Proliferation of troublesome and herbicide-resistant weeds warrants weed management practices that aim to deplete weed seedbanks in soil because seedbanks are the source for weed population persistence and thus an ideal target for long-term sustainable weed management. Understanding the phenology of weed seed production can help inform the appropriate timing of weed management practices with the goal of limiting weed seed production and return to the soil seedbank. This project aimed to identify the critical period for weed seed control (CPWSC) for seven common weed species in western Canada based on two component curves that define the phenology of seed production: (1) the critical period for weed emergence (CPWE) and (2) the critical period for seed production (CPSP). The CPWE identifies how late a weed species is capable of emerging and producing viable seed before the end of the growing season, while the CPSP identifies when during the growing season viable seed production begins. Overlapping these two component curves identifies the CPWSC or the period of the growing season during which management can effectively mitigate weed seed production and return to the soil seedbank. Experiments to determine the CPWE and CPSP were conducted at Lethbridge AB, Lacombe AB, Saskatoon SK, Indian Head SK, and Carman MB in 2021 and repeated on adjacent sites in 2022. Overall, ten site-years of research were conducted for green foxtail, wild oat, lambsquarters, wild mustard, and redroot pigweed, while eight site-years were conducted for kochia (excluding Lacombe) and four site-years for wild buckwheat (at Lethbridge and Lacombe only).

Based on a 5% seed threshold, wild oat, wild mustard, wild buckwheat, kochia, green foxtail, lambsquarters and redroot pigweed were capable of producing seed up until emergence dates as late as July 31, August 01, 05, 08, 13, September 07, and October 08, respectively (equivalent to 1537, 1561, 1685, 1753, 1779, 2236, and 2633 GDD, respectively). Also based on a 5% seed threshold, seed production began for wild mustard, wild oat, green foxtail, lambsquarters, redroot pigweed, wild buckwheat, and kochia on July 16, 21, 28, August 05, 06, 15, and September 18, respectively (equivalent to 1252, 1346, 1482, 1636, 1656, 1879, and 2444 GDD, respectively).

The CPWSC (20% seed threshold) ranged from July 10 to 28 (1118 to 1477 GDD) for wild oat, July 10-24 (1127 to 1409 GDD) for wild mustard, July 14 to August 03 (1213 to 1596 GDD) for green foxtail, July 17 to August 29 (1313 to 2121 GDD) for wild buckwheat, July 25 to September 19 (1445 to 2451 GDD) for kochia, July 29 to August 14 (1509 to 1811 GDD) for redroot pigweed, and August 03 to 15 (1611 to 1840 GDD) for lambsquarters. Therefore the CPWSC generally takes place later in the growing season than typical post-emergence weed management practices because all weed species were capable of emerging after this time frame and producing viable seed before the end of the growing season. Thus the pre-harvest and post-harvest windows may be more critical for management aimed at reducing weed seed production and return to the soil seedbank. The current study suggests that some annual weed species may be managed by growing crops with earlier maturity such as winter wheat or fall rye. Future research should aim to identify which management practices could be most effective when implemented during the CPWSC for target weed species