Spinach is a quick-maturing, cool-season, hardy vegetable crop that is grown for both fresh markets (field packed in bunches or lightly processed in cellophane bags) and processing markets (frozen products). A fresh-market spinach crop can be grown in 30 to 50 days, while the same variety grown for freezer processing takes 70 to 120 days. In California, spinach is planted at distinct times in three production areas: (1) year round in the Salinas Valley and Central Coast; (2) fall and winter in the Central Valley; and (3) fall and winter in the Coachella and Imperial Valleys. Weed populations in spinach fields differ by geographic location and by planting season.
The spinach planting configuration and low tolerance for weed contamination make spinach susceptible to crop losses caused by weeds. In coastal California districts, spinach is frequently planted in dense stands of 16 to 24 seed lines per 80-inch bed. This dense planting arrangement makes cultivation of the bed top difficult if not impossible. Spinach grown in the Central Valley is often planted two seed lines per bed; a configuration that does permit cultivation. (Hand-weeding is commonly practiced, but it is very expensive. Growers are commonly faced with the choice of high hand-weeding expenses or rejection of the crop because produce buyers have a very low tolerance for spinach contaminated with weeds.)
Weed management in spinach is necessary throughout the season to achieve a sufficient marketable yield. Weeds that are present in and around fields at planting sometimes harbor insects, pathogens, nematodes, or vertebrates that can invade or spread to the spinach plants when they emerge. Early-season weed control is especially important in precision-planted crops like spinach because compared to other vegetable crops (sweet corn, tomatoes, and even some cole crops), spinach is a relatively poor competitor against weeds. Loss of spinach seedlings to weed competition can substantially reduce the vigor and uniformity of the overall stand.
Economically acceptable weed control can only be achieved with a weed management program that integrates several methods because no single weed control practice provides complete weed control in spinach. An integrated weed control program relies on both cultural and chemical management methods to keep weed populations at tolerable levels.
Herbicide options for spinach are extremely limited. Although seven herbicides are available for use, only four (cycloate, phenmedipham, clethodim, and sethoxydim) are selective and are used during the cropping season. None of these herbicides provide control of all weeds that infest spinach fields, so correct identification of weeds is important in selecting the best herbicide to use.
A typical weed control program includes the use of preplant fumigation with metam sodium or a preemergent herbicide. Postemergent herbicides such as phenmedipham (Spin-Aid) and sethoxydim (Poast) are only used in specific situations such as grass weed control or for broadleaf weeds in processing spinach or seed crops. Most spinach herbicides are applied to the bed top and the bandwidth of the herbicide spray varies depending on whether cultivation will be employed.
Poor or erratic weed control can occur with any herbicide used in spinach. Unsatisfactory herbicide performance may be the result of several factors, such as poor field selection, poor land preparation, faulty herbicide application, or the presence of resistant weeds. In addition, spinach herbicides are not 100% selective and can, under certain conditions, cause stunting, chlorosis, necrosis, or even death of spinach seedlings. Neither the fresh market nor the processing industry tolerates retardation of growth, discoloration, or deformity of spinach. It is therefore critical to use spinach herbicides carefully to avoid undesirable side effects from their use.
Selection of the best weed management program is governed by several factors:
- Geographic location, which determines planting date, weed spectrum, and irrigation/rainfall patterns;
- Date of planting, which determines weed spectrum and irrigation/rainfall;
- Weed species present (or anticipated to be present), which determines choice of weed control method and choice of herbicides;
- Availability and cost of hand-weeding, which determines if hand-weeding can be considered within the program;
- Planting configuration, which determines if cultivation can be considered in the program and if herbicides must be incorporated into the soil; and
- Irrigation method, which determines how the herbicide is incorporated, i.e., with overhead irrigation, or mechanically if overhead irrigation is unavailable.
Monitor spinach fields for weeds during the cool and warm seasons to determine the species of weeds that are present. Geographical and field differences in weed populations vary significantly and most herbicides are selectively effective on weeds, so it is important to know which weeds are present. Seeds can remain viable in the soil for many years and cause recurring problems in fields. The weed spectrum in a field reflects the effectiveness of weed control programs over several seasons. Therefore, monitor weeds in the previous crops to determine the weed spectrum in the field. Never allow weeds to set seed in a field intended for spinach production.
