By: Russ Wallace and Larry Stein
Weed control is critical to spinach growers’ profitability; however, no new preemergent herbicides have been registered for use in spinach since the late 1990s. Over the past decade, only one new postemergence herbicide, Stinger, has been registered for spinach. This lack of new registered products puts the spinach industry at risk, especially if current registrations and uses are ever withdrawn, or if these products become less effective. The reason fewer new herbicides are being developed specifically for vegetables is that chemical companies are concerned with potential injury, lack of profitability, and lawsuits. Instead, much of the agriculture industry has concentrated research on herbicide-resistant or herbicide-tolerant seed technologies in large-acreage crops like corn, soybeans and cotton
Weeds as few as one per square yard can reduce spinach yield and quality. For processed spinach, there is zero tolerance for weeds at harvest because they are very difficult to separate from the spinach. Weeds are also problematic in fresh-market spinach, because workers must spend time separating the weeds by hand before harvesting, bunching, or packaging.
The Texas Wintergarden Spinach Producers Board has funded weed control research for the past several decades (Figure 1). Since 2002, research in the Wintergarden region has focused on new and older herbicide chemistries and has sought the best ways to combine specific herbicides with cultural practices such as wider seedbeds, higher spinach density, application timing, and using multiple low-rate applications.
Weed control in Texas-grown spinach
Spinach (Spinacia oleracea) is a cool-season crop that is typically planted in the early fall through early winter in Texas. It may be harvested after 30 days for baby leaf, six to eight weeks for fresh use or up to 12 to 13 weeks after planting for processing. Spinach is generally harvested two or three times from a single planting, except for baby which is a once over harvest. Spinach production has been largely concentrated in the Wintergarden area although some is now produced on the High Plains.
In the past, weeding and harvesting spinach was done by hand. Though much of the production is mechanized today, field workers are still needed to hoe escaped weeds during production in both fresh-cut and processing spinach.
From the 1970s to 1990s many spinach herbicides were removed from the market for economic reasons or were banned by the EPA. Ro-Neet (cycloate) continues to be available to growers but is expensive and only effective for 2 to 3 weeks after planting. In 1985, research showed that Dual (metolachlor) herbicide was effective at controlling most weeds in the Wintergarden, but the manufacturer was not interested in a registration. Later trials, at the Del Monte Research Farm near Crystal City, showed Dual’s continued effectiveness as a potential replacement or companion for Ro-Neet.
As a result of that research, today Dual Magnum (s-metolachor) is used as a preemergent on almost every acre of spinach in the Wintergarden. Ro-Neet continues to be used sporadically as a preplant incorporated (PPI). Ro-Neet is used more in double-cropped baby-leaf spinach fields because Dual Magnum can only be applied once a year. Because there are so few options available, research since 2002 has emphasized finding products to control weeds when Dual Magnum and Ro-Neet are not appropriate or can’t be used. This has been challenging because manufacturers are not eager to register new uses for older chemistries or to spend research funds on new chemistries. The registration of the postemergence herbicide Stinger (clopyralid) in the early 2000s has helped, but it is expensive and has limited use. Spin-Aid (phenmedipham) and other selective grass herbicides are also available for use today.
Economically-damaging weeds in spinach
Weeds compete with spinach for moisture, nutrients, space and sunlight; they can reduce yield and quality. Weeds at harvest can contaminate processed crops, increase labor costs and/or cause fields to be abandoned. In Texas, conditions that are optimal for planting spinach also plague the crop with warm- and cool-season weeds. Warm-season weeds are problematic during the first few weeks after the fall planting. As the season progresses, these weeds complete their life cycle while competitive cool-season weeds emerge if not controlled by preemergent herbicides. The eight weeds listed below are problematic to Wintergarden spinach production.
Henbit (Lamium aplexicaule) is a member of the mint family and is a very common broadleaf weed in Texas. It is also called Purple Deadnettle. Henbit is a cool season annual weed that has unique upper leaves surrounding a square stem (Figure 2). Its many stems develop from a single taproot with sparsely hairy leaves and small purple flowers. In spinach, henbit stems can grow 6 to 18 inches tall along the edges of beds or between the plants. High-density plantings with excellent growth should help to reduce the henbit problem through competition and shading. Henbit can be controlled with Ro-Neet preemergence and Spin-Aid postemergence. Dual Magnum does help to suppress it.
London rocket (Sisymbrium irio) is a very competitive winter annual broadleaf weed that can infest spinach fields. If not controlled, it will emerge with the spinach crop. It has a rosette development pattern during early growth (Figure 3). Mature plants grow 2 feet tall, with stems emerging from the base of the plant. The lower leaves are usually lobed. Flowers have four yellow petals and form in clusters. Seeds are in long, thin, tubular pods. It can only be suppressed with Ro-Neet; Dual Magnum gives very little control. Spin-Aid may control this weed well when applied postemergence to very young plants.
