Two species of cleavers, Galium aparine L. and Galium spurium L., are known to inhabit croplands in western Canada. The latter is the more abundant of the two species. An increased abundance of these species over the most recent decades warrants a more comprehensive understanding of their developmental phenology and the cause of this increase. This study aimed to identify the base temperature and emergence characteristics of Galium spp. across different populations from western Canada. A thermal gradient plate experiment was conducted using five G. spurium populations collected from various Saskatchewan (SK) and Alberta (AB) locations. One known G. aparine reference sample was also included. A common garden experiment was conducted using the six Galium spp. populations to determine emergence characteristics. The base germination temperature identified was 2 C for all populations of G. spurium and 4 C for G. aparine. The median germination temperature for G. aparine was 8.34 C, whereas G. spurium had a similar median germination temperature of 6.5 C. Despite similar germination characteristics, the field emergence study revealed differences between populations’ initiation of emergence (150 to 250 growing degree days [GDD]) and time to 50% emergence (275 to 470 GDD) in spring. Highly variable emergence among years and populations within the year (200 to 600 GDD in 2013 and 100 to 200 GDD in 2014) were observed during fall, probably due to differences in moisture availability. Cumulative emergence among populations in fall was very low (1% to 9%) compared with spring emergence (2% to 17%). Overall, this study provides evidence for a low base temperature and differences in emergence periodicity among populations, both of which may be significant factors contributing to the seasonal success of this species.

The predictability of the spring emergence period of kochia is a potentially exploitable attribute that may be applied to weed management. The emergence timing of kochia was monitored, during the spring and summer of 2005 and 2006, in 12 farmed fields in the northern region of the Northern Great Plains (NGP) of North America. Kochia emergence began at 50 cumulative growing-degree days (GDD Tbase 0 C, starting January 1) and continued into late summer. The onset of kochia emergence occurred well before the emergence onset of other weed species common to this region. Fall soil samples revealed few kochia seeds remained in the seedbank. In growth room studies, kochia seed placed at the soil surface had greater emergence compared with seed burial to a 10 mm or greater depth. Kochia did not emerge from seed planted 80 mm deep; however, radical emergence was evident indicating fatal germination. Kochia should be controlled early in the growing season, and burying seed will greatly reduce future kochia populations.

With the recent interest in genetically engineered (GE) wheat and the commercialization of novel-trait imidazolinone herbicide-resistant wheat in North America, volunteer wheat as a weed has also been the subject of renewed interest, specifically, its recruitment and persistence in annual cropping systems. The recruitment of seed from a wheat seedbank established the previous autumn was monitored in a flax crop at two field sites in southern Manitoba, Canada, in 2003 and 2004. Seeds of eight Canadian Western Hard Red spring wheat cultivars, which exhibit a range of preharvest sprouting-resistance characteristics, were broadcast and incorporated into the soil in the autumn at 500 seeds m−2. Tillage treatments consisted of autumn tillage only, and autumn and spring tillage. Recruitment the following spring occurred very early in terms of accumulated growing–degree days (base temperature of 0 C) but expressed as a proportion of total seeds broadcast was low and variable. Total cumulative emergence of wheat over all 4 site yr ranged from 0.9 to 13.1%, with an overall average of 4.3%. There was no relationship between preharvest sprouting-resistance characteristics and recruitment proportion, and no significant influence of tillage treatment on wheat recruitment. Wheat seed that did not recruit was rapidly degraded in the soil and did not persist for more than 12 mo. However, some emerged volunteer wheat plants escaped all control measures normally used in establishing and growing a typical flax crop, and these escaped volunteer wheat plants set viable seed. Therefore, results of this study indicate that efforts and attention should be directed toward achieving very high levels of volunteer wheat control in subsequent rotational crops and that reseeding by escaped volunteer wheat plants may be a more important persistence mechanism for spring wheat than multiyear soil seedbank persistence.

Observational studies of weed emergence provide essential data for the creation and testing of predictive emergence models, with data ideally being collected from a wide range of sites representing a range of environments under which the seedlings of a given species emerge. The spring emergence of genetically engineered glyphosate-resistant volunteer canola was monitored in 20 farmers' commercial wheat fields over 2 yr, 2003 and 2004, in the southwestern region of Manitoba, Canada. Three different tillage systems, low-disturbance direct-seeded, high-disturbance direct-seeded, and conventionally tilled fields, were represented in this sample. Tillage system did not significantly influence the emergence periodicity of volunteer canola, but did influence total densities observed. Total volunteer canola densities in the surveyed fields were variable and ranged from 6 to 2,015 seedlings m−2. Conventionally tilled fields had the lowest densities of volunteer canola seedlings, and high-disturbance direct-seeded fields (no autumn tillage) had the highest densities. Volunteer canola is a relatively early spring emerging plant species requiring minimal accumulated growing degree days for emergence (calculated using a base temperature of 5 C). Emergence curves were steep, with a short window of volunteer canola emergence either prior to or within a spring-seeded wheat crop. To determine what proportion of autumn-broadcast canola seed (simulated harvest losses) would recruit the following spring and the influence of fall or spring tillage on this recruitment, a small plot experiment also was conducted at three sites. The following spring the percentage of canola seedlings that emerged ranged from 1.3 to 9.4% of the seed broadcast, depending upon the tillage treatment. The effect of tillage treatment on canola densities in the small plot experiment was similar to the field survey results.

Although the prolonged emergence pattern of common waterhemp is well known, its effect on flowering phenology and success is less well understood. The ecological significance of later common waterhemp cohorts could have been underestimated. Ecological knowledge of common waterhemp, such as reproductive phenology and success, would help us better understand the invasiveness of this weed and thus facilitate the development of more targeted control methods. Field studies were conducted at Ames, IA, to evaluate temporal variation in flowering phenology (e.g., date of flower initiation, number of flowering plants per day) and reproductive success (e.g., seed production) of natural common waterhemp cohorts. Later-emerging common waterhemp cohorts flowered quicker and had a relatively shorter flowering period than early cohorts. Common waterhemp cohorts showed very large variations for the duration of their vegetative phases, indicating different photoperiod sensitivity among cohorts. Furthermore, common waterhemp cohorts exhibited a pulsed flowering pattern, which was probably influenced by temporal distribution of rain events; up to seven distinct flowering pulses within 40 d were observed in 2009, and about eight flowering pulses scattered over a 60 d period were observed in 2010. Common waterhemp maintained high seed production throughout the growing season. Seed production for the entire cohort was influenced more by emergence timing, whereas individual plant fecundity was more affected by plant population densities. Common waterhemp demonstrates strong phenological plasticity by evolving a pulsed flowering pattern, which rendered it plastic enough to tailor flowering to variable environmental conditions thus facilitating effective pollination. Common waterhemp later cohorts have a strong seed production potential, which may be ecologically significant with regard to herbicide resistance evolution.