Weed Management Before Planting
Management of weeds in spinach requires a combination of control strategies. Cultural controls including sanitation and rotation, and mechanical controls including tillage, cultivation, and hand weeding along with herbicide use are considered the core of a weed management program.
Choose fields known to have low weed pressure and few problem weeds such as burning nettle, little mallow (cheeseweed), chickweed, London rocket, and shepherd's-purse. Selection of a field that has low weed pressure will make subsequent weed control operations more efficient and economical. Additionally, herbicide choices are very limited for spinach growers, which makes spinach more vulnerable to weed losses or high hand-weeding costs.
Also, when choosing fields, it is important to pay careful attention to plantback intervals because small amounts of selective herbicides used on the previous crop may remain (carryover) in the soil long enough to damage a subsequent spinach planting. Spinach is particularly sensitive to soil residues of benefin (Balan), chlorsulfuron (Glean), isoxaben (Gallery), pendimethalin (Prowl), and trifluralin (Treflan).
Weed control in previous crops in the field can dramatically affect weed pressure in subsequent crops. Keeping fields as weed-free as possible and preventing weeds from going to seed in previous crops helps to reduce weed pressure in subsequent spinach crops.
Clean all field equipment when moving from weedy fields to clean fields. Preventing the introduction of weed seeds into an otherwise uninfested field can greatly assist in the economical control of weeds.
Spinach should be grown in rotation with crops that have effective weed control programs. The importance of an effective crop rotation program is that the overall weed pressure is maintained at low levels. In coastal districts spinach rotates with lettuce, however if pronamide (Kerb) or bensulide (Prefar) are used in the lettuce crop, the field may not be cropped to spinach for 6 or 4 months, respectively. In the Central Valley, common rotation crops are processing tomatoes, beans (blackeyes, limas), and sweet corn. More herbicides are available to use in these crops than in spinach, but when rotating with spinach, always check the herbicide label for spinach plantback restrictions.
A well-prepared seedbed that is free of large clods permits precision planting with rapid and uniform emergence of spinach seedlings. Uniform depth of seeding is critical when using preplant-incorporated herbicides because if the spinach seeds are planted too deeply, the seedlings may be killed by the herbicide. Well-prepared seedbeds also permit proper and accurate incorporation of preplant-incorporated herbicides, leading to improved weed control and reduced phytotoxicity to the seedlings. In situations where cultivation can be used, uniform beds with level bed tops are essential.
Soil solarization can be used to control most weeds in spinach grown in California's interior valleys. It will also control or suppress some other important pests such as nematodes. Soil solarization requires a summer fallow season for treatment; it fits best with a fall-planted crop. The effectiveness of solarization is best in areas of the state where temperatures are consistently hot in summer; consequently, solarization may not be as reliable in the coastal production districts as in interior valleys, but most times effective weed control results.
When possible, preirrigate before seedbed preparation or preirrigate prepared seedbeds. Preirrigation followed by cultivation, flaming, or use of a nonselective herbicide such as glyphosate, kill an initial flush of weeds. If cultivating, do not work the soil too deeply (more than 2 inches) in order to avoid bringing up soil from greater depths that may contain ungerminated weed seed. For organic fields or for weedy fields, preirrigation can be repeated to deplete the weed population in the seed bed.
There are three major types of herbicides that are used for weed control before the crop is planted. The first group is fumigants that kill weed seeds and germinating seeds before planting. The second group is herbicides that kill weeds that have emerged after the beds were formed; these herbicides are referred to as preplant foliar herbicides. The third group, preplant incorporated, consists of herbicides that control weed seeds as they germinate. As the name implies, the preplant-incorporated herbicides must be incorporated into the soil soon after application to prevent volatization of the chemical and to move the herbicide into the root zone.
Metam sodium is available as a soil fumigant to control soilborne diseases and nematodes, but it can also be used to control weeds, although results are not always consistent. Be sure the soil is well cultivated and moist before its application. Wait at least 14 days before planting spinach and avoid moving untreated soil onto the bed top or seed row.