Purslane (Portulaca oleracea) is a summer annual broadleaf that will emerge with late summer or early fall plantings. Purslane is now more prevalent because the widespread use of Dual Magnum only suppresses it. Purslane can survive under dry conditions, but does best in irrigated fields. The plants grow prostrate but will grow up to reach sunlight above the spinach crop (Figure 4). The leaves are egg shaped, smooth and succulent; they do not have stems. Flowers are tiny, yellow and grow at the stem tips. Uncontrolled plants may spread over 3 feet wide. Ro-Neet will delay purslane emergence for about 3 weeks. While Dual Magnum may suppress purslane, a postemergence application of Spin-Aid may be needed.
Common lambsquarters (Chenopodium album) is a summer annual broadleaf that can infest fields during the early season, especially during warm periods. It can be very difficult to control since it is in the same botanical family as spinach (Figure 5). Lambsquarters can grow to 5 feet if left uncontrolled. Leaves on newly emerged plants are generally pale greenish gray and covered with clear, glistening granules that develop into a fine white powdery coating. The leaves are oblong to lance shaped. Ro-Neet will control lambsquarters for a short period, but Spin-Aid may be needed to control small plants. Dual Magnum does not control lambsquarters well.
Fumitory (Fumaria officinalis) is a cool season annual that is often called wild carrot (Figure 6). It grows in the Wintergarden region, but is not as widespread as other weeds. The plants are prolific and stand out well in spinach fields. They can grow several feet tall and have carrot-like to feathery light green leaves and small pink flowers. Fumitory stems can grow just below the spinach crop and appear to emerge 12 to 18 inches away from the main stem. Dual Magnum and Ro-Neet have provided partial control during some years. Spin-aid does not list fumitory on its label, but it may provide control if applied to very young plants.
Curly dock (Rumex crispus) is perennial broadleaf weed that is generally found in wet or moist regions. It is frequently found along roadsides, ditches, and other unmanaged areas and is associated with high water use or overwatering (Figure 7). Curly dock is a member of the buckwheat family and can grow from 2 to 5 feet tall. Its leaf pattern is rosette and alternate on the stem. Seeds are flat and covered with a papery membrane that is dispersed by wind and rainfall. Dual Magnum and Ro-Neet are not labeled for curly dock. Stinger will control it applied postemergence, but works best applied to young weeds. The Spin-Aid label does not indicate control.
Hophornbeam copperleaf (Acalypha ostryifolia), a member of the spurge family, is a summer annual weed but does not produce the milky sap. A broadleaf weed, it has leaves that are heart shaped and serrate (Figure 8). Hophornbeam copperleaf can range from 1 to 2 feet tall. As a summer annual, it is more problematic in early spinach plantings. No control is mentioned for either labeled preemergence or postemergence spinach herbicide. Roundup and gramaxone will control it preplant, but emerging weeds should be cultivated out or removed by field laborers.
Thistles including sowthistle (Sonchus spp.), Canada and bull thistles (Cirsium spp.) and musk thistle (Carduus nutans) can be annuals, perennials or biennial weeds. They typically grow in or along the edges of spinach fields during the late fall or early spring. Cool, moist conditions favor thistle growth and development (Figure 9). Thistles form rosette leaf patterns and can range from 1 to 6 feet tall. Leaves are irregularly shaped with prickles and ooze milky latex when broken. Neither Dual Magnum nor Ro-Neet will control thistles. Spin-Aid will control sowthistle, while Stinger will control all thistles only when applied postemergence. Handweeding should provide control but deep roots should be pulled out for perennial thistles.
Rescue grass, cheat grass, annual bluegrass and foxtails can infest spinach production fields (Figure 10). These annual grasses are generally easy to control with postemergence herbicides like Poast Plus and Select Max, as well as their generic equivalents. To control annual grasses, they should be actively growing and most of the plant should be above the spinach crop. This allows maximum absorption and movement of herbicide throughout the plant. Stressed grasses are more difficult to control than healthy grasses. Dual Magnum and Ro-Neet control many grass species well. Spin-Aid and Stinger have no effect.
