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Timothy W. Miller

Diquat was tested to determine its suitability for use as a preharvest desiccant of selected vegetable seed crops during 1997 and 1998. In separate studies, diquat was applied at 0,0.56, or 1.12 kg·ha-1 ai. to spinach (Spinacia oleracea L.), table beet (Beta vulgaris L.), and coriander (Coriandrum sativum L.) plants at usual swathing time. Except for beet seed in 1998, there was no clear trend toward reduced seed weight with increasing diquat rate. Spinach seed germination in 1998 and coriander seed germination in 1997 were reduced by diquat at 1.12 kg·ha-1 compared to seed from nontreated plants or plants treated with 0.56 kg·ha-1. In all crops, diquat at 0.56 kg·ha-1 was adequate for crop desiccation purposes. However, seed producers should consider the potential benefits from chemical desiccation that may potentially lower germination of the harvested seed. Chemical name used: 6,7-dihydrodipyrido[1,2-α:2′,1′-c]pyrazinediium ion (diquat).

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Timothy W. Miller and Carl R. Libbey

Mid-summer, foliar-applied paraquat is often used to control weeds and desiccate foliage of field-grown narcissus (Narcissus pseudonarcissus) prior to bed reshaping in the autumn. Paraquat-treated narcissus plants sometimes display chlorotic foliage the subsequent growing season. A trial was conducted to determine if paraquat causes that injury and if so, under what conditions paraquat may be safely applied to narcissus. Narcissus (‘Flower Carpet’ hybrid) was treated with two rates of paraquat at three summer application timings and was then evaluated for damage to new foliar growth the following spring. Flower number and flower stem length was also measured and bulb yield was determined. Narcissus foliage displayed 50% to 77% chlorosis in February after half-green foliage was treated with paraquat at 0.47 or 0.78 lb/acre, respectively, the previous summer. Foliage was still 13% to 63% chlorotic, respectively, at flowering. Paraquat at both rates applied to half-green foliage also reduced flower number and flower stem length in one of two iterations, as well as reducing average bulb weight 18% to 33%. If applied when leaves were 75% dry, foliar damage and reduction in average bulb weight was limited to the 0.78 lb/acre rate, while flower number and stem length were not affected at either rate. When desiccating late-season narcissus foliage and weeds with paraquat, therefore, growers are advised to delay application until narcissus foliage is about 75% dry and most of the flower stems have fallen, and to use a maximum of 0.47 lb/acre.

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Timothy W. Miller and Carl R. Libbey

Tulip, narcissus, and bulbous iris are grown on about 1600 acres annually in western Washington. These bulb crops are poor competitors with winter annual weeds that continually germinate from September through July in the mild maritime climate of this region. Because they do not adequately control emerged weeds but can injure bulb crop foliage, herbicides are applied in the fall. Unfortunately, fall-applied herbicides lack the soil persistence necessary for season-long weed control. If nonselective herbicides could safely be applied after emergence of bulb foliage, emerged weeds would be killed and the application of residual herbicides delayed until spring, thus lengthening the period of weed control through bulb harvest. Glyphosate was tested for selectivity at three postemergence timings (early, middle, and late) on four cultivars each of tulip and iris and three narcissus cultivars. Middle and late glyphosate treatments caused severe injury to tulip foliage and flowers and reduced bulb count and weight, but early glyphosate did not significantly injure most varieties. Narcissus and iris were more tolerant to glyphosate than tulip, but these species also were most tolerant when glyphosate was applied early. In a separate study on iris, carfentrazone, paraquat, and glufosinate were applied postemergence at the same three timings. Glufosinate initially caused moderate injury to foliage (about 20%), but plants quickly recovered. Injury from carfentrazone and paraquat was much more severe (more than 50%), although plant recovery from carfentrazone damage was greater than from paraquat. Bulb yield was not adversely affected by either glufosinate or carfentrazone if applied early. Paraquat at all timings significantly reduced total bulb count and weight.

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Yushan Duan, Thomas W. Walters and Timothy W. Miller

A large percentage of North American processing red raspberry (Rubus idaeus L.) is produced in maritime areas of western regions in Washington, British Columbia, and Oregon. Most of these raspberry plantings are caneburned; they are treated with herbicides in early spring to remove the first-emerging primocanes in an attempt to reduce their competition with floricanes during flowering and fruiting and thereby increase fruit yield. Trials were conducted in northwestern Washington to determine the effect of carfentrazone and oxyfluorfen applied for primocane management in three currently grown raspberry cultivars. Although caneburning did provide early season weed control in treated plots, weed control fell below 70% in the commercial ‘Meeker’ field more quickly each year: at 138 days after treatment (DAT) in 2010, 71 DAT in 2011, and 47 DAT in 2012, compared with greater than 80% control at the end of each season when the soil residual herbicide terbacil was applied. Oxyfluorfen provided longer suppression of primocane growth than did carfentrazone, particularly in ‘Cascade Bounty’ but also to a lesser extent in ‘Meeker’. Caneburning did not improve raspberry yield in five of the six tested years for ‘Meeker’, or in any of the five tested years for ‘Cascade Bounty’ or the single year for ‘Coho’, although the trend was for greater yield with caneburning across all cultivars. Oxyfluorfen improved 3-year ‘Meeker’ average yield 43% from nontreated raspberry in the commercial field trial, compared with a 29% increase with carfentrazone.

