In the Organic Orchard

Summer 2010
Well-mulched tree
A well mulched apple tree at MOFGA’s Common Ground Education Center. English photo.

By C.J. Walke

One of the foundations of organic agriculture is the belief that healthy soils grow healthy crops. We can spend a lot of time battling insects and diseases in the orchard, but if the tree is not rooted in a healthy, biologically active soil, then it will always be under some stress. Lacking nutrients, moisture and/or good aeration, the health of the tree will be compromised, making it more susceptible to pest pressures.

This new MOF&G column will focus on home-scale organic orcharding, using MOFGA’s educational orchards as an example and beginning at the ground level.

Understory Management – The “Orchard Floor”

In the MOFGA orchards I maintain an 18- to 24-inch weeded area around the tree trunk. This exposes the trunks for easy inspection for insect pests (discussed below) and limits shelter available to voles from predators (hawks); voles like to eat the tender bark and can quickly girdle and kill a young tree. It also provides a clear area where I can gently scratch in any needed soil amendments within the drip line of the tree, where most of the tree’s feeder roots are concentrated.

Eliminating grass or sod within the drip line also reduces competition for moisture and nutrients, since sod is very dense and high in carbon dioxide, causing the tree’s feeder roots to run deeper and work harder. To smother the sod, I overlap layers of newspaper and cover them with mulch hay or ramial wood chips (chips from hardwood tree branches that are 2.5 inches in diameter or smaller). This is very effective. After a year, most of the grass under the mulch is dead and anything that grew in the mulch is loosely rooted in the newspaper, not the soil, and is easily pulled. This mulching method also helps manage quackgrass, a persistent weed with rhizomes that can grow many feet long, since the rhizomes tend to run between layers of newspaper and are easily pulled. (Note for certified organic growers: Only newspaper without glossy or colored inks is allowed as mulch. [NOP 205.601 (b)(2)(i)])

Ramial wood chips, which can include healthy orchard prunings, are full of nutrients in buds, leaves and cambium layers, all of which recycle nutrients and help build the orchard soil. I don’t recommend using bark mulch, which tends to come from softwood sawlogs and is often dyed different colors. Softwood tissue is broken down by brown rots that tend to have allelopathic properties that deter hardwood growth (your fruit trees!) and encourage softwood growth. Any orchard prunings infected with fungal disease should be removed from the orchard and burned, not chipped, to prevent the spread of fungal inoculum.

Doing this work beneath the fruit tree builds a healthy, biologically active soil that supports fungal dominance, compared with a garden soil that favors bacterial dominance. In our vegetable gardens we frequently cultivate the soil to incorporate compost and green manures. Limited cultivation also mixes air into the soil, enabling aerobic soil microbes to thrive and break down organic matter into available nutrients for vegetable crops.

On the orchard floor, I try to replicate the floor of the forest edge, where leaves drop in autumn, dead branches fall, plants die back, and organic matter piles up and is decomposed by fungi, without any tillage. If you find mushrooms or puffballs (the fruiting bodies of certain fungi) on your orchard floor, you are on track to a diverse, biologically active orchard soil!

Pest Management – Round-headed Apple Borer

Numerous insect pests can occur in the orchard, ranging from those with limited impact that don’t require specific control to those that can render fruit inedible or even kill young trees. Identifying specific insects and understanding their biology is key to managing pests in the organic orchard and will determine the most effective time to intervene with cultural methods or sprays.

The round-headed apple borer, a major pest of young apple trees, can seriously damage or kill trees. The adult beetle is about an inch long with longitudinal brown and white stripes, but you will most likely never see the beetle. The adult makes a small slit in the bark of the trunk at the soil surface and deposits an egg in the slit. As the larva develops, it eats the cambium layer just under the bark; you can see its moist, orange/brown, sawdust-like frass coming from the hole in the bark where the egg was laid. (Frass is the waste that larvae excrete after eating plant tissue.) The larva lives for two or three years in the tree, creating roughly a dime-sized cavity in the first year, but excavates around the tree, even into the roots, the following two years, severely weakening or killing the tree.

In early June, before adult borers start laying eggs in mid- to late June, I mix Surround WP (a refined kaolin clay product), white milk paint (available online and from the Green Store in Belfast) and water to a thickened white paste and slather it on the trunks of apple trees from just below the soil line up to the first scaffolding branches. The slurry doesn’t kill the beetles or borers, but Surround is an irritant that flakes off onto pests, creates discomfort and deters them from laying eggs in a seemingly inhospitable environment. Also, the white color of the slurry, coupled with the understory management discussed above, makes the orange/brown frass a lot easier to see. When I find frass, I use a sharp knife to shave off a small slice of bark and clean out the cavity until I find the larva. For older larvae I also poke a small piece of wire into borer tunnels that are too deep for the knife; a slight squishing sensation is the sign of success!

C.J. Walke is MOFGA’s landscape coordinator, organic orchardist and librarian. His future columns will focus on organic insect and disease management in the small or home orchard, but will also apply to larger orchards. Readers may contact Walke at (207) 568-4142, [email protected], or talk with him at one of MOFGA’s educational events.

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