Introduction

Salad greens provide market gardeners a high value crop that brings a premium price if grown and marketed successfully. Salad mixes have become particularly popular among customers seeking convenience. Salad mix is a collection of leafy greens pre-mixed and sold together. This idea originated in France centuries ago where a mix of immature “baby” greens was called “mesclun.” The greens can be grown as a mixture in the field or combined after harvest.

Because of their highly perishable nature, salad greens are better suited to local production than long distance shipping. Growers should be aware that the inputs necessary to produce a quality crop can be high and include a lot of hand labor.

The crops discussed in this publication include four types of lettuce (crisphead, butterhead, romaine, and leaf), spinach, endive, escarole, radicchio, witloof, and Swiss chard.

Plant Descriptions

Most of the salad greens grown in Wisconsin belong to one of three plant families: Asteraceae (composite family), Brassicaceae (mustard family), or Chenopodiaceae (goosefoot family). The majority are cool-season annuals, although some varieties can tolerate summer heat. As a result of the natural growing conditions suited for salad greens, some market gardeners only offer them early in the season and again in the fall when temperatures cool down.

Lettuce

Lettuce (Lactuca sativa) is a cool-season, herbaceous annual in the Asteraceae family, native to the eastern Mediterranean and Asia Minor. Early records of lettuce cultivation have been found on Egyptian tombs dating back to 4500 B.C. Lettuce was cultivated for the medicinal properties of its milky sap and for its seed oil. Lettuce is thought to have arisen from Lactuca serriola, a widely dispersed, primitive, spiny form. The word “lettuce” is derived from lactis (Latin for “milk of plants”).

Despite the low level of vitamins and nutrients in most lettuce varieties, lettuce is considered the fourth most significant U.S. crop from a nutritional standpoint because of the large volume consumed. Lettuce contains fiber, vitamins A and C, calcium, and iron. Leaf lettuce has more nutritional value than head lettuce, and dark green leaves have more nutritional value than light colored leaves.

The different varieties of lettuce have a huge variation in leaf shape, texture, color, and overall plant form.

Spinach

Spinach (Spinacia oleracea) originated near Iran and was introduced into China via Nepal in 647 A.D., and into Spain via N. Africa in 1100 A.D. Spinach is a member of the goosefoot family, Chenopodiaceae.

The genus name Spinacia refers to the prickly seed capsule and is derived from spina which means “spiny” in Latin. This hardy, cool-season annual is an important source of vitamins A and C as well as calcium, phosphorus, iron, and potassium. Spinach is also high in fiber and relatively high in protein.

Spinach leaves are borne in a rosette with many leaves attached to a short fleshy stem. Leaves may be smooth or savoy (crinkled). Savoy types have a layer of cells that continue to grow between the leaf veins causing a puckering effect. Spinach cultivars may be upright or spreading.

Endive and Escarole

Endive (Cichorium endivia) is a hardy annual or biennial with two forms: curly and broadleaved. Endive is a member of the composite family, Asteraceae. Records of its cultivation date back to ancient Egypt. Curly endive is also known as chicorée or frisée and has narrow, lacy, deeply cut curly leaves. Broadleaved endive is called escarole and has large, crumpled, smooth-edged leaves. Both curly endive and escarole have a slightly bitter taste, although much less so than radicchio. Curly endive withstands heat better than escarole, radicchio, and witloof. Escarole is more cold tolerant than witloof or curly endive and also tolerates low light conditions, making it potentially well-suited for winter greenhouse production.

Radicchio and Witloof

Radicchio (Cichorium intybus) is native to the Mediterranean region and is a cultivar of chicory. Radicchio forms loose, small heads similar to a pointed cabbage. It typically has red leaves that are white along the midrib and near the base. The bitter leaves sweeten slightly in cooler weather. Radicchio is somewhat frost tolerant.

