Background 

Unlike crops, which have been selected for uniform emergence, weed species have evolved variability in their emergence timing. Even seeds maturing on the same plant may germinate at different times. This “bet-hedging” strategy, with which a weed avoids putting all its “seed in one basket” of emergence timing, enables weeds to escape control measures. Post-emergence management carried out too early will yield low returns for the effort, investment, and ecological cost of the management (herbicide off-target effects, soil compaction, etc.), as weed seeds that have yet to germinate are often unaffected. On the other hand, most weed management tactics such as post-emergent herbicides and cultivation are most effective when weeds are small, at or near the seedling stage (Norsworthy et al. 2012). Therefore, delayed management operations may result in reduced weed control efficacy and greater yield loss (Davis 2006). Thus, weed management should be timed to occur soon after the emergence of most problem weeds.   

The timing of weed emergence is not easy to predict because it reflects a multitude of speciesspecific parameters (e.g., base temperature, base water potential) and environmental factors (e.g., soil temperature and moisture). While many weedy species can germinate under a broad range of environmental conditions (Baker 1974), weeds in most agricultural systems have evolved to germinate when the appropriate microenvironmental cues are detected (Long et al 2016) to maximize fitness during favorable environmental conditions (Grime 1977). Weed growth, competitiveness, and fecundity are strongly influenced by emergence timing relative to the crop (Hartzler et al 2004, Wu et al 2014). Temperature is a particularly dominant influence on emergence timing in many annual weed species (Werle et al 214a; Werle et al 2014b). Understanding how changes in winter temperatures affect weed germination and emergence is crucial to designing weed management systems that are resilient to climate change. 

Changing winters in northern climates.

In northern latitudes, winters are warming faster than any other season (Hayhoe et al 2007, Karmalkar et al 2017, Brown et al 2010, USGCRP 2017). Winters are also shortening as spring advances earlier in the year and autumn senescence is delayed, resulting in a longer growing season (Piao et al 2015, Monahan et al 2016, Contosta et al 2020). Against this backdrop of milder winters and longer, warmer growing seasons, climate change is driving greater winter weather variability (Chen et al 2018) related to changes in the Arctic jet stream (Francis et al 2015, Cohen 2016, Overland et al 2016). This variability may result in extreme cold temperatures (Cohen et al 2013, Kug et al 2015, Overland 25 al 2011, Cohen et al 2018). One extreme occurred in 2018–2019, when the incursion of the polar vortex plunged temperatures below –20°C across the Northeast. While swings between extremely cold and warm temperatures are somewhat rare, freeze-thaw cycles in which temperatures fluctuate around 0°C may become increasingly common as the climate warms (Henry 2008) with significant implications for agricultural productivity (Rotz et al 2016) and concomitant weed management. 

Weed seed bank dynamics and their responses to temperature. 

Weeds are a persistent challenge for crop production. They reduce crop yield and quality, sometimes even causing stand failure (Grekul and Bork 2004, Baker and Mohler 2014, Hatzler 2004, Rosenbaum et al 2011). Weed seed banks are the primary source of weed recruitment in most agroecosystems. Weed issues are likely to be exacerbated by a warming climate (Hatfield et al 2011, 2014), which will impact weed seedbanks as well as emerged weeds. Weed seed persistence within the soil seedbank is strongly regulated by soil temperature (Smith et al 2018, Kreyling et al 2010, Walck et al 2011). For this reason and others, changes in temperature are likely to drive shifts in weed community composition and abundance that could pose new challenges for cropping systems in the region.   

Warmer temperatures affect weed seed dormancy.

Most weed species exhibit seed dormancy (Cavers et al 1989). Seed dormancy, which prevents germination at times that would result in low survival, is controlled by species-specific physiological, physical, and/or chemical mechanisms that may confer both dormancy and defense (Baskin and Baskin 2014, Davis et al 2016). Dormancy mechanisms are strongly influenced by temperature (Benech-Arnold 2000). Maternal plants exposed to warmer air temperatures during seed set can produce seeds with lower dormancy levels (Gutterman 2000). Warmer temperatures following seed dispersal can increase the rate of afterripening and thus the fraction of germinable weed seeds within the soil (Dwyer 2016). 

Increased soil freeze-thaw cycles and warmer temperatures affect seed longevity.

Soil freeze-thaw cycles directly affect weed seed persistence by breaking down hard seed coats (Baskin and Baskin 2014). In species with physical dormancy, fractures to the weed seed coat release dormancy and thereby increase germination, emergence, and recruitment (e.g., velvetleaf (Abutilon theophrasti)). Fracturing of the seed coat also increases vulnerability to soil pathogens and decay (Connolly and Orrock 2015). Indirect effects of increased soil temperatures and freeze-thaw cycles on seed longevity may be mediated by increased activity of pathogenic fungi and other microorganisms (Classen et al 2015). Another indirect effect is that soil heaving associated with freeze-thaw cycles moves weed seeds in the soil profile (Chambers and Macmahon 1994). Some seeds are moved into deeper layers where they are more protected from seed predators (Omani et al 1999, Korres et al 2018).   

