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WSU-TFREC Orchard Management Forum

Weed Better Spray in the Fall

Tim Smith -- WSU Extension, North Central Washington

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I've never met anybody that applied their weed control in the Fall and wasn't happy that they had done so the next Spring. It is always easier to put off something that can be done later. However, if you decide to try to control weeds next April or May, you are moving an operation from a flexible period in the Fall to a very busy time in the Spring. You can very quickly finish the season by applying your contact + residual mix, then get on to the three major reasons why we tend to avoid work post-harvest: 1. Deer, 2. Elk, and 3. upland game birds.

Why control weeds?

They just grow right back. It's a good thing we don't think the same way when it comes to pruning the trees. Managing the growth of plants under the trees is a yearly effort, but necessary. Why? Without much explanation, let's list some reasons:

Water management: weeds use water, lots of it. Keeping your weed strip fairly clear of weeds saves 50,000 to 100,000 gallons of water per acre per year. Water that goes through the trees, producing quality fruit, not weeds. Weeds also tangle sprinkler heads, stopping them from turning. Most importantly, they block the sprinkler pattern, causing uneven, therefore, inefficient irrigation.

Mouse management: Thinking about controlling mice in the Fall is something like thinking about the status of your boats drain plug after you have reached the middle of the lake. It's a useful thought, but maybe a bit late for best effect. Mice love weedy orchards. They can live and raise families all summer, and mouse control efforts may not be adequate under these extreme conditions. We must keep mouse cover to a minimum all season if we are to get the best effect in mouse control on bad mice seasons.

Haul out the sprayer and get it ready

O.K., so let's say you are now convinced that weed control is a good idea this Fall. If your sprayer is ready when you have the proper spray conditions, you can spray about 2 orchard acres per hour (2 MPH, 20 foot rows). You don't make much progress when you're tinkering with the sprayer until 10 or noon, then the wind comes up.

Uneven application is the most likely cause of poor weed control. Spend some time getting the boom set up right. Most people use flat fan nozzles, which is the best idea for residual products. You must have even application to assure tree safety. Most people use 8002 or 8004 nozzles in the fall, for 50 or 100 gallons per acre. I recommend the lower gallonage, as you will get better effect with lower rates of contact or systemic herbicides.

Take all the nozzles apart, clean them, with special attention to the strainers. Are all the strainers the same design? The same mesh (50 or 100)? Clean and flush the tank and line prior to reassembly.

Adjust the boom height until you have about 1/3 if each nozzle pattern overlapping the neighboring spray pattern on either side.

Use a tall, skinny container to catch the spray from each nozzle for one minute. At about 35--40 psi, the 8002 should put out about 25.6 ounces (.2 gallon) per minute, the 8004 about 51.2 ounces. If they are below this amount, take them apart and reclean them. If the 8002 is 2 or 3 ounces over the proper amount, or the 8004 is 3 or 4 ounces over, replace it. It is wasting more product per acre than it is worth, plus the spray pattern will be uneven. You can not eyeball a pattern that is only 10--20 percent uneven, trust your measurements and throw that worn nozzle into the trash.

When applying the product, keep systemic products off of young tree trunks and low hanging branches. Remember, if you wet the weed, then the weed slaps against sensitive parts of your tree, damage is possible. Stop spraying if the wind is over about 5--7 MPH. We have many calm days in the Fall (not so the Spring), so there is no need to push the conditions. If weed growth is heavy, use higher spray gallonage, and drive a bit slower. Dropping from 2.5 to 2 MPH will only cost you about 2 or 3 minutes per orchard acre.

Once the job is done you can pursue the Fall recreational activity of your choice with a conscience as clean as your orchard will look next May.

--Tim Smith

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Wenatchee WA, Friday, October 20, 2000