Witloof (Cichorium intybus) is closely related to radicchio and is a common European salad crop that can be grown in the Midwest. It is also sometimes referred to as Belgian endive or chicon. Witloof is Flemish for “white-leaf.” It does not form heads but produces a tight cluster of individual upright leaves in the second year of growth. Plants are grown in the dark to blanch the leaves so they’ll remain tender and less bitter. Witloof is eaten raw in salads and also steamed, boiled, or baked. Witloof is very perishable and does not ship easily. Since it is not commonly grown in Wisconsin, you may need to test your potential market before you plant a lot of it. Try forcing a few roots on a small scale, eating them yourself so you can describe the flavor, and then visiting potential customers with your sample product. Witloof does not contain many vitamins or nutrients.

Swiss Chard

Native to the Mediterranean region, Swiss chard (Beta vulgaris var. cicla) is a member of the Chenopodiaceae family and technically a variety of beet (Beta vulgaris). Swiss chard is grown for its large crinkled edible leaves and fleshy leaf stalks (petioles) which taste similar to spinach. It is an herbaceous biennial which is grown as an annual. Unlike most other salad greens, chard does not bolt (form a premature seed stalk) when exposed to high temperatures. Therefore, chard is often grown during the heat of summer.

Specialty Greens

Numerous crops are grown as salad greens in the Midwest. Some, such as arugula, mizuna, and dandelion, are used in small quantities to add flavor to salad mix. Others, such as New Zealand spinach and purslane, grow well in hot weather and are used to fill out summer salads, while cold-tolerant crops such as mâche, claytonia, and cress extend the growing season. For more information about these crops, consult the references at the bottom of this publication.

Where to Grow Salad Greens

Because salad greens are planted early in the spring—in most cases as soon as the ground can be worked, it is important to select a site where the soil warms rapidly. Salad greens transpire a lot of water and also have shallow root systems, so soil with good moisture retention is beneficial for these crops. Adequate drainage helps prevent diseases.

Early season crops are often planted on sandy soils which warm up quickly and can be worked early. Mid- to late-season crops should be planted on heavier soils or soils with a high organic matter content so they have better moisture-holding capacity. Growing leafy crops on muck soils reduces the amount of grit caught in crinkled leaves such as those of spinach. Salad greens respond well to rich, friable soil high in organic matter. Small seeds common to salad greens cannot germinate in crusted soil.

Salad greens are especially attractive to deer. Consider using portable electric fencing to protect your crop. Consult your county Extension office for information.

Choosing the Best Cultivars for Wisconsin

There are four main types of lettuce commonly grown in the Midwest: crisphead, butterhead, romaine, and leaf. Lettuce is bred for disease resistance, leaf or head color, size, shape, texture, flavor, earliness, uniformity, and heat tolerance (resistance to bolting).

Some varieties of lettuce perform better under long day conditions while others are day-neutral and not affected by day length. Table 1a lists some recommended cultivars for Wisconsin.

Note: Cultivars of salad greens perform very differently depending on soil temperature, air temperature, and day length. Choose cultivars according to your own situation and needs. Consider what your market demands, the length of your growing season, your soil, pests, diseases, irrigation, cultivars other growers like, and cultivars you personally like. When trying a new cultivar, do not use it exclusively. Grow new trials next to old standbys so you may compare the characteristics objectively

Crisphead lettuce

Crisphead lettuce cultivars (Lactuca sativa var. capitata) are often synonymous with “iceberg” lettuce, which is the name of a cultivar. Leaves are large, heavy, and brittle. The outer leaves are green while the interior leaves are white or yellow. After initial rosette development, leaves begin to overlap each other and become tightly folded to form a dense head which is usually more than 6 inches in diameter.

Butterhead lettuce

Butterhead lettuce cultivars also belong to L. sativa var. capitata, but the heads are smaller and more compact, the leaves are loosely crumpled and have less prominent veins and midribs, and the lettuce has more flavor and a buttery texture compared to crisphead lettuce. There are two distinct types of butterhead lettuce: Boston—a day-neutral cultivar which produces large heads with light green leaves, and bibb—a smaller, dark green, short-day cultivar. Bibb lettuces do not travel well since their leaves bruise and tear easily, so these types are well suited to local production. A third type, Batavia, has characteristics that are intermediate between Boston and bibb.