Preplant Foliar Herbicides
The postemergent herbicides, glyphosate (Roundup, Touchdown) and pelargonic acid (Scythe), are used to kill existing weeds on preformed beds before planting spinach. Pelargonic acid has contact action only, and therefore it is most effective on young seedlings. Glyphosate has systemic action and is effective on most established weeds. Three key weeds, little mallow, purslane, and burning nettle, are relatively tolerant to glyphosate and are not well controlled by it.
The herbicide cycloate (Ro-Neet) performs best when the soil is incorporated immediately after application. Follow label directions regarding depth and method of incorporation. Incompleteincorporation of cycloate (Ro-Neet) results in poor weed control because much of the herbicide is lost to volatilization. If beds have not been shaped properly, precise depth of incorporation may not be possible and herbicide performance may be erratic. Preplant incorporation does not work well in cloddy soil and herbicide performance will usually be poor under such conditions. Immediate incorporation to prevent volatility losses is especially important if the soil is moist.
Disc incorporation of cycloate (Ro-Neet) can provide adequate control of grass weeds but often results in only partial control of broadleaved species. Disc incorporation also runs the risk of mixing the herbicide too deeply in the soil, thus increasing the risk of crop injury.
Weed Management After Planting
A healthy, vigorous crop provides substantial competition that suppresses weed growth and acts as part of the weed control program. Therefore, proper fertilization, irrigation, and insect and disease control measures promote good crop growth and compliment other weed control measures.
Spinach stands that are uniform are better able to compete against weeds. Gaps in the stand provide space for weeds to become established before the spinach canopy grows and completely shades the soil.
No specific systems of biological control have been introduced for control of weeds in spinach. Some weed seeds in the soil may be attacked by insects, bacteria, and fungi, which help reduce the number of some weed seeds in the soil if new crops of weed seed do not replenish it. However, no consistent control methods have been developed with these organisms.
Cultivation is an effective method of weed control. Good bed preparation is essential to allow close cultivation of spinach. Repeated shallow cultivations dislodge small weed seedlings that emerge after each irrigation.
In bed-planting configurations that have two seed lines per bed, up to 80% of the bed can be cultivated early in the crop cycle. In bed-planting configurations that have more than 2 seed lines per bed, the percentage of the bed that can be cultivated is greatly reduced.
Adjust cultivators to disturb only a shallow layer (ideally not over 2 inches) of soil to minimize bringing weed seeds up from deeper layers. Timeliness in cultivation is essential. Seedlings are easier to kill than larger weeds.
Hand-weeding is an expensive component of the crop production budget. If a weeding crew is sent into a field immediately before harvest when weeds are mature, weed removal is typically slow and expensive. Ideally, good cultural practices and careful use of preemergent herbicides will result in minimal hand-weeding requirements.
After planting the crop, there are two periods in which herbicides may need to be applied, depending on the weed species present. Postplant preemergent treatments are applied after planting but before the first irrigation and the emergence of the crop; postemergent treatments are applied when the spinach is in the seedling stage or older.
Cycloate (Ro-Neet) and S-metolachlor (Dual Magnum) are applied to the soil surface after planting but before the crop has emerged. The best management practice is to follow the application immediately by sprinkler irrigation. Depending upon the soil texture, 0.5 to 0.75 inch of water is applied to activate the material.
Phenmedipham (Spin-Aid) can be applied in one application when the crop has four to six true leaves or as a split application when the spinach has two true leaves followed by a second application 4 to 6 days later. Temporary stunting, yellowing or tip burn may be observed after the treatment. Phenmedipham is only registered for use on spinach grown for processing or seed.
Sethoxydim (Poast) and clethodim (Select Max) are used for grass weed control and must be applied with a crop oil concentrate adjuvant when grasses are actively growing to obtain satisfactory activity. Clethodim effectively controls annual bluegrass. Make sure that the grasses are growing vigorously and are not stressed for moisture to ensure good control; low soil moisture reduces control substantially. Follow label restrictions in relation to spray volume.