Importance of the Work 

Weed management is a priority issue for Northeastern farmers, particularly given the increasing prevalence of organic production, the rise of herbicide-resistant weeds, and the recent increase in small farms and urban farming. Interest in local food is also increasing, so specific, regionally focused data and tools for the Northeast could provide great benefits to growers and consumers while reducing negative impacts on the environment. Weeds are the major cause of yield losses in organic production (Baker and Mohler 2014; Jerkins and Ory 2016). Yield losses to weed competition are an increasing problem for conventional farmers as well, as the incidence of herbicide-resistant weeds continues to increase (Heap 2023). Preventing yield losses requires weed management operations such as cultivation or herbicide applications conducted at the proper time. The failure to account for the temporal variability of emergence can result in mistimed application of these control measures, leading to poor efficacy. Poor efficacy may necessitate repeated operations that are not only costly to the farmers, but also detrimental to our environment. Thus, better timed, and more effective use of herbicides and/or cultivation will protect yield and minimize unintended consequences like the spread of herbicide resistance in weed populations.       

As the climate warms, changes to weed emergence patterns or weed community composition are likely to impact crop yield and farm profitability. Accurate predictions about near-term effects of increased temperatures on weed communities will allow farmers throughout the Northeast region to proactively respond to these changes. 

Technical Feasibility

We need effective and affordable methods to simulate increasing temperatures and increasing weather variability in the field to better understand the impacts climate change may have on weed emergence. Our current research has shown that hexagonal open top chambers (“OTCs”, shown in Figure 1 (Marion et al 1997)) meet this need and passively increase air and soil temperature, while having a minimal effect on soil moisture. OTCs have been used to simulate warming throughout a wide range of climates and environments (Bjorkman et al 2017; Seipel et al 2019). We found that our OTC design was relatively easy to implement and effective at modifying temperatures. The OTCs had substantial effects on air and soil temperature (Figures 2 and 3) and are built of plastic that allows 95% light transmission (Carolyn Lowry, pers. comm.). On average we obtained an increase in air temperature within the OTC of approximately 0.5°C, and soil temperatures by approximately 0.4°C. If funded, we will test light transmission and the frequency of transmitted light, to test whether collected data are impacted by a shift in light ratio. 

For example, fall maximum air temperature within the OTC was up to 5°C warmer compared with the control plots, while the OTC increased maximum air temperature by as much as 10°C in spring. The change in autumn air temperature within the OTC decreased cold hardiness accumulation by 20 chilling degree day units (base 5°C). Interestingly, the magnitude by which our OTCs decreased chilling degree days is consistent with the predicted decline in chilling degree days for future climate change scenarios in northeastern North America (Bélanger 2002). In the spring, the OTC increased the number of days that maximum air temperature was above 15°C (the temperature at which alfalfa breaks dormancy) by 40% compared with the control. While the OTC did not impact winter maximum air temperature as much as in autumn and spring, we did observe up to an 8°C increase in the minimum daily temperature in winter. Throughout the spring, the OTC doubled the number of days that air temperature was greater than 30°C compared with the control plots without an OTC. 

This OTC design is already being implemented by two multistate collaborators in an AFRI-funded grant focused on alfalfa management in Pennsylvania and New Hampshire. Importantly, the proposed work will provide complementary information to the existing project focused on weed responses in a wider range of Northeastern US states and climate conditions.   

Figure 1, found in Attachments section. Picture of OTCs at Rock Springs, PA (photo credit A. Isaacson).

Figures 2 and 3, found in Attachments section. Figure 2. Average monthly difference in temperature (ºC) for year one (A. 2020-2021) and year two (B. 2021-2022) in State College, PA. Data are monthly averages of the daily average temperatures of temperature sensors at a 10 cm height in an alfalfa-orchardgrass mixture. Figure 3. Daily maximum (top) and minimum (bottom) soil temperature at 5 cm depth in Pennsylvania (PA, left) and New Hampshire NH, right) buried in a plot with an OTC (“constant warming ’’) and without (“control”).

Multistate Advantages

Weather patterns, soil types, and weed communities are highly variable across the Northeast, making the collection of data from across the region critical for understanding the response of agricultural weed emergence to climate change. Additionally, Cordeau et al. (2017) found that populations of weed species had different emergence patterns in different Northeastern states. It is not yet clear whether that difference is due to genetic variability within the species or plasticity in emergence patterns depending on climatic conditions. A multistate project will allow us to replicate the same weed emergence experiment at multiple sites across the region. Participating researchers will include Richard Smith (New Hampshire), Carolyn Lowry (Pennsylvania), Mark VanGessel (Delaware), Antonio DiTommaso (New York), and Thierry Besancon (New Jersey). Our thorough coverage of the region will ensure that results capture regional variability in weed emergence and climate.   

Likely Impact

Current research on Northeastern weed emergence does not account for warming temperatures which are occurring at a much more rapid rate than predicted (Karmalkar and Horton 2021). The findings from this research will allow weed scientists to model changing patterns of weed emergence under warming conditions and will provide more accurate information for our growers and other stakeholders in the region. 

The findings of this research will empower farmers to better predict the emergence patterns of common weeds, thereby improving weed management efficacy and efficiency. While weed emergence timing is one of many factors that determine treatment windows for farmers, application outside of the optimal window of emergence and early growth is drastically less effective. Better information on emergence timing will be especially helpful for newer farmers; with many of our experienced farmers aging out of farm management, new farmers are likely to become more common in the next decade. Improving farmers’ ability to manage weeds effectively in a rapidly changing climate will be an important aspect of cropping system adaptation and resiliency to deteriorating stressors of climate change. Optimized weed management programs will simultaneously enhance farm profitability and reduce negative environmental impacts.