Romaine

Romaine, also known as cos (Lactuca sativa var. longifolia), produces elongated upright heads with long leaves that have prominent midribs. Romaine lettuce has coarse outer leaves and tender, flavorful inner leaves. It is more tolerant of adverse environmental conditions such as temperature and moisture extremes than crisphead and butterhead lettuce.

Leaf lettuce

Leaf lettuce (Lactuca sativa var. crispa) cultivars come in all types of shapes, colors, and textures. They are particularly well suited to local production since they do not ship well and would otherwise not be available commercially from other states. When selecting cultivars for salad mix, keep in mind that frequent dense plantings are required, so you will need a lot of seed. Since seed prices vary, you may wish to grow the more expensive varieties for mature heads or leaves and use less-expensive varieties for salad mix.

Spinach

Spinach cultivars are selected for their vigor, uniformity, disease resistance, nutrition, and resistance to bolting. Varieties are classified based on leaf texture: smooth or savoy (crinkled).

Endive, escarole, radicchio, and witloof

Endive, escarole, radicchio, and witloof cultivars are bred for less bitterness, resistance to tipburn, uniformity, nutrition, vigor, and appealing colors or leaf textures.

Swiss Chard

Swiss chard cultivars are bred for flavor, disease resistance, vigor, and color such as red, pink, orange, white, and yellow.

Table 1a. Recommended lettuce cultivars for Wisconsin

Table 1b. Other recommended cultivars for Wisconsin

Long-Day vs. Short-Day Lettuce Cultivars

Some cultivars require long days to initiate flowering (at least 15 hours of light) while others require short days (less than 12–15 hours). “Short day” is actually a misnomer as darkness rather than day length triggers blooms. Plants that are unaffected by day length are known as day neutral.

Planting, Transplanting, and Culture

Refer to table 2 for the estimated amount of seed required, seed planting depth, yield, planting date, spacing, and days to first harvest.

Salad greens produce a marketable crop relatively quickly and are typically planted in succession so that growers can offer customers salad crops throughout the season. Sow seed as soon as possible in the spring. (Figure 1 shows approximate dates of the last spring and first fall killing frosts of the growing season.)

To help keep track of plantings during the hectic growing season, take some time in the winter to make a calendar that lists proposed yearlong planting dates. Keep in mind that the number of days to harvest for fall crops may actually be longer than that listed on the seed package because environmental conditions may slow growth. Some crops such as spinach can be overwintered, or grown under shelter such as under floating row covers, in a cold frame, or in a tunnel or hoophouse with one to two layers of clear plastic. Escarole and some other specialty crops such as claytonia, mâche, and land cress may also be grown under winter protection.

If you grow heat-sensitive leafy crops in the summer, you can try to create a cooler microclimate by shading plants with a lightweight shade cloth, which allows some light to pass through, or by planting between rows of taller crops oriented in a north to south direction. Since high soil temperatures can prevent germination, you may wish to pre-germinate seed for late-summer plantings by soaking them in water until the first root (radicle) just begins to emerge. Such seed needs to be planted by hand.

Soil Preparation

Work beds 6–7 inches deep to promote good rooting. Just before seeding, use shallow cultivation to prevent soil crusting. For early season crops on sites where erosion is negligible, prepare the site the fall before planting. Preparing the soil in the fall will prevent any planting delays caused by cold, wet soils and will help reduce soil compaction associated with working wet soils. Compacted soils restrict root growth, reduce the amount of oxygen available to roots, and limit water penetration, all of which can hurt yield potential.

Raised Beds

Raised beds are an alternative to the conventional field planting method. They improve soil drainage and allow access to the crop without causing soil compaction. Raised beds are typically 4–5 feet wide and 100 feet long. The width is determined by the type of equipment used and by the crop. Leave a 1-foot aisle on either side of each bed to accommodate foot traffic.

Seeding

Seeds for salad crops tend to be very small. Some companies offer pelleted seeds that are coated and rolled to make them uniform and easier to handle, especially in precision seeders. Some pelleted seeds include pesticides in the coating. Precision seeding requires specialized planters and uniformly sized seeds, and prevents the need for thinning.

