Blueberry Council of Missouri
Blueberry Council of Missouri Newsletter
WEEDS, THEY NEED TO DIE
By Howard Thompson
November 2009
After only 6 years of growing blueberries, there are many areas of blueberries cultivation that I am still a learner but when it comes to weeds and hoofed vermin, I am fairly comfortable talking about either. Since landmines and automatic weapons with computer driven night vision targeting devices are illegal, I will look at the legal weeds and their control. By definition a weed is an undesired plant in an undesired location. This would make a rose a weed if it was in a corn field or a blueberry patch. Weeds compete with the production plants for nutrition, water and sunlight. Weeds can supply cover and nutrition to insects that may find you plants very tasty and carry diseases that could affect you plants. Also if the weeds are tall, they will interfere with your harvest. This makes weed control an important part of having healthy and productive plants. A well weeded berry patch can have twice the production of a weedy patch which means weed control will pay for itself.
Although complete eradication of weeds is not possible, it starts even before planting. Adjusting the pH will select out some of the weeds. Tilling and the use of nonspecific herbicide for a few years before the actual planting will minimize the perennial weeds which would leave mostly annuals. If seed production is controlled the propagation of the weeds is controlled. Then with a heavy application of mulch after planting, the remaining seeds are smothered. In my experience, mulch is much more effective on controlling weeds in new plantings when the plantings are in the fall.
Weeds can be categorized several ways: by early leaf formation/cotyledon structure (monocot= grasses and nutsedges, dicots=broadleafed) and by survivability (annuals, biannual and perennials). No matter how you categorize the weeds, there are those that are a major nuisance while others have a minimal impact on blueberries which makes them not worth the bother. Weed control can be divided into in-row and between-row management. Between row management is the easiest once your plants are established. Seeding this area in the fall with a perennial grass like fescue will help control annual weeds and prevent erosion. The between row space is easy to care for by simply mowing. The mowing controls seed production of the fescue and of the weeds. Unfortunately grass between the rows does increase the habitat for Japanese beetles and June bugs. It has been reported that these beetles do not like patches of clover which could make a short white clover or birdfoots tri-fold an alternative to grass between the rows. This is in contradiction to the recommendation presented by Rutgers University which strongly recommends against the addition of clover into the seed because it is difficult to control (keep out of the plant rows)
The goal of in-row weed management is to establish and maintain a 3-4 foot “weed-free” row with the blueberry plants in the middle of this space. In-row management is literally where the money is at and can be divided into mechanical and chemical means. The mechanical means of control is the old fashion method of just pulling the weed out by it roots or tilling. Unless you have a small patch, this is very time consuming, painful and may not be that effective other than for use as spot weeding. Cultivating around the base of the plant with gas powered tiller is strongly not recommended because the root systems of blueberry plant are TOO SHALLOW to use these devices without damaging the roots, You might be able to use a hoe if you are careful. The third mechanical means is to use fires to literal burn up the weeds. This method is not without major risk: burning up your entire patch. Since I have already had to call the fire department over a burn pile that turned into a “forest fire”, my wife will not let me play with fire. The final mechanical method is the regular application of sawdust or wood chip mulch. The mulch will also help with moisture control during the summer while slowly fertilizing the plants. From a mechanical point of view, I find mulching and spot hand weeding acceptable. This means adding several inches of mulch every fall/ winter.
The use of chemicals for weed control is divided into pre-emergent and post-emergent that covers nearly two dozen mechanisms of actions to kill plants. This accounts for what seems like an endless list of herbicides. In both cases, germination usually occurs but with the pre-emergent herbicide, the new seedling plants are killed shortly after germination but before the cotyledon break out of the ground due to a residual in the soil. The pre-emergent herbicides are effective against annuals and some broadleaf weeds. The pre-emergent efficiency is dependent on the soil type since it has to remain in the soil to work. This means more is needed on sandy soil and less on compact clay.
The post-emergent herbicide damages plants on contact or is carried to the entire plant. They are most effective against plants that are less then 6-8 inches tall. These herbicides have to dry several hours on the plant before rain which can decrease their effectiveness. Young plants and recently cultivated plants are at increase risk of unintended killing because their roots may be exposed or have not developed a “protective” bark. Due the many modes of action, post-emergent herbicides can be selective effecting only a certain group of plants or a non-selective which kill almost everything. No matter what type of herbicide that you use, there is a chance of the weed either being resistant to it or developing resistance. According a presentation last year by the Missouri extension, about 4% of the land in Missouri has some herbicide resistant weeds. This suggests a need to rotate you herbicides.
