What to Do When Lettuce Bolts? Cause Analysis and Countermeasures
Lettuce (scientific name: Lactuca sativa L. var. angustana Irish.), also known as celtuce, is a biennial or annual herbaceous plant in the genus Lactuca of the Asteraceae family, a variety of Lactuca sativa that can form a fleshy tender stem. Also known as stem lettuce, asparagus lettuce, green lettuce, and lettuce vegetable. Harvest period: January to April. It originated in Central or North China. The aboveground stem is edible; the stem skin is white-green, and the stem is crisp and tender, with young stems being emerald green and turning white-green after maturity. The main edible part is the fleshy tender stem, which can be eaten raw, in cold dishes, stir-fried, dried, or pickled; the young leaves are also edible. The stems and leaves contain lactucin, including lactucin (C11H14O4 or C22H36O7). It tastes bitter and has an analgesic effect. Lettuce has strong adaptability and can be cultivated in spring and autumn or overwintered, with spring cultivation being the main method, harvested in summer.
Morphological Characteristics of Lettuce
The stem looks ringed, like a bamboo shoot. It has a taproot system, and after transplanting, many lateral roots develop, shallow and dense, mainly distributed in the 20-30cm soil layer. The stem is shortened. Leaves are alternate, lanceolate or long ovate, etc., with colors ranging from light green, green, dark green, or purplish red; the leaf surface is flat or wrinkled, with entire or notched margins. The shortened stem gradually elongates and thickens as the plant grows; after the stem tip differentiates flower buds, the stem thickens while the flower stem elongates, forming a club-shaped fleshy tender stem. The flesh color is light green, emerald green, or yellow-green. It has conical capitulum inflorescences with pale yellow flowers, each inflorescence having about 20 flowers, self-pollinating, though cross-pollination sometimes occurs. The achene is dark brown or silvery white, with a pappus.
Causes of Bolting in Lettuce and Countermeasures
Lettuce has achieved year-round cultivation and four-season supply in many places. However, during cultivation, plants often exhibit premature bolting, characterized by small and few leaves, thin and sparse stems, and tender stems growing excessively tall into a club shape, affecting yield and reducing quality. Based on recent production experience, the causes of tender stem bolting and excessive height in lettuce and effective countermeasures mainly include the following 10 aspects.
1) Improper variety selection. There are many lettuce varieties, with different adaptability and sensitivity to light and temperature. Early-maturing varieties tend to have stronger cold tolerance and weaker heat tolerance than mid-to-late-maturing varieties, and are more sensitive to high temperatures and long daylight.
Countermeasures: Therefore, suitable varieties should be selected according to the cultivation season. For example, lettuce planted in spring and harvested in summer, or planted in summer and harvested in autumn, grows under high temperature and long daylight conditions, so mid-to-late-maturing varieties with heat tolerance and insensitivity to long daylight should be chosen; using early-maturing varieties would easily lead to excessive height and bolting. Early-maturing varieties planted in early spring will be marketed early, with high yield and good benefits; if planted late, due to rising temperatures and long daylight in the later growth period, they will easily grow tall and bolt, with low yield and poor benefits.
2) Unsuitable sowing time. For spring lettuce cultivation, if autumn sowing and seedling raising are too early, the pre-winter growth period is long, seedlings are too large, flower bud differentiation is early, and they are prone to frost damage during overwintering and early bolting the following year; if sowing is too late, the pre-winter growth period is short, seedlings are small, nutrient accumulation is low, they are also prone to frost damage during overwintering, and marketing is delayed the following year. If summer lettuce is sown too late or autumn lettuce is sown too early, the seedling period has high temperatures and long daylight, making excessive seedling growth likely.
Countermeasures: Therefore, lettuce cultivation should pay attention to mastering the appropriate sowing time. Generally, spring lettuce is sown in open fields in early October, or in protected fields around late January of the following year; summer lettuce is sown in early April; autumn lettuce is sown in early August; and winter lettuce is sown in late August.
3) Poor seedling management. Too much sowing, overly dense emergence, and delayed thinning can easily cause crowding of seedlings; excessive nitrogen fertilizer in the seedbed, excessive watering, and overly heavy shading or delayed removal of covers during shaded seedling raising can all cause excessive seedling growth. Therefore, seedling management should aim to control excessive growth and cultivate short, sturdy seedlings.
Countermeasures: The base fertilizer in the seedbed should mainly be organic fertilizer, with an appropriate amount of compound fertilizer; sow thinly, thin timely, control watering, and keep the seedbed soil alternately dry and moist; for shaded seedling raising, gradually cover later and uncover earlier after full emergence, and stop shading after 3 leaves; for summer and autumn lettuce seedlings, spray 1-2 times with 500 mg/kg chlormequat solution, which can effectively prevent excessive growth.
4) Delayed transplanting. If transplanting is delayed, seedlings tend to grow too fast, with elongated hypocotyls in a excessive growth state, making it difficult to obtain plump tender stems after planting.
Countermeasures: Generally, suitable seedlings with 5-6 leaves should be transplanted timely. The seedling age for autumn sowing and spring harvest is about 40d (days), for spring sowing and summer harvest or summer sowing and autumn harvest is about 25d (days), and for autumn sowing and winter harvest is 30-35d (days).
5) Overdense cultivation. If planting density is too high, the rosette leaves close the rows before formation, causing plant crowding, insufficient light, and plants competing for light by growing tall.
Countermeasures: Under normal conditions, for spring-harvest lettuce, moderate dense planting can be used for early benefits, with 8000 plants per mu; for summer-harvest lettuce, due to high temperature and long daylight causing easy excessive growth, sparse planting is suitable, with 5000 plants per mu; for autumn-harvest lettuce, the later temperature is suitable, and 7000 plants per mu can be planted, with good commodity quality and high yield.
6) Insufficient intertillage and loosening of soil. After planting, fertilization, watering, and rainfall can easily cause soil compaction on the bed surface, high humidity, and poor aeration. If intertillage and loosening are not done timely, it will affect root growth and absorption of water and fertilizer, causing leaves to lack sufficient water and fertilizer, leading to weak plants with thin stems.
Countermeasures: Therefore, after transplanting and seedling recovery, intertillage should be done timely, deep and thorough, to promote seedling establishment, root development, increase rosette leaves, and robust tender stem growth.
7) Improper watering amount. After planting lettuce, if watered too much under insufficient fertility or if rainwater is not drