Weeds cause 34 percent of losses in global crop production. As climate change intensifies, weed pressure is expected to increase. Higher temperatures and changes in precipitation patterns are already creating new conditions worldwide. Although higher levels of CO2 can boost growth and yields in some crops, rising atmospheric CO2 can also make the widely used herbicide glyphosate less effective and boost weed growth, adding to the potential for increased competition between crops and weeds. Several weed species benefit more than crops from higher temperatures and CO2 levels. These include kudzu, a vine that attacks trees and shrubs; Johnson grass, which attacks numerous crops; morning glory annual vines, which attack trees, shrubs, field and vegetable crops; and velvetleaf, the bane of corn.

The immediate concern is managing or eliminating weed pressure this season. You can also benefit from connecting with ag extension programs, researchers at land-grant universities and other resources to learn what to expect from weeds in the changing climate and how to adapt. The first step is to define weed. Today, farmers are growing naturalized and native species as crops. Dandelion greens and burdock are two examples of plants commonly known as weeds that are now cultivated commercially.

Both crops command prices comparable to other health products and sell in health food stores like Whole Foods and in major grocery store chains like Hannaford and Stop & Shop. Cornell University Weed Ecologist Antonio DiTommaso said a weed is any plant that is out of place and competes with the cash crop for resources. Sweet corn that pops up in a field of broccoli could be considered a weed. Thus, my crop may be your weed and vice versa.

Managing weeds

Herbicides have become the tool of choice in intensive farming because the weed control effect of tillage has proven to be insufficient in the long term, according to a report released by the United Nations Food and Agriculture Organization (FAO). “The problem of tillage is that by creating a good seedbed for the seeds, it creates the same conditions for the weeds. While weed seeds are buried deeply with the mouldboard plough, the same plough brings to the surface the weed seeds that had been buried the season before… Weeds propagating through sprouts or roots can even be multiplied by tillage implements, which only cut and mix them with the soil, so that the number of potential weed plants is increased. Through soil carried with tillage implements from one field to another, the weed population is also spread throughout the entire farmland,” the report stated.

Tilling for weed control is not the ultimate answer, experts noted. It is often not necessary to eradicate the weeds completely, but only to avoid the setting of seeds and competition with the crop. Leaving weeds in a crop at a stage where the crop can suppress them and where there is no damage or problem for the harvest can help with managing other pests, such as termites or ants, which, in the absence of weeds, would damage the crop. According to DiTommaso, weeds can also protect and restore soil, and effectively provide “surgery” when areas are torn up, burned or otherwise altered.

Completely nonchemical weed control is possible, according to the FAO. As reported in earlier columns, such practices are already successfully applied in commercial farming. Weed germination declines in soil that has not been tilled for several years. The superficial weed-seed bank depletes. If no new seeds are added, seeds still remaining in the soil will not germinate as they will not receive the light stimulus or soil temperature fluctuations needed for germination. This happens because crop leaves filter out specific wavelengths of light that “tell” the weed seeds in the soil it is not in their best interest to germinate. “It turns off a specific pigment, a phytochrome, from turning germination on, and indicates to the weed seeds: ‘If you germinate, you’ll have to compete with this crop above you,'” explained DiTommaso.

In bare, pristine soil, that light is not intercepted and the entire visible wavelength goes right through. “That’s why I encourage vegetable growers who want to know how to manage weeds to minimize how much bare soil is there and limit the amount of time bare soil is there,” DiTommaso said. “If you’ve planted and you know you have weed seeds there, try to get the soil covered as fast as possible by either the crop leaves or by a cover crop. The weed seeds can detect the difference in how much far infrared light is getting through. You need to know; we’re not just saying grow cover crops to physically keep weeds out but also to reduce their germination. That’s important.”

DiTommaso studies the impact of climate change, principally temperature and precipitation alterations, on the impact of weeds in cropping systems and natural areas. The increased frequency of extreme weather events (heavy rainfall, early frost, warm spells) in the Northeast interests the researcher because such events are difficult to defend against. “How do weeds compete with crops during extreme events?” he asked. “I’m looking at species that are a southern weed species, like Sorghum halepense (Johnson grass) and Ipomoea hederacea (ivy-leaf morning glory). These are major weeds in the South and Mid-Atlantic crops. Our concern is, with warmer temperatures, they’ll be able to establish in places like the Northeast where they previously didn’t do well. These species are more able to adapt to extremes than crops.”

Johnson grass is a perennial weed that’s closely related to sorghum. In the northern United States. and Canada, it has adapted to the colder climate by becoming an annual plant. In the South, it’s still a perennial that survives by spreading rhizomes. In the North, it survives by seed. “This ability to adapt to a new environment and eventually proliferate in it is something we should be looking for,” DiTommaso said. “I look at weeds as a genetic resource for us because usually their genetics aren’t very restricted. Many of our crops are the same hybrid and the same line, so if it’s susceptible to disease, we lose everything. Weed species, even individuals within the same species, can have very different genetics – like an apple tree. We have to graft if we want to get the same apple.”

DiTommaso is not a breeder though he encourages breeders to ask: What can we learn from these highly adaptable weeds that can help our crops? “If one looks at what makes weeds what they are, their ability to adapt, particularly in ag systems, to disturbance is impressive,” he said. Traits like fast growth, ability to grow in soils of differing fertility, drought tolerance, and resistance to certain diseases and insect pests have been bred out of crops through over-domestication, as breeders focused mostly on yield. “These traits that make these plants so problematic might be some things that plant breeders should be thinking about. They’re wild relatives of our crops. There’s something to be said about having some weeds around still. They do contain some traits that hopefully we’d be able to incorporate in some of our crops.”