3 results
Seasonal changes in annual bluegrass (Poa annua) germinability and emergence
- Shachar Shem-Tov, Steven A. Fennimore
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- Journal:
- Weed Science / Volume 51 / Issue 5 / October 2003
- Published online by Cambridge University Press:
- 20 January 2017, pp. 690-695
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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.
Weed Management in Lettuce (Lactuca Sativa) with Preplant Irrigation
- Shachar Shem-Tov, Steve A. Fennimore, W. Thomas Lanini
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- Journal:
- Weed Technology / Volume 20 / Issue 4 / December 2006
- Published online by Cambridge University Press:
- 20 January 2017, pp. 1058-1065
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During dry weather, preplant irrigation of raised beds followed by shallow tillage to create a smooth planting bed is necessary to establish a good lettuce stand. Depletion of germinable weed seeds in the top 5 cm of soil by the sequence of preplant irrigation, followed 7 to 14 d later by shallow tillage to prepare a seedbed, reduces weed density in the subsequent crop. Preplant irrigation and tillage reduces weed density and, when used together with preplant herbicide, provides effective weed control in the cropping season. Preplant and in-crop weed densities resulting from furrow or sprinkler preplant irrigation, followed by shallow tillage and lettuce planting 7 or 14 d later, were compared with the no preplant irrigation control. During the 14-d preplant period, up to 127 weeds/m2 emerged and were removed by shallow tillage before crop planting. Preplant irrigation and shallow tillage reduced in-crop weed density by up to 77% and reduced hand-weeding and crop-thinning time by up to 50% compared with the no preplant irrigation and no herbicide control. Delaying tillage for 14 d following preplant irrigation provided sufficient time for adequate heat unit accumulation (>120 growing degree days, base 10 C), allowing for many weeds to germinate and be killed by shallow tillage. However, 7 d between preplant irrigation and tillage resulted in less heat unit accumulation (<50 growing degree days, base 10 C) and less weed germination before tillage. Preplant irrigation together with pronamide at either 0.67 or 1.34 kg ai/ha reduced weed density compared with the no preplant irrigation. Effective use of preplant irrigation and preplant weed removal may increase the consistency of weed control with lower pronamide rates. Preplant irrigation followed by shallow tillage is an effective cultural practice to control in-crop weeds for conventional lettuce production.
Burning Nettle (Urtica urens) Germination and Seedbank Characteristics in Coastal California
- Ran N. Lati, Shachar Shem-Tov, Steven A. Fennimore
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- Journal:
- Weed Science / Volume 64 / Issue 4 / December 2016
- Published online by Cambridge University Press:
- 20 January 2017, pp. 664-672
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Burning nettle is a noxious weed that commonly infests coastal California vegetable fields. Weed control programs for lettuce and fresh spinach grown in this area do not adequately control burning nettle, and escaped weeds that mature are highly problematic during hand weeding and harvesting. Information on the biology and ecology of burning nettle is limited, and work was conducted to develop information about this weed. The objectives of this study were to evaluate the effect of temperature on burning nettle germination and to determine its base temperature value, to characterize the germination pattern of this weed and seedbanks under local California coastal conditions, and to estimate the optimal timing for burning nettle removal by herbicides and physical methods. The upper optimal temperature for burning nettle germination was 22.8 C, but there was no difference in the final germination percentage between 4 and 22.8 C. The base temperature was determined to be 3 6 0.2 C, and this information allowed the development of temperature-based optimal control timing models. In the field, burning nettle emerged throughout the year without any seasonal pattern, and germinable seeds were also found in the seedbank throughout the year. Burning nettle was able to complete a growth cycle throughout the year in coastal California. Burning nettle has a short growth cycle that allows it to set viable seeds within 466 6 13 growing degree days (GDD), and this timing is critical for burning nettle removal by herbicides, cultivation, or hand weeding. The optimal timing for phenmedipham application at 180 g ai ha–1 was estimated to be 205 GDD. The germination and seedbank field studies indicate why burning nettle is so well adapted to the mild climate of coastal California. However, results presented here suggest strategies to reduce the burning nettle seedbank, improve its control, and allow more efficient lettuce and fresh spinach production.