Specialized planters include the gravity-feed cone seeder, the belt-drive seeder, and the vacuum seeder. Walk-behind plate-type seeders are also used. Some growers who do not use precision seeders thin salad greens by hand and sell the thinnings. Prevent a crust with shallow cultivation, mulch, or by supplying enough moisture to wet the soil at germination but not to saturate it.

Starting Seeds

Salad greens can be started in the greenhouse and transplanted outdoors to get a head start on the growing season. Start your transplants 3–4 weeks before you plant them outdoors. You can buy or mix your own sterile potting mix for starting transplants. The mix should include peat, sphagnum, or compost to retain moisture; vermiculite or perlite for aeration; and mineral and nutrient sources to encourage growth after the first roots form.

Fill plastic or polystyrene cell trays with potting mix, or make individual blocks with a soil blocker. Cells as small as 1 inch in diameter are fine. Plant seeds and cover with a fine layer of media. Label transplant trays with cultivar and planting date. Keep media moist but not wet. Refer to the culture section to find the best temperature for germination. Typically the nighttime temperature should be 10–15°F cooler.

Thin seedlings to one plant per cell or plug, or one plant per inch if grown in undivided flats. Ten days before transplanting, move plants into a cold frame to harden them off, or put them outside for a few hours each day during the warmest time of the day.

Figure 1. Approximate dates for first and last killing frosts

Table 2. Planting Guide

Lettuce Culture

Lettuce prefers temperature of 65–70°F during the day and 45–55°F at night. High light intensity and long days increase the growth rate and hasten leaf development. However, long days associated with high temperatures will cause lettuce to bolt. Lettuce, like most salad greens, can be planted out as early as April 15 in southern Wisconsin with successive plantings made every 2–3 weeks to assure a continuous crop as long as cool weather permits. Some heat-tolerant varieties of lettuce can be planted as late as early July. Under optimum conditions, lettuce seed will germinate in 2–4 days and emerge shortly thereafter. Cooler soil temperatures will slow the rate of germination, and germination stops at temperatures above 77–86°F (depending on cultivar). For head lettuce, plant three seeds every 8 inches in rows 12–18 inches apart and thin to one plant every 8 inches. For leaf lettuce, plant densely and thin to 1 inch between plants. Plant in rows 2–4 inches wide or in beds.

Decide whether you would like to combine seeds to mix cultivars within the planting beds or grow them separately to be mixed postharvest. An advantage to planting individual rows to only one cultivar is that if diseases or insects affect one type of lettuce, then you can more easily manage the problem than if they are all grown together.

Light is required for germination so lettuce seed is typically only lightly covered with soil after planting. Sprinkle lightly for even moisture.

Head lettuce may be started from seed indoors and transplanted out in early May to ensure adequate time for maturity before high temperatures cause plants to bolt. Transplants can be planted in a honeycomb fashion in the field whereby alternating rows of 3 plants and 2 plants equidistant from each other are planted in the bed to maximize the use of available space. Transplants should be watered well at the time of planting.

Spinach Culture

Spinach leaves grow best at 55–65°F. Under long days and warm temperature, spinach quickly bolts and stops producing large leaves. In the Midwest, spinach must be grown in the spring or fall when short days trigger leaf production instead of flowering. Plant spinach from mid-April through early May and again from early August through early September. At colder temperatures growth is slowed. Spinach is very hardy and if the plant is acclimated to cold it can withstand some freezing down to 14–20°F. Spinach seeds can be sown in late fall the year before and, depending on temperature, they will grow to maturity before frost or grow to a size (3–4 inches diameter) that will survive the winter and begin growing new leaves early the next spring. Spinach seeds can also be sown in late fall or early winter and covered in mulch to sprout early the next spring.