A perennial weed, Johnsongrass (Sorghum halepense) is a very aggressive grass that reproduces by seed and underground rhizomes. It grows in small or large clumps and can grow 5 to 6 feet tall if not controlled (Figure 11). The large rhizomes as well as its hairless leaf blades easily identify it. Because hoeing and cultivation spread Johnsongrass rhizomes easily, it is best to control it with postemergence selective herbicides. Stinger and Spin-Aid have no effect on grasses. Ro-Neet and Dual Magnum provide some control, but only on emerging Johnsongrass plants from seed. Rhizome seedlings must be controlled with Poast Plus or Select Max.
Integrated weed management strategies
Integrated weed management uses preventative, cultural, mechanical, biological, and chemical strategies for controlling weeds. Control improves when two or more of the strategies are used. The elements of these strategies are as follows:
- Obey local, state, federal, and international laws that apply to seed transportation.
- Use only certified weed-free seed.
- Use only weed-free manure or compost.
- Clean field and harvesting equipment when traveling between fields.
- Eliminate weeds near or around field edges and equipment yards.
- Know your field’s weed history and scout for escaped weeds.
- Practice zero tolerance. Do not allow difficult-to-control weeds to produce seed.
- Rotate crops between fields.
- Avoid overwatering the spinach crop or irrigating fallow areas.
- Fertilize crops appropriately. Healthy spinach competes with weeds more effectively than stressed plants.
- Remove live weeds by plowing or cultivating the fields before shaping beds.
- Cultivate before weeds grow too large.
- Use handweeding and hoeing as needed.
- Plow under weeds immediately following the final harvest to prevent reseeding.
Biological control technology
- Use natural predators to suppress or kill weeds. This practice can, however, be difficult to maintain and may be costly. Some pathogens and insects have been researched as potential biological pesticides. However, biocontrol agents may interact with other pesticides and with climate changes.
- Use herbicides judiciously. Always read and follow label instructions to avoid human exposure, spinach injury, or development of weed resistance and build-up on uncontrolled species. Uses include:
– Preplant burn-down herbicides
- Products containing glyphosate, paraquat, or pelargonic acid
– Preplant incorporated
- Dual Magnum
- Ro-Neet (sprinkler applications only)
- Stinger, Spin-Aid, Poast Plus, Select Max
Current Weed Control in Spinach
Spinach growers currently have only Dual Magnum and Ro-Neet for preemergence applications. However, overusing these two herbicides could cause future problems unless producers follow good weed management practices and rotate production fields with other herbicides and crops. Overuse of a single herbicide can promote weed resistance. It can also lead to increased populations of uncontrolled weeds (population shift). Incorporating before bed shaping then using after planting, may reduce the potential shift in weed species.
Additional research is needed to evaluate new ways to use older products. As well, newer and older products need further evaluation in terms of chemical rates, tank-mixes, timings, and use patterns. Over the past decade,Wintergarden trials and other university research projects have included thiobencarb, flufenacet, EPTC, triallate, oxyfluorfen, KIH-485, linuron, ethofumesate, dimethenamid-P, quinclorac, pendimethalin, imazamox, terbacil, and fluroxypyr among others. Some formulations caused significant crop damage while others showed little injury but failed to control weeds acceptably. Still others showed promise for low crop injury and good weed control, but the manufacturers have shown no interest in spending money to register them.
The table below describes current herbicide options available for spinach growers in Texas. Parameters include the application method, the expected season-long weed control, percent crop injury and potential yield reduction from the herbicides applied alone or in combination.
Table 1 shows the advantages and disadvantages of each herbicide as well as their potential in-field combinations. However, there are no herbicides that will control all the weeds in spinach; some will have to be removed by hand.
Ro-Neet is very safe to spinach but only controls most winter weeds for about 3 weeks. Its advantage over Dual Magnum is that it controls henbit. Dual Magnum improves overall weed control but experience shows that there is generally up to 10 percent yield loss compared to untreated spinach. The losses are generally due to Dual Magnum’s tendency to cause root injury when spinach is planted at cooler temperatures. This loss is seen as acceptable due to the product’s weed control advantage over Ro-Neet. Some spinach varieties may be more sensitive to Dual Magnum than others.
Broadleaves: Spin-Aid and Stinger are two broadleaf herbicides that are available for use in spinach. Spin-Aid is a contact herbicide that burns the leaves while Stinger moves throughout the plant and creates symptoms such as leaf twisting and curling. Each of these products controls weed best when they are small, from cotyledon to five leaves.
Spin-Aid will control London rocket, shepherd’s purse, and purslane. This product requires a 40-day preharvest interval (PHI) and using it at temperatures above 75 degrees increases the risk of spinach injury. If daytime temperatures are high, it is best to spray during the evening. Adding a surfactant to Spin-Aid may increase crop injury, which shows up as marginal leaf burn. This type of injury can delay harvest and lower yields (Figure 12).
