Ecological characteristics of Ventenata dubia in the Intermountain Pacific Northwest

Ventenata dubia is an exotic winter annual grass that has invaded Conservation Reserve Program (CRP) lands, improved pastures, intensively managed hay fields, and rangelands within the Intermountain Pacific Northwest (PNW). Currently, producers are attempting to develop V. dubia management strategies with little knowledge of its life history traits. We conducted several studies to characterize V. dubia life history patterns. Preliminary germination trials were completed to describe primary and secondary dormancy characteristics. Field studies were conducted to evaluate (1) seed bank persistence patterns, (2) seedling emergence patterns under V. dubia litter, and (3) seedling emergence and phenological development patterns within timothy hay, CRP, and rangeland habitats. Preliminary germination trials suggest that the after-ripening period required for loss of dormancy does not exceed 30 d and that dormancy breakdown peaks at approximately 90 d, after which germination occurs over a wide range of temperatures (9 to 29 C). A small fraction (< 1%) of the seed bank remained germinable up to 3 yr after burial at 2 cm depth in a grassland habitat. Seedling emergence and survival was significantly greater under high V. dubia litter layers (100% cover) compared with bare surface during the drier study year because of higher soil moisture levels maintained under litter. Across habitat types, mean seedling emergence (50% of total) occurred between 33 and 94 growing degree days (GDD) after soil moisture rose above the permanent wilting point in the fall. Seedling emergence periodicity varied among habitat types in relation to spring seedling emergence, ranging from 0 to 13% of total emergence per year. Phenological development differed across sites and years by up to several hundred GDDs but was closely aligned to Julian days. This collection of studies improves our understanding of V. dubia life history traits and will aid integrated weed management strategies in the Intermountain PNW. Nomenclature: North Africa grass; Ventenata dubia (Leers) Coss.; timothy; Phleum pratense L. Management Implications: Ventenata dubia is an exotic annual grass that has invaded established CRP lands, improved pastures, intensively managed hay fields, and rangelands within the Intermountain PNW. Significant economic and ecological impacts have resulted from V. dubia invasions across perennial grass habitats in recent years, underscoring the need for development of integrated control strategies. Control of V. dubia using selective herbicides within perennial grass stands, as well as cultural control strategies, will benefit from greater understanding of V. dubia life history patterns. Within the Intermountain PNW, V. dubia seedling emergence occurs after fall rains have increased soil moisture above the PWP in timothy hay, CRP, and rangeland habitats. The greatest proportion of fall seedling emergence occurs within 6 wk of initial emergence. Spring seedling emergence is more likely in timothy hay and rangeland habitats where fall environmental conditions are less conducive for seedling emergence and survival in the Intermountain PNW. At a local scale, seedling emergence and survival is likely mediated by the amount of residue at the soil surface. Higher V. dubia litter levels will increase seedling emergence and survival compared with bare surface during drier or colder fall growing seasons. A small fraction of V. dubia seed banks (< 1%) may remain persistent, or germinable, for up to 3 yr at shallow soil depths (2 cm). These life history traits should inform V. dubia management strategies in the Intermountain PNW. Ventenata dubia may be controlled with early postemergence herbicide applications in late fall. However, this application timing is often a narrow window for growers because of unpredictable weather conditions. Predictive models of seedling emergence patterns can be used to time pre-emergent or early postemergent applications to temperature and soil moisture conditions. Additional research is needed to identify herbicide control options for fall applications that could potentially provide soil residual control of spring-emerged seedlings. Predictive models of phenological development may also be used to time cultural control strategies that aim to reduce or prevent V. dubia seed production, such as targeted mowing in CRP or altering hay harvest schedules. Cultural control strategies that target the seedling phase of V. dubia's life cycle should focus on litter management techniques, such as prescribed burning, to eliminate the benefits of litter on seedling recruitment. Because of low levels of seed bank persistence, integrative control strategies should be planned for several years (> 3 yr) to reduce the negative effects of V. dubia on managed perennial grass systems across the Intermountain PNW.