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Timothy W. Miller, Carl R. Libbey and Brian G. Maupin

Propane flaming and organic amendments were evaluated for usefulness in matted-row strawberry (Fragaria ×ananassa Duch.). Flaming was used once before transplanting ‘Hood’ strawberry (PRETR), twice before transplanting (PRETR + PRETR), or once before and once after transplanting (PRETR + POSTR) and compared with rototilling before transplanting in 2000–02. Organic amendments tested across flame treatments included corn gluten meal (CGM) at two rates, wheat gluten (WG), and mustard seed meal (MSM) with high or low glucosinolate content, and herbicides included oxyfluorfen, pendimethalin, and a combination of oxyfluorfen + pendimethalin. Amendments/herbicides were applied immediately POSTR in Year 1 and again to established plants in late winter of Year 2. All plots were weeded by hand after weed evaluations were completed and weeding hours recorded. The trial was conducted twice: Iteration 1 and Iteration 2. Effect of flaming on grass and broadleaf weed ratings was brief during Year 1 of both iterations, with only slight differences observed in June and no differences by September. Total weeding time was reduced 12% by flaming PRETR once compared with rototilling in Iteration 1 and was reduced 10% by all flame treatments in Iteration 2. Rototilling reduced total berry yield and average individual fruit weight compared with flaming treatments in Iteration 1; there was no significant effect of flame on strawberry yield or individual fruit weight in Iteration 2. Organic amendments did not reduce weeding time in Iteration 1 compared with the nontreated control, although weeding time was increased 18% by CGM at 487 kg·ha−1 compared with synthetic herbicide treatments. In Iteration 2, total weeding time was reduced 14% for the two pendimethalin treatments and for high-glucosinolate MSM compared with nontreated control plots. First-year strawberry leaf area was reduced by oxyfluorfen + pendimethalin compared with nontreated strawberries (802 and 1086 cm2/plant, respectively) and was generally increased with organic amendments. Strawberry yield in Iteration 1 was increased ≈14% by CGM at 974 kg·ha−1 and WG and low-glucosinolate MSM compared with nontreated strawberry. Oxyfluorfen and oxyfluorfen + pendimethalin reduced strawberry yield by ≈20% and average individual fruit weight by ≈9% (14.8 and 14.5 g/fruit) compared with nontreated strawberry (16.1 g/fruit); high-glucosinolate MSM also reduced average individual fruit weight to 14.8 g/fruit. There were no significant effects of amendments/herbicides on strawberry yield parameters in Iteration 2.

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Mercy Olmstead, Timothy W. Miller, Callie S. Bolton and Carol A. Miles

Consumer demand for organic and sustainably produced products has increased the interest in organic wine grape (Vitis vinifera) production. However, organic production can be challenging, and weed management is a critical issue during the establishment of an organic vineyard. In 2009, the effectiveness of five cover crop treatments and cultivation regimes was evaluated for two years for weed control in a newly established organic vineyard of ‘Pinot noir précoce’ and ‘Madeleine angevine’ grape cultivars in northwestern Washington State. Alleyway management treatments were cultivation in alleyways with hand weeding in the vine row (control), grass cover crop which included perennial ryegrass (Lolium perenne ssp. perenne) and red fescue (Festuca rubra ssp. arenaria) seeded in the alleyway and in-row tillage with a specialty offset-type cultivator, winter wheat (Triticum aestivum) cover crop with in-row string-trimming, austrian winter pea (Pisum sativum ssp. sativum var. arvense) cover crop with in-row string-trimming, and winter wheat–austrian winter pea cover crop mix with in-row string-trimming. In 2009, weed dry biomass was lowest in the alleyway of the control (0.8 g·m−2) and offset cultivator treatments (6.3 g·m−2) on 3 Aug. and tended to be lowest in the alleyway of the control (4.8 g·m−2) and offset cultivator treatments (16.0 g·m−2) on 27 Sept. In the second year of establishment (2010), winter wheat and austrian winter pea were eliminated from the plots by mid-July, and white clover (Trifolium repens) and perennial ryegrass were the dominant weed species and accounted for a majority of the total weeds. On average over the two-year period, the control treatment required the most time for alleyway management (92 h·ha−1) followed by the offset cultivator treatment (64 h·ha−1), while the winter wheat, austrian winter pea, and winter wheat–austrian winter pea mixture required 32 to 42 h·ha−1. ‘Madeline angevine’ produced more shoot growth than ‘Pinot noir précoce’ in Sept. 2010 (42.3 and 25.9 cm respectively), and shoot growth of both cultivars in the control treatment was significantly longer (125.0 cm) than under any other treatment (55.4 to 93.0 cm), illustrating the importance of weed control during vineyard establishment. In this study, the most effective weed management regime, although also the most time consuming, included a vegetative-free zone around the vines (e.g., in-row) maintained by hand weeding and a cultivated alleyway.