In reviewing the sources in preparation for this article, I noted several cautions that need to be observed particularly for the post-emergent herbicide. They all have warning about using them less the 2-4 weeks before harvest but the instructions do not give a reason for this application to harvest interval. In my experience with these three herbicides, it does take 1-2 weeks from application to clearly see the die-back. Also they appear to keep the weeds down for 2-4 weeks. When I mentioned this concern to my wife, she said, “Don’t eat the grass”. This writer has requested the scientific reasoning for the application to harvest interval on Round-up, Rely and Poast from the manufacturer but at the time of this writing have not received a response.
The other generalized warning has to do with the weather and attire. You do not want to apply when the humidity is low and the temperature is high. This contributes to the formation of a smaller droplet thereby increasing drift which can kill the productive plants. This is similar to spraying on a windy day without a shield. Since your feet and legs are likely in the spray area, this means at least boots, socks and long pants. Since you are handling a poison, gloves (preferably not leather), long sleeves and eye protection are to be used. All clothing needs to be washed after use and before wearing again. Some of the herbicides also suggest waterproof coveralls which mean you need to READ THE INSTRUCTIONS for you safety.
In my discussion with many of growers the primary means of weed control is a combination of mechanical and post-emergent herbicides. The primary mechanical means entails simply mowing the grass between the rows short and regular application of tree based mulch (straw has TOO many seeds). The herbicide of choice is glyphosate (Round-up, Kilz-All) which is non-selective and is carried down the plant to the roots. Poast and Rely have also been mention by the growers. Poast is selective to grasses but Poast should not be use during late July and August when we are in our drought period. It does not work well on heat stressed plants. Rely is non-selective but is not transported in the plant which means it only kills what it touches. Several of the members have been thinking about using one of the pre-emergent herbicides. Other members refuse to use pre-emergent herbicide because they feel they have problems when they have to replant in a previously treated row.
At this point in my blueberry career, I have two expertises: weed control and praying to the blueberry gods every October when I plant. I have my plants set on breams which are nearly 12 inches high. The plants are mulched with 3-4” of saw dust and 2” of wood chips. There is a 5-6 feet of grass between the edges of the breams. This sets up a natural demarcation which it easier for herbicide application. Although I would like to kill everything in the row that is not a blueberry plant, but that is not practical or realistic. I have come to peace with some weeds like Henbit, purple deadnettle, catchweed and chickweed that act as winter cover crops and die out before blue berries draw from the soil and before most of the fertilizer has been applied. I also have come to peace with the clover which put nitrogen into the soil. The caveat here is that the weeds are can not be overwhelming. My biggest nemeses are the grasses (Bermuda, fescue and crabgrass) and the sedges. I have settled into a routine of mowing the grass to keep a height of 2-3” and using a 4 gallon backpack sprayer with a fan shield about once every 4 to 6 weeks along the edges of the grass and the breams. Because of the fan shield, I can get very close to the plants.
My spraying starts the first of week of MAY which usually still gives me time for a second application shortly after Memorial Day which is usually nearly 2 weeks before harvest and carries me through the harvest. My last spray is usually mid September unless we had an extremely wet summer which may require another spray. I rotate glyphosate, Relay (glufosinate) and Poast (sethoxydim). In the late fall/early winter, I apply more sawdust.
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References:
- Bradley K, Johnson B, Smeda R & Boerbom . IPM: Practical Weed Science for the field scout, Corn and Soybeans IPM1007 Univ Mo Ext 2007
- Fertility and weed management in organic High bush blueberry production. Organic Agriculture center of Canada{oacc.into}, 2006)
- Fishell, et.al. Early Spring Weed of No-till Production, Univ Mo- Extension Publications .2000
- Ward,D & Majek,B. 2007 Blueberry Weed Control Recommendation of New Jersey. Rutgers Coop Ext NJ Agri Exper Station. Rutgers, the State University of NJ, New Brunswick
- Williamson,J. “Weed Management in Blueberries. “ HS90 Dept Horticul Sc. Flo Coop Extent.Service, edis.ifas.edu, 2007
- http://www.kswildflower.org Good source to locate weeds.