Stinger is registered for many broadleaf weeds including dandelion, prickly lettuce, thistles, and volunteer legumes such as alfalfa, clover, etc. It may suppress London rocket when applied to the weed just after it emerges. It must be applied between the 2nd and 5th leaf stage of spinach and has a 21-day PHI. Stinger does not require a surfactant, but if you include one, follow all label instructions. There is a 10.5 to 18 month rotation restriction for legume crops after applying Stinger. Flat leaf spinach varieties are more sensitive to Stinger than Savoy types. However, though flat leaf varieties can experience mild to moderate leaf twisting/curling, it does not affect the quality of processed spinach.
Grasses: The two postemergence grass herbicides available for spinach in Texas are formulations of sethoxydim and clethodim; their trade names are Poast Plus and Select Max, respectively. Generic formulations for these two products may also be available so check labels for registered uses. Both herbicides control annual and perennial grasses in spinach. Annual grasses may be a problem during early plantings. Perennial grasses like Johnsongrass, or volunteer corn, or wheat are controlled with either product. Surfactants help with herbicide uptake and may improve control. Crop injury is rare with these products.
Improving weed control with high-density plantings
A healthy field of spinach will compete with weeds much better than a field that is emerging poorly. Crops stressed by insufficient water, low fertility, or herbicide injury will compete with weeds less effectively. In the early 2000s when growers switched from 40-inch beds to 80-inch beds and increased crop densities, spinach competitiveness with in-row weeds improved. The table below shows the potential weed management benefits of treated and untreated spinach at higher seeding rates. It also shows the combined effect of higher density seeding when treated with low and standard herbicide applications. Even when no herbicides were used, higher planting densities reduced weeds by 85 percent. Fumitory (wild carrot) was the prevalent weed for this particular data.
Effects of herbicide and handweeding on the cost of weed control
Weed control consists primarily of preplant cultivation followed by preemergent herbicides and handweeding in spinach. Eighty-inch beds and high-density seeding make cultivation during the season less likely, although some use cultivation to keep the bed furrows clean. As a result, weed control depends mostly on herbicides and quick coverage by the spinach crop.
No herbicide controls all weeds in spinach, so handweeding is often needed. Research in the Wintergarden region evaluated registered and potential herbicides for high-density spinach in terms of overall weed control, crop injury, and yield. The study also documented the effects of post-herbicide handweeding and its effect on weed populations, yield, and grower profitability.
In the two years of study, there was no effect on yields by treatment, but the numbers of weeds removed from each ton of spinach harvested was lowest with Dual Magnum applied once at the full rate. Though each handweeding raised weed management costs, they reduced the numbers of weeds per ton and the cost of hand removal at the processor.
Drift potential from off-site herbicides
In the past 20 years, chemical companies have shifted research priority to herbicide-resistant seed in corn, soybeans and cotton. Given the spread of glyphosate-resistant weeds over the past decade, chemical companies have increased their focus on developing seed technologies to produce crops that will tolerate 2,4-D and dicamba. The potential for increased use of these herbicides poses a concern to the specialty crop industry.
Given Texas’ windy and warm conditions, widespread applications of these products could significantly damage spinach production and yields by contaminating off-site spinach fields. Recent trials evaluated simulated drift of 2,4-D, dicamba, and glyphosate at very low rates to determine their effects on yield, quality, and visual response (Figures 13, 14, and 15). While the spinach crop may sometimes recover from herbicide drift injury, it is illegal to sell crops that have been exposed to nonregistered products or improper uses. In those cases, the crop should be destroyed. Table 4 shows estimated crop injury and yield responses from those trials.
The future of weed control in spinach
Widespread use of Dual Magnum plus handweeding will likely continue to be the first choices for weed control by spinach producers in the Texas Wintergarden and throughout the state. Ro-Neet, Stinger and Spin-Aid plus grass herbicides will be used as needed. As of 2014, chemical manufacturers have shown little interest in expanding their product uses to spinach, especially where there are perceived risks of crop injury. Additionally, revenue potential to the companies does not justify registration expenses.
What is the future of weed control in processing and fresh-cut spinach?
- It is unlikely that herbicide-resistant spinach will be developed in the near future.
- The continued reliance on Dual Magnum may lead to greater populations of weeds it does not control, or possibly even weed resistance that will add to the cost of weed management in spinach.
- With the development of dicamba- and 2,4-D-resistant row crops, and potentially others in the future, off-site drift to nontarget food crops like spinach is increasingly likely and will require growers’ strict attention.
Improving weed control in spinach is the only way to meet these continuing challenges and will require continued collaborative efforts among all stakeholders in the spinach industry.
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