grapes can be tricky anytime — even more so in a drought year like the one last year in Washington. The key to ensuring vineyards receive the necessary water and nutrients lies underground at the roots.

For two years, Washington State University soil scientist Joan Davenport and a research team sampled Concord roots on 42-year-old vines at different points in the growing season to better understand when roots are working most actively to support the vine.

While Davenport focused on Concord grapes for the study, the findings relate to wine grapes as well.

The takeaway: Growers must remember their roots are distributed over a much wider area than they probably think, which affects how and when they apply nutrients and water, Davenport told growers at the Washington State Grape Society annual meeting in November.

The density of fine roots (the feeder roots that take up most of the water and nutrients) on grape plants changes according to plant needs, with the fewest fine roots found during bloom. (Source: Joan Davenport/WSU Illustratioin: Jared Johnson/Good Fruit Grower)

Types of roots

Grapevines have a lot of different roots. The big roots, called coarse roots, help serve as the sturdy backbone of the plant, while the finer roots are the feeder roots that take up the majority of water and nutrients.

Overall, root distribution decreases as they grow farther from the trunk at deeper depths, but that growth is influenced by water.

To better understand how those roots develop during the season, Davenport and her research team dug up an average of four Concord vines from a furrow-irrigated vineyard at different points in the season: in winter, at bud break in early spring, when vines showed three to four leaves in late April, at bloom, at veraison, at harvest (in about mid-September) and at postharvest, before the vines had gone dormant.

They excavated the entire root ball, measured the length of roots and weighed coarse roots. They also collected soil samples, in a radial pattern out from the trunk, for fine and some coarse roots and separated them from the soil.

Davenport said she expected to find fine roots in the first 8 inches below the surface but was surprised to find them even beyond that. Overall, roots were found 1 yard deep. However, roots decrease farther from the trunk and deeper below the surface of the soil.

“As a general rule of thumb, the further out from the trunk you go and the deeper you go, the fewer roots you have,” she said.

Coarse roots

The density of coarse roots was greatest at bud break, with 6 linear feet of roots in a cubic yard of soil.

The second-highest density of coarse roots was found directly preceding bud break in late winter. The lowest point was at harvest.

“When we have some of that early growth, the plants are actually using a little bit of those root materials to feed the growth,” Davenport said.

The team generally found that more coarse roots were found closer to the vine trunk and diminished with greater distance from the trunk. They also decreased at greater depths.

Fine roots

Because fine roots are so tiny and would be destroyed or lost in the process of collecting and measuring coarse roots, the team collected soil samples in a radial pattern around the trunk to assess the distribution of fine roots.

They then separated these roots from the soil. Fine roots were measured separately in the first 12 inches in depth and again at a depth of between 12 and 30 inches.

Fine roots grow and die. They decreased with depth, Davenport said, but depending on the time of year, they increased or decreased the closer or farther away they were from the trunk.

Fine roots were densest at postharvest, with the fewest found at bloom. “By the time we get to bloom, the fine root density has declined dramatically at the surface and pretty much disappeared at the deeper depth,” Davenport said.

Fine root numbers then increased incrementally through veraison, harvest and postharvest, with fine roots growing at the expense of the coarse roots, she said. “This tells you how much the fruit is utilizing these resources and allows for more root growth after the fruit is harvested.”

People who manage crops tend to focus on the plants above ground and sometimes forget about the root systems, Davenport said, but they shouldn’t assume that roots are growing just because they don’t see them. “Know your roots,” she said.

In terms of the fine, feeder roots, it would be nice to have a lot of them during blooms, but the plant is so busy funneling its resources to its buds, that it “robs Peter to pay Paul,” Davenport said.

However, the exact opposite is happening at postharvest: The fruit is off the vines, and with shorter days and colder temperatures, the plant begins to send carbohydrates and nutrients to the trunk and roots to store for spring, causing a resurgence in fine root development.

“Make sure you pay attention to that root growth and development when you’re developing your irrigation management strategies,” Davenport said. “Because there’s a time when roots are growing or developing, and when you’re stressing them with less irrigation, you’re actually doing more damage than you know.” •