Similar to lettuce, spinach is direct seeded in rows or may be broadcast in beds. For salad mix, sow seeds 3⁄4 inch apart in rows 2–4 inches wide and thin to 1 inch between plants. Clip when small. For full-sized leaves, plant 11⁄4 inches apart in rows 12–18 inches apart and thin to one plant every 6–8 inches. Seed should be planted 1⁄2–3⁄4 inch deep. Unlike lettuce, spinach is slow to emerge and temperature affects the percentage of seeds that germinate. In temperatures of 40–45°F, almost all of the seeds will germinate, but they will take up to 3 weeks to emerge. In warmer temperatures, germination is faster (7–10 days) but fewer seeds will germinate.

Radicchio Culture

Plant radicchio by seed or from transplants in late May or early June, and seed a fall crop in mid-July. Plant three seeds 1⁄4-inch deep every 8 inches in rows 18 inches apart, and thin to one plant every 8–10 inches. Harvest small heads in 9–10 weeks. Some growers cut back plants in late summer and let them resprout, forming heads 4–6 weeks later.

Witloof Culture

Witloof, or Belgian endive, is direct seeded outdoors in late June in rows 18–24 inches apart. Plants should be thinned to 6–8 inches apart within the row. In the fall dig the roots and place them in cold storage. In early fall, when roots are 6–7 inches long and 1–2 inches wide at the crown, lift them from the ground and trim the tops to 2 inches. Smaller roots will not produce a tight bud of leaves, and larger roots will produce several buds that are too loose and small. Store the roots at 32–40°F to encourage dormancy and high relative humidity so they do not wilt.

Check your seed package for the length of time needed for cold storage, since cultivars vary from a few weeks to several months. After cold storage, place roots upright in boxes or pails of damp sand, compost, or peat and cover them 1 inch deep. Keep the boxes at 50–60°F and 95% humidity in the dark. In 30 days or so, buds form. Lower temperatures will produce tight, round buds more slowly, and higher temperatures will produce elongated looser buds more quickly. Cut the buds just above the soil line with a sharp knife when they are 4–6 inches long. Keep them in the dark until consumed, since light will turn leaves green and make them more bitter. Roots will resprout for a second and third harvest.

Endive and Escarole Culture

Cultural practices for curly endive and escarole are similar to those for lettuce, but they tolerate greater temperature extremes than lettuce. Both grow well in 60–75°F weather, and unless they are bred for day length adaptability, will tend to bolt under long summer days with hot temperatures. Also, hot weather tends to make leaves taste more bitter. Curly endive tolerates heat well and escarole tolerates cold. Blanching (preventing light from reaching the leaves) will reduce the bitterness of endive and escarole. Planting close together helps self-blanching cultivars keep out light. Plant 1⁄4-inch deep in spacings as you would lettuce, 8 inches apart for heads and 1 inch apart for salad mix. You can direct seed or start transplants indoors. To blanch interior leaves on a head, wait late enough in the day to dry the leaves and then tie the outermost leaves together for 2 weeks before harvest. Endive and escarole do best with constant soil moisture, which reduces tip burn on leaves.

Swiss Chard Culture

Chard can be planted as early as mid-April and planting can continue every four weeks or so until 30 days before frost for salad mix and 60 days before frost for full sized leaves. Cold temperatures can induce bolting which stops leaf growth and makes leaves bitter. Plant either directly in the field or use transplants. For salad mix, seed 1⁄2-inch deep and thin to 1 inch. For full-sized leaves, space plants 8–12 inches apart in the row.

Extending the Growing Season

You can lengthen the growing season by protecting plants from late spring frosts and early fall frosts. There are a variety of ways to prolong the growing season, including planting on a southern slope, creating a warmer microclimate using floating row covers, clear plastic tunnels, cold frames, or greenhouses made of plastic sheeting (“hoophouses” or “polyhouses”).

Floating Row Covers

Floating row covers of spunbonded polypropylene allow sunlight and water to pass through the fabric. They can be used to protect low-growing crops from frost, to serve as windbreaks, and to protect against insect pests. Depending on the fabric weight, row covers can provide 4–8°F of frost protection. Row covers allow you to plant salad greens 3–4 weeks earlier in the spring and extend the season 3–4 weeks later in the fall. They can be reused two to three seasons.

