Research in the Pacific Northwest suggests that mulch placed in the tree row can cut a young apple tree’s water needs by more than 50 ­percent.

Dr. Eugene Hogue, a retired researcher with Agriculture and Agri-Food Canada in British Columbia, Canada, conducted experiments using a paper slurry mulch that hardens into a barrier on top of the soil, but he believes that other mulches can have similar effects in terms of reducing water use.

“The sprayed mulch offers a very good barrier, but if you put on enough grass mulch or alfalfa mulch or shredded paper mulch, it comes out pretty close to the same thing, depending on how thick you have it and how you maintain it,” Hogue said. “You would have something in the same ­ballpark.”

Hogue conducted the paper mulch research in 2001 at the Summerland research center in British Columbia using a lysimeter to calculate the trees’ actual evapotranspiration rates. Mulched young trees, with a trunk diameter of one inch, used 54 percent less water than non­mulched trees. For older trees with a trunk diameter of two inches, the mulch reduced water use by 15 percent.

The paper mulch was a byproduct of paper recycling that growers could obtain just for the cost of transportation. At one time, Hogue thought that it might be useful for controlling weeds in organic orchards, which has been a major focus of his work, but he found that it contained a surfactant used for de-inking newspaper that was not environmentally friendly.

Climate change

Hogue, who is still conducting research, believes that other types of mulches could play an important role in addressing the effects of climate change and limited water supplies. He is experimenting with growing a cover crop of legumes and grasses in the orchard alley and moving the mowed clippings into the tree rows as a mulch. The legume contributes nitrogen for the trees, while the grass is more persistent as a cover crop, will provide more biomass, and stands up better to being driven over by tractors.

However, because of the tight row spacings in modern orchards, less ground is available for producing biomass than in traditional widely spaced plantings. And, many orchards nowadays are irrigated by drip, which also makes growing a cover crop between the rows more problematic. Hogue said some growers are using plants that can grow with little water, such as Canada mix, which includes crested wheatgrass, pubescent wheatgrass, and perennial ryegrass seeds.

David Granatstein, with Washington State University’s Center for Sustaining Agriculture and Natural Resources, said that in his trials, a wood chip mulch resulted in a 20 to 25 percent reduction in irrigation water use over the season compared with bare ground. However, he’s also interested in mulches that can be generated in the orchard, rather than brought in from outside. He’s testing the idea of using tree prunings as a source of mulch material. In one orchard this spring, prunings were flail mowed in the alley, and the chipped wood and other plant residue was swept into the tree row. “It actually made a good mulch,” he reported.

Granatstein has also tested  “living mulches”  planted in the tree row.  Dwarf white clover looked particularly promising, using less water than the bare ground control.  But all mulches have trade offs, he noted. On the positive side, the living mulch fixed nitrogen in the soil, kept weeds down, and did not appear to compete with mature trees. However, it did result in a much higher vole ­population.

“We keep coming back to voles being a major barrier to some of these alternative orchard floor management ideas,” he said.

Granatstein would like to find a legume species that is also repellent to voles and would be suitable as a living mulch. For example, sweet clover contains a coumarin-like compound, which is avoided by voles, but it grows tall and would block irrigation sprinklers. It also has an aggressive tap root that could compete with the tree roots. Hogue said other ways to address vole problems are to encourage predators, by providing nesting boxes for owls, for example, or to devise a way to remove the mulch from under the trees before the winter.

Boosts growth

Over the years, Hogue has experimented with many different types of mulches used for a variety of purposes. He found that any good mulch that doesn’t have negative side effects is likely to boost tree growth (measured by trunk size) and increase the yield potential. He recommends using mulches on young trees.

In terms of the chemical characteristics of the soil, municipal biosolids  provided the greatest carbon and nitrogen boost of all the mulches in his trials. The biosolids combined with a  paper mulch had the greatest positive effect on soil structure, judging by the water infiltration rate, while a black polypropylene geotextile mulch had the worst effect. The polypropylene makes no contribution to organic matter and causes wide fluctuations in the temperature of the soil surface, Hogue said. An absence of earthworms under the geotextile changed soil texture for the worse.

Compost scores highly in terms of its influence on soil nutrients, organic matter, water infiltration, and water retention, but its greatest drawback is its relatively high cost unless it can be made at the orchard, Hogue said. In his own orchard, where he did a lot of his own composting, he applied compost as a mulch on areas with poor or rocky soil with extremely good results. Trees that were falling behind caught up with the rest after the compost treatment. “A lot of the poor soils are coarse and don’t have good water retention,” he said. “It’s important to have something that will keep whatever moisture you’re putting on there.”

Another drawback of compost as a mulch is that it can contain weed seeds and augment the weed problem in an orchard. Putting a compost amendment down with a different mulch over the top would be the best combination, Hogue said. “If you have the mulch that will seal it in there, you get a combination of increased fertility with the weed control.”