Biology is the key to controlling weeds

by Kristen M. Castrataro
The Massachusetts Cultivated Blueberry Association held its annual spring seminar at Vandervalk Farm & Winery in Mendon, MA on June 17, 2017. Two dozen attendees from Massachusetts and Rhode Island gathered to learn about weed management in blueberry plantings from Hilary Sandler, UMASS Cranberry Station and State IPM Coordinator.
“What is a weed?” asked Sandler. “A weed is a plant that is growing contrary to what we want it to do…It’s really in the eye of the beholder.” Joe-Pye weed, a plant which can become a plague in pastures and hay fields, is commonly grown in the landscape as an ornamental. It is what Sandler terms “a good weed.”
Good weeds grow rapidly from seeds. They reproduce quickly and vigorously. The reproductive potential of weeds is epitomized by pigweed (also known as redroot). A single pigweed plant can produce up to 117,400 seeds per plant.
It is one thing to produce seeds. It is another thing to disperse seed widely enough to ensure maximum survival. Some plants have such admirable dispersal capabilities they have inspired inventions we now take for granted. “We steal from nature,” Sandler said.
Should the seeds land in an unfavorable location or face adverse conditions, all is not lost. Many plants create seeds that germinate over time — the majority in year one, a lesser amount in year two and a still smaller amount in year three. If one year is too wet or too dry, the weeds have a chance to make up for lost time the next year. Sandler credited this staggered germination with creating the phrase, “A year of weed equals seven of seed.”
Some seeds even possess the ability to move in and out of dormancy within a single season. Initial conditions may initiate germination and then suddenly change. The seed, sensing the changing environment, can halt germination, move back into dormancy and wait for better days.
Not all plants reproduce via seeds, however. Many perennials reproduce vegetatively, spreading out and creating daughter plants that are still connected to the main plant. Some weeds creep across the ground via stolons and create daughters by forming roots at places where the plant touches the ground. Strawberries reproduce via stolons.
Others form rhizomes, which act like stolons except the plant spreads under the ground and emerges from the soil at a seemingly random location. Then there are weeds that proliferate via bulbs or nutlets, like bittersweet.
The most insidious weeds are what Sandler calls “two-barreled.” They can reproduce both sexually (through seeds) and vegetatively. Virginia creeper, yellow nutsedge, and cinquefoil are a few examples.
Once they have spread, “good weeds” are able to adapt to a variety of environmental conditions and thrive in bare or disturbed soils.
The very characteristics which allow weeds to survive and proliferate are the ones that make them difficult to combat. Creeping and spreading weeds can be incredibly resistant to manual control because daughters can survive even if separated from the mother. Chemical control can be less effective as well due to the vast system of vegetation the herbicide must travel in order to eradicate the family.
Weeds are not only resilient. They also have something in common with the main crop: both are plants. It may seem self-evident but weed control presents more hazards than insect control because the very treatments which kill the weeds can often kill the cash crop.
To maximize damage to the weeds and minimize damage to the crop, it is critical to learn as much about the enemy as possible. Proper identification is essential. Sandler recommends several resources. Newcomb’s Wildflower Guide is a guide based on the characteristics of the plant’s flowers. Weeds of the Northeast by Richard H. Uva uses a different key that relies on vegetative characteristics, enabling growers to determine the identity of plants when they are not in flower.
Sandler was responsible for introducing another guide to American growers: Identification Guide for Weeds in Cranberries published by CRAAQ. The guide was originally published in French. A feature Sandler especially liked was the inclusion of Priority Groups. Based on rate of spread, effectiveness and impact on yield, weeds were assigned to one of four groups, with one being the most impactful on yield and four being the least.
Working with a bilingual cranberry grower and the French authors, Sandler helped adjust the rating system. The English edition evaluates four categories: Crop impact, biological family/type (which includes the number of seeds produced), invasive/reproductive capacity and adaptation to habitat (measured in part by ease of pulling).
For each weed, each category receives a number from one to eight, with eight being the most noxious. The weed is then assigned a box score based on the total points of the four categories. One box is the least problematic and four boxes is the most. This approach was designed to work as an IPM aid, allowing growers to determine which weeds need to be treated and which can be ignored because they pose little to no economic or long-term impact.
What Sandler likes most about the new version is its flexibility. “You can tailor it to how it works for you,” she said. CRAAQ’s publication may rate a weed as one box, but a particular farmer may feel that it is more problematic on his farm. He can self-score the weed and perhaps find it becomes a three or four box weed demanding treatment.
“Plants can definitely behave differently in different locations,” observed Sandler. The variability can be affected by local climate, soil type, soil health and even the operator. She told the growers, “You play a part in the whole ecosystem sense of your farm.”
After identifying the weed, the most important step is learning about the plant’s life cycle because certain strategies will only be effective on certain weeds at certain times.
Manual strategies include mowing, flaming, cultivating and pulling. These strategies can be effective for many weeds, but the timing is important. Mowing, for example, should be performed before the plant begins to set seeds and should be done with a catcher. Even seeds that appear too young to reproduce can sometimes perform. “Otherwise,” cautioned Sandler, “you’re just being a dispersal agent.”
Flaming can be performed with infrared flamers or open flame. The latter is faster, but open flame can more easily get out of control. Sandler recommends irrigating first to minimize the potential for wildfires. She also warned against flaming poison ivy, as the resulting reactions to eyes and lungs can be devastating.
Cultivating and pulling are especially successful with simple or solitary weeds. Those same techniques can actually increase the amount of weeds for plants with rhizomes, stolons, bulbs or nutlets as they can reproduce with only tiny portions of the plant intact.
Then there are herbicides. Even herbicides work better in certain situations than others. Pre-emergents are preferred for summer annuals. Post-emergents can work as a follow-up to a pre-emergent or as a stand-alone emergency treatment.
For perennials, herbicides such as glyphosate are actually most effective in the fall. Glyphosate moves with the carbohydrates in the plant’s xylem and phloem. In the fall, the carbohydrates are moving from shoots to roots, so the herbicide literally attacks the plant from the ground up.
As with any pesticide applications, Sandler reminded growers to carefully review and follow product labels. The new “WPS has become more rigorous,” she said.
Controlling weeds is an important — and sometimes frustrating — part of farming. According to Sandler, there is also a bright side to having weeds: they are an excellent diagnostic tool for soil health. Healthy weeds indicate fertile growing conditions. Additionally, vigorous weeds respond better to chemical treatments. “The healthier your weed is at the time of application, the better control you’re going to get,” Sandler said.
With proper plant identification, a firm understanding of weed biology and accurate selection of management techniques, farmers should be able to successfully combat even the most aggressive weed pests.

2017-06-30T10:46:19+00:00June 30, 2017|Grower East, Grower Midwest, Grower West|0 Comments

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