Don’t skimp on site prep
This is the only opportunity to control replant disease.
There is no part of site preparation that a grower can afford to be lackadaisical about, says Dale England (right), who runs a fumigation business with his brother Len.
Site preparation, including fumigation, is an opportunity to optimize tree growth for the entire life of the orchard you’re planting.
Site preparation begins with tree removal. In the past, when fumigation was less common, growers used to cut trees down to the stump and use the wood for firewood. They then came in with a bulldozer to get the tree roots out before replanting, recalls Dale England, who operates Custom Orchard Fumigation in Manson, Washington, with his brother Len.
The Englands recommend having the entire trees, roots and all, removed by an excavation contractor. Then, the ground should be ripped about two feet deep, which requires a bulldozer of some sort. For broadcast fumigation, the soil should be ripped every four to six feet in both directions, and a crew should be brought in to clean up whatever debris surfaces. Dale said the ground can be ripped as many as five or six times, but more than twice is probably not cost efficient.
Ripping the soil after tree removal is one of the most important parts of site preparation, Len said. In the case of the fumigant Telone C-17 (a liquid that vaporizes in the soil), it will also ensure that the fumigant is applied deep enough in the ground and is able to spread through the soil and control the organisms that cause replant disease.
“With the fumigation, we’re pulling the shanks through the soil, and as we do so, it’s like raking underneath the soil level, and as those shanks move along and hit roots or pipes, we have to lift up our shanks, and that doesn’t place the chemical down where it should be,” Dale said. “In addition, we don’t get a good seal.”
“Ripping is an essential part that should not be overlooked,” Len added. “As clean as anyone can get it, the better. It’s to their benefit to have it as clean as possible. Ripping is just as important as tree selection and irrigation selection. There isn’t a step in the process that isn’t important.”
The Englands, who have been fumigating orchards for 20 years, apply only Telone C-17 (dichloropropene and chloropicrin), which is a broad-spectrum fumigant that controls soil diseases as well as nematodes and other pests. Telone C-35, which contains more chloropicrin, will also do the job, but it is more expensive and provides no additional benefits, the Englands said.
They have two applicators—one six-feet wide and the other ten-feet wide— that they designed themselves to do the fumigation process in a single pass. The applicators have injectors that put the fumigant 18 inches into the ground, with tillagers behind the injectors, and a packer, in the form of a roller, that seals the ground and prevents the fumigant from venting before it’s done its job.
Whether the grower intends to plant nursery trees or grow bench grafts or sleeping eyes in place, the soil preparation and fumigation is the same. When it’s time to fumigate, the ground should look as if it’s ready to plant.
Some growers have just the tree rows fumigated, rather than the entire area, and in that case, ripping in one direction would be sufficient, Dale said. “Farmers who are trying to do a quality job but not spend extra money will just treat their tree rows.”
Strip fumigation is cheaper than covering the whole area, but the grower must know where the trees will be planted and mark the rows, which is an additional step and cost. And some people feel that if the replant organisms in the soil are particularly mobile, they could move into the tree row from the untreated strips. The Englands use a six-foot-wide applicator for the strip treatment, and Dale said when the row spacing is tight, leaving little untreated area between the rows, it might be best just to broadcast the area.
With either broadcast or strip fumigation, the effects only last for a period of time. Fumigation simply gives the young trees a chance to grow for a couple of years before the organisms reinfest the soil, Len said. For this reason, fumigation should be done months or weeks before replanting, but not years. If an orchard site is being replanted in stages over a number of years, it should not be fumigated all at once.
The fumigant Vapam (metam sodium) is applied with water, preferably while the irrigation system is operating. Telone C-17 does not require water, but the soil must not be too dry. Dale said the moisture level is just right if you can make a ball from the soil in your hand, throw it in the air six or eight inches, and it breaks apart when you catch it, rather than staying in a muddy clump. If the soil is too dry, the fumigant won’t be sealed in, and if it’s too wet, the vaporized fumigant won’t move well enough in the soil to do a thorough job. The soil temperature should be at least 40°F at a depth of 18 inches.
Dale warned that after fumigating, the ground is loose and therefore has less water-holding capacity than it had before. It’s important, therefore, not to delay too long before irrigating the newly planted trees.
“If you rip, you fumigate, you plant, and then you don’t put your sprinkler system in until the middle of June, you probably have stunted your trees by then. There isn’t a step you can be lackadaisical or late on, and expect the trees to grow.”
Len urged growers not to miss the opportunity to fumigate before planting the orchard. “You can put the best trees in the world in the ground, and if the ground is not good, and you ought to have fumigated, the trees are not going to grow anyway.”