The seasonal emergence characteristics and seedbank ecology of annual bluegrass were evaluated in a vegetable field in the central coast of California. The emergence and germinability of annual bluegrass were monitored continuously for over 3 yr to detect seasonal variation in weed emergence. Weed emergence and seedbank densities were measured every 45 d for 41 mo. Weed emergence was monitored simultaneously in the field and in a growth chamber that was adjusted seasonally for day length and temperature. Samples were incubated in the growth chamber for 45 d and then elutriated to measure remaining viable ungerminated caryopses and relative germination potential. Seedbank densities ranged from 2,000 to 20,000 caryopses m−2 during the study period. Weed emergence and germinability were highest from October to November and lowest from March to July. Emergence from soils collected in the spring and fall and incubated under both spring and fall conditions in the growth chamber indicated that seed dormancy state determines germinability rather than environmental conditions during incubation. Soil samples collected in the spring had no emergence (< 1%) when incubated under spring or fall conditions, whereas samples collected in the fall had high emergence (> 95%) under both spring and fall conditions. A waveform regression fit the repeated emergence pattern with a cycle length of 364 d (365 d for the growth chamber). This model predicted that weed emergence would peak on November 5 and be lowest on June 20. A survey of organic vegetable fields found that caryopses were more common in soils with higher clay content than in lighter soils (R2 = 0.77, P = 0.02). Our data suggest that annual bluegrass is more likely to emerge when the soil is wet during winter. The timing and location of annual bluegrass emergence may increase the probability of reproductive success during periods when seasonal rains and wet conditions limit tillage, weeding activities, and competition with other weeds.
Nomenclature: Annual bluegrass, Poa annua L., POANN.