Mulling over mulches
Mulches can conserve water and control weeds, but there are tradeoffs.
David Granatstein, who has studied mulches in tree fruit and organic systems, believes that some of the benefits seen in tree fruit—water conservation, weed control, and improved organic matter and tree growth—may be valuable in grape production. However, he explains that grape growers will have to pay attention to the mulch interactions with nitrogen, soil moisture, rodents, and spring frosts.
"The bottom line with mulches is that they can conserve water and control weeds," said Granatstein, statewide coordinator for Washington State University's Center for Sustaining Agriculture and Natural Resources. "But there are usually tradeoffs from other system effects. Initially, the costs can be high, but you need to look at multiyear benefits."
Granatstein discussed his mulch research with grape growers attending the annual meeting of the Washington Association of Wine Grape Growers in Kennewick, Washington. Through the years, he has experimented with all types of mulch, from shredded paper to black geotextile material to spray-on paper mulch.
Mulches serve as physical barriers and are used for various reasons, including reducing water loss, controlling weeds, and influencing the chemical and biological properties of soil. "Dead" mulches are those that come from inert material like synthetic fabric or stone. Organic mulches include things like straw, wood chips, and other crop residues layered over the soil. Living mulches are also known as cover crops and include companion crops, perennial plants, self-seeding annuals, and annuals.
After years of research, Granatstein has found that some materials do not make good mulches. In his trials, chopped alfalfa hay provided too much nutrient, little weed control, and delayed senescence in trees in the fall. Oriental mustard, planted in the fall, had more annual weeds in it the following year. Winter rye didn't reseed, although it provided good weed control.
Mulches can have physical, chemical, and biological influences in the orchard or vineyard. Granatstein explained that physical influences include modifying soil temperatures, increasing chance of frost (bare soil is less of a problem than covered soil), changing reflectance, increasing water evaporation, and improving infiltration. Chemical influences of mulches can affect nutrients, carbon, and salts, while biological influences impact things like crop performance and root growth.
"And physical influences usually affect other things like insects, weeds, pathogens, and more," he said.
In a tree fruit trial in Wenatchee, Washington, Granatstein found a difference in soil temperatures of 10 to 15°F between soil with mulches and without in the spring time. However, temperatures were warmer in the mulch treatment in the fall. Bulk density of the soil was improved from the mulches, and moisture retention improved. He measured water savings from trees with mulch of up to 50 percent compared with nonmulched trees.
"But for vines, you may want to dry soils out quicker than tree fruit," he acknowledged.
Research from orchard floors in New York that used straw mulch for five years showed a boost in organic matter, as well as nutrients like nitrates, potassium, and phosphorus. "This is something you have to watch because you can easily overshoot your potassium and phosphorus when adding nitrogen," he said.
Trials with living mulches of dwarf clover and white clover have shown good weed control and nitrogen release after the clover was cut. In the first year of the living mulch, most of the biomass was weeds, he said, adding that in the second year, the biomass was mostly mulch with few weeds.
Nearly half of the clover nitrogen mineralized in the soil after it was cut, with nitrogen released in the soil over two weeks, he added.
An orchard trial in New Zealand that used a "mow-and-blow" strategy for four years with a living mulch also showed an effect on the pH, nitrogen, and organic matter of the soil.
The major downside to living mulches is increased rodent activity.
"When you put things under trees and vines, rodents are very thankful," Granatstein said. "Rodents have been a real Achilles' heel with living mulches."
Meadow voles seem to be the biggest problem, he said, adding that they like the cover and food from the living mulch. In one clover stand, voles made it disappear within two years.
He has experimented with Galium odoratum, a ground-cover species derived from a similar plant known to the Greeks for its ability to curdle milk. He found a significant drop in vole activity from the Galium ground cover. Trials have also shown that a mulch of wood chip material does not increase vole activity.
Living mulches have also been studied for their value as a habitat for beneficials and nematodes.