To use row covers, drape them over the crop or support them with wire hoops. Gather the edges in a loose accordion-type fold and loosely bury them in soil along the crop row. As the crop grows it will push up enough fabric to maintain a “floating” cover. With crops that will grow to market maturity under row covers, consider supporting the row cover fabric to prevent abrasion damage to plants. Use 9-guage wire hoops spaced 6 feet apart and buried 1 foot deep on each side of the row.

Row covers can be held in place by burying the edges with weights such as rebar. Completely seal all four edges to the ground if you use row covers as an insect barrier. If not using row covers as an insect barrier, remove row covers when the average daily temperature is warm enough for crop growth. Be sure to vent the beds on hot days and to let plants harden off for a few days to prevent burning before completely removing the row covers. Harden plants by removing covers on overcast days or for a few hours on sunny days.

Tunnels and cold frames

Tunnels are large, unheated plastic-covered hoops that can be used to extend the growing season. Slitted clear poly tunnels increase daytime temperatures by 10–30°F and provide 1–4°F of frost protection. Cold frames can be used to grow crops to maturity earlier in the spring and later in the fall than would normally be possible. Consult the references at the end of this publication for more information on cold frames and tunnels.

Hoophouse, polyhouse, or greenhouse production

Salad greens lend themselves well to season extension with greenhouses or plastic hoophouses. You can plant under shelter 4–6 weeks earlier than directly outdoors; some crops as early as February. Similarly, fall crops can be extended through November before natural light levels become too low and supplemental lighting becomes necessary. Floating row covers on top of crops grown under plastic can give additional insulation. Crops grown in these structures can be grown from transplants or direct seeded.

Soils and Nutrient Management for growing Salad Greens

Obtain a soil test for available nutrients before planting a field for the first time and test routinely thereafter at least once every 3 years. After 3 years, soil conditions can change enough to make your current fertility management program obsolete. For information on how to collect good samples and where to send them for analysis, see Sampling Soils for Testing. Routine soil tests include pH, organic matter content, phosphorus, and potassium. Special tests are available on request for nitrate-nitrogen, calcium, magnesium, sulfur, boron, manganese, and zinc. You will receive the results of your soil test along with fertilizer recommendations based on your cropping history and planned use of the field. You can also test your potting mix and the soil you intend to use for indoor production of salad greens.

Soil pH

Soil pH varies between the various salad greens but generally should range between 6.0–8.0 for optimum nutrient availability and to prevent deficiency symptoms. For lettuce, 6.0–8.0 is optimal; for spinach, chicories, endive, and escarole, 6.0–7.5 is optimal; and for chard 6.5–7.5 is optimal. If the pH is below 6.0, apply aglime to raise the pH.

Fertilizer Needs

Plants take up nitrogen as nitrate (NO ^–) orammonium (NH₄⁺), phosphorus as phosphate (P₂O₅), and potassium as potash (K₂O). These chemicals, as fertilizers, can come from organic or inorganic sources. With adequate environmental conditions, soil microbes break down organic matter and supply the chemical nutrients that plants need to their roots. Organic fertilizers can also improve soil tilth and health. Inorganic fertilizers can be used to quickly supply nutrients to plants.

Organic fertilizers can come from a variety of sources such as manures, compost, fish meal, and bone meal. Each material contains varying amounts of specific nutrients. Table 3 provides fertilizer recommendations and table 4 lists organic fertilizers and the amounts of nutrients available in each. For more information on this subject, refer to Extension publication Organic Soil Conditioners (A2305).

Table 3. Annual nitrogen, phosphate, and potash recommendations for leafy crops

Nitrogen

Salad greens have a relatively shallow and small root system, so they cannot explore a wide area of soil for their nutrient needs. Also, rapid, continuous growth encourages crisp high-quality leaves. Frequent small applications of fertilizer are more effective than a few large applications. Adequate nitrogen is necessary for optimum growth of all salad greens. On mineral soils, broadcast no more than one half of the nitrogen during field preparation and band or sidedress the remainder later, such as at thinning. On muck soils, apply all of the recommended nitrogen after field preparation. Supplemental nitrogen may be needed throughout the season to meet the plant’s needs, since 75% of the plant biomass is accumulated in the last few weeks before harvest. A combined nitrate and ammonia-based nitrogen fertilizer will provide both rapid and sustained nitrogen release.

