Lettuce, Endive, and Escarole

Introduction

Lettuce (Lactuca sativa) and endive (Cichorium spp.), which includes frisée and escarole (C. endivia) as well as Belgian endive and radicchio (C. intybus) are members of the Asteraceae family which originated in the Mediterranean region. Their closest crop relatives are artichokes and sunflowers. 

Lettuce grows best at cool temperatures, making spring and fall the major production seasons in New England. While endives are also cool-season crops, some are more tolerant of high temperatures than lettuce and therefore make a good substitute for lettuce during the warmer mid-summer weather. 

Types and Varieties

There are five common types of lettuce. Crisphead, or iceberg, is commonly found in produce markets. The leaves are thin and crisp, often with curled or serrated edges, and well-developed heads are firm. Butterhead, or bibb, lettuce has green or red outer leaves and tender cream to yellow inner leaves. Butterhead leaves are delicate and require careful handling as they bruise and tear easily. For this reason, it is best suited to local market sales. Romaine, or cos, is an upright plant with smooth outer leaves and crisp, whitish-green inner leaves with prominent midveins. Leaf lettuce (sometimes called loose leaf or loose head) does not form a head and the leaves may be serrated, deeply lobed, or crinkled. Leaf lettuce color varies from light green to red to speckled, adding attractive color to salad mixes. Finally, “one-cut” lettuce varieties has a high leaf count and narrow leaf attachment that enable both whole head harvesting (by cutting at the base) or loose-leaf (by cutting just above the base, sometimes for multiple harvests). These varieties tend to be miniature so that loose leaf cuttings integrate well into salad mixes and are suitable for dense plantings. Unlike most leaf lettuces, one-cut lettuces are usually transplanted, to allow for quicker bed turnover. One-cut lettuces can have variable leaf shapes (smooth, lobed, serrated), colors (green, red) and textures (tender/butter-like, crisp/romaine-like).

Endive encompasses many diverse types. Escarole and frisée form loose and leafy heads with sturdy leaves; frisée adds loft to salads while escarole is often cooked. Belgian endive form generally small, tight heads as do radicchio. Radicchio has many types unto itself with a wide array of leaf sizes, shapes, and colors. Italian (or "culinary") dandelion is also a member of Cichorium intybus that forms long leaves that are usually bunched. While these crops are mostly grown like lettuce, Belgian endivein particular must be forced and is therefore grown in two stages. During spring/summer, the seedlings produce large dandelion-like leaves and a large taproot, similar to carrot. In the fall, the taproots are dug, potted, and placed in the dark. The new growth from the roots produces small, dense, and blanched heads called chicons. Because of the labor involved and the specificity of the product, Belgian endive is typically sold to fine dining markets.

Soil Fertility

In general, lettuce and endives require the same fertility. Apply lime according to soil test results to maintain soil pH at 6.5-6.8 and maintain soil calcium levels. Low plant calcium levels, caused by either inadequate water or low soil calcium levels, may increase the incidence of tip burn, a disorder that causes the margins of leaves to turn black and decay. It is of particular concern with iceberg and romaine lettuces, where tipburn on internal leaves may not be immediately obvious. Over-application of nitrogen on fertile soil can result in very rapid growth that can also induce tipburn. Banding preplant fertilizer at planting is preferable, but if not possible, then broadcast and incorporate the initial application. Less nitrogen fertilizer will be needed if legume sod was plowed down or if manure was applied (see Table 1 and Table 7).

Plant Nutrient Recommendations According to Soil Test Results for Lettuce, Endive, and Escarole

Planting

These crops generally share spacing and seeding requirements. Final spacing on crisphead lettuce and endives should be 12-18" between plants (67-100 plants per 100 row feet) usually in double rows 12-24" apart. Other types of lettuce can be 10-16" apart (75-120 plants per 100 feet of row) with 10-18" between rows. Spacings should allow good air movement around the plants to minimize disease development. Miniature types can be grown more densely. Many growers plant these crops on 4' wide, 4" high raised beds with 2-4 rows per bed (with high density used for mini or one-cut varieties). Crop rotation within and between seasons is recommended for disease management. For direct seeding, 10-18 oz of seed are needed per acre (0.0625-0.125 oz per 100 feet of row).

Lettuce and endives will germinate at low soil temperatures, but the optimum germination temperature is 75ºF. Temperatures above 80ºF will inhibit seed germination and cause bolting in lettuce. Temperatures below 70ºF will promote bolting in endives, as will very hot temperatures after transplanting. Lettuce seed requires light to germinate and should therefore be seeded shallowly. 

For once-over harvesting, uniformity at harvest is essential. Producers for direct retail markets and CSA might prefer variable maturity. Precision seeding with modern planters and coated/pelleted seed can enhance uniformity. Irrigation immediately after direct-seeding also promotes uniform emergence.

Since lettuce matures quickly (40-50 days), and temperature affects days to harvest, plant several successions for consistent production throughout summer. 

These crops are often started as transplants, which can be set out from mid-April to early August for field production, or in an extended season in high tunnels. Some growers use floating row covers over beds with early transplants for earlier crops. Hardened transplants should be set out when they are 3-4 weeks old. Hardening is accomplished by withholding water and gradually reducing temperatures for 10 days before the planned transplant date. Use of a liquid starter fertilizer at transplanting time can reduce shock and provide immediately available nitrogen and phosphorus in cold soil. Use a dilute solution to avoid injury to roots. In hot summer conditions, starting transplants in the greenhouse or shade house provides better germination than in the field.

Field Culture 

Hot temperatures cause lettuces and endives to develop a bitter flavor and promote bolting. White-on-black plastic mulch (white above, black below) can be used to cool soil temperatures and reduce these problems. Shade cloth can also be used to reduce heat stress. Choose heat-tolerant varieties for mid-season production. 

Harvest and Storage 

Head lettuces should be harvested when heads are well-formed and dense by cutting at the base. A few wrapper leaves should be left on each head for protection during packing. Leaf lettuces can be harvested by hand or mechanically.

Food safety issues from microbial contamination are often linked to leafy greens that are eaten raw. It is important to follow some basic practices that are in accord with the guidance outlined in the Food Safety section of this guide.

Leafy lettuces and endives are fragile and highly perishable. Conversely, heading lettuces and endives are more easily stored than leaf lettuces. Cool to 32-34ºF as quickly as possible after harvest and store at 98-100% relative humidity. Temperatures below freezing will damage crops. Some radicchio varieties can be stored through winter much like storage cabbage.