Micronutrients

Lettuce is prone to micronutrient deficiency when the levels of the essential nutrients are low. In particular, boron, copper, zinc, and molybdenum deficiencies may occur in lettuce grown on alkaline soils. Spinach is prone to many of the same micronutrient deficiencies including boron, copper, molybdenum and magnesium. Under droughty conditions, calcium may become deficient, causing tipburn on salad greens. This physiological disorder occurs when high temperature and low water uptake causes rapid growth and decreased transpiration, reducing calcium transport to the developing tissues.

Table 4. Nutrient composition of various organic fertilizers

Irrigation

Because of the large leaf area, their high transpiration rate, and their small shallow root system, salad greens require regular irrigation, particularly on sandy soils. Irrigation is often helpful prior to germination to prevent a crust from forming on the soil. If leaves begin to wilt mid-day, plants are moisture stressed. Plants that wilt intermittently produce smaller yields than plants that are not allowed to wilt, while plants that wilt frequently or for too long will often die due to irreversible cell damage. Lettuce is particularly prone to wilting. Spinach is less demanding of supplemental irrigation, because it has a short lifespan which occurs during the cool part of the growing season.

Both drip and overhead sprinkler irrigation systems are effective. These systems include trickle tape, solid set, and traveler hose wheel. Drip irrigation systems will reduce the incidence of foliar diseases caused by excess leaf moisture.

Harvest, Handling, and Storage of Salad Greens

Almost all salad greens are hand harvested in the Midwest. Growers either pick the outer, older leaves from each plant or clip all the leaves at once. With both methods, new leaf growth occurs and several harvests are possible from each planting. The days from planting to harvest vary depending on the crop being grown and the weather conditions. Sample your own crop frequently to determine the best time to harvest under your conditions. You will be your own best sales representative if you learn to recognize and describe produce quality. Most leafy greens can be harvested from immature stages until they just begin to lose peak flavor. Some become bitter if harvested too late. Also, plants harvested late are more susceptible to postharvest diseases. Some growers sell “micro” greens: tiny leaves harvested when the first true leaves appear. Micro greens are used in salad, in place of sprouts, or as a garnish.

Harvest individual leaves by plucking them from the plant, bunches of leaves by cutting them with a sharp knife or shears, and lettuce and radicchio heads by cutting them with a sharp knife. Cut heads below the lowest leaf and remove any damaged leaves. Harvest salad greens into a container lined with a polypropylene mesh bag, such as a laundry bag. This will reduce handling time postharvest.

Salad greens retain their fresh crisp taste if they are hydrocooled and kept under high humidity. Hydrocool them by submerging the mesh bagfuls of crop in a cool water bath for several minutes immediately after harvest. If the leaves are gritty or dirty, change the water for a second or third rinse.

Or choose a bigger tank to wash greens, such as a recycled stainless steel bulk milk tank from a dairy farm. Some growers have fitted motors from whirlpool baths into bulk milk tanks to create a bubble-type washing system for salad greens.

Dry greens by spinning them for a few minutes or laying them out on screen tables. You can spin dry a small amount of washed greens by swinging the mesh bag in circles over your head lasso-style. Larger quantities can be dried by placing two mesh bags of greens into a secondhand washing machine set to spin cycle for a few minutes. Some growers remove the center agitator from the machine. Other growers use large restaurant salad spinners powered by an electric motor.

Pack greens into waxed cardboard boxes and store them in a cooler. Maintain humidity and provide evaporative cooling by lining the box with a clean damp cloth. Make sure that temperatures do not fall below 32°F, since freezing can harm the crops. Ethylene, a gas given off by ripening apples, melons, pears, plums, and tomatoes will also reduce the quality of salad greens and cause them to yellow. Do not store these crops in the same cooler as salad greens. Sell greens bulk by the pound or in plastic produce bags as soon as possible after harvest. They can be kept in a cooler for 1–2 weeks.

Harvesting Lettuce

Lettuce should be harvested when plants reach a suitable size for the cultivar grown. For crisphead lettuce, the heads should yield slightly to pressure. Crisphead lettuce is ready to harvest approximately 60–120 days after planting, depending on weather conditions and cultivar grown. Butterhead lettuce is mature in 42–70 days, romaine in 50–70 days, and leaf lettuce in 48–58 days. All of the lettuce types except crisphead have excellent quality as an immature crop, and can be harvested starting approximately 3 weeks before full maturity. Butterhead and leaf lettuce have more tender leaves and more exposed leaf surfaces, so they are more perishable than crisphead or romaine lettuce. Lettuce should be stored at 33–35°F and 90–95% relative humidity.

Harvesting Spinach

Spinach matures in 35–50 days and can be harvested once five to seven leaves have formed. Continue harvesting until the new leaves produced become too small or a seed stalk forms. Typically an average of 25 leaves per plant can be expected. Spinach keeps well if stored at temperatures of 32°F and a relative humidity of 85–95%.

Harvesting Radicchio

Harvest radicchio when heads are the appropriate size for the cultivar grown, approximately 60–90 days after planting, and store for up to 2 weeks at 32°F and 95% relative humidity. Endive and escarole reach full maturity in approximately 52–90 days depending on cultivar and growing conditions. They can also be harvested as immature greens beginning 3 weeks earlier. Like radicchio, they should be kept at 32°F and 95% relative humidity and will keep for up to 2 weeks. Harvested witloof maintains freshness for only a few days. Store it in the dark at 32°F and 95% relative humidity.

Harvesting Swiss Chard

Swiss chard matures in 50–60 days but can also be harvested as immature leaves approximately 3 weeks before full maturity. Store chard at temperatures of 32°F and a relative humidity of 85–95%.

Tips for Harvesting and Packing Lettuce

Growing salad greens can involve a lot of stoop labor during thinning, weeding, and harvesting. Change your position often to minimize stress and fatigue to your body. Some growers sit on a one-legged field stool that straps on with a waist belt. Others sit on a cart that straddles the crop rows. Slow-moving tractors are available that straddle the crop row and allow workers to lie face down with their hands free to harvest. Some growers have fitted these machines with a handheld power band saw and vacuum system to cut and collect leafy greens.

With a smooth level floor in the packing area, a palletized packing and storage system can be designed to fit small-scale operations (small pallets moved with a hand-pushed pallet truck) or large operations (pallets moved by forklift). Heavy boxes of produce can be moved from one area to another on roller tables.

Lay out your washing and packing area to minimize stooping, lifting, and carrying. Set up work stations and water baths at table height. Ideally tables could be adjusted to match each worker, so that work is performed at a height between wrist and elbow when the arm hangs at the worker’s side.

Common Insect Pests of Salad Greens

Aphids

Aster Leafhopper

Black Cutworm

Spinach Leafminer

Common Lettuce Diseases of Salad Greens

Aster Yellows

Cercospora leaf spot

Mosaic viruses

Damping off

Downy mildew

Lettuce bottom rot

Sclerotinia drop

Weed Management for Salad Greens

Weed management is essential for crops to produce maximum yields. Weeds compete with crop plants for sunlight, water, nutrients, and space. It is essential to eliminate perennial weeds before planting. Do this by smothering with a cover crop (such as buckwheat), by heating the soil with black plastic, by hand removal, or by using herbicide sprays. Cultivate or hoe regularly to control annual weeds. Cultivating early in the season reduces weed impact the most. Pre-emergent herbicides may be used to clean up any annual weeds present in the field at the time of planting.

Salad greens are poor competitors with weeds because of their small shallow root system, and the relatively small leaf canopy they produce as compared with beans, tomatoes, or vine crops.

Weeds get in the way of harvest and easily contaminate the finished product. Shallow cultivation will help reduce root damage to the crop. Sometimes hand cultivation or hand weeding is necessary. The laydown tractors described in the harvest section are also useful to carry workers comfortably for hand weeding.

Refer to Commercial Vegetable Production in Wisconsin for specific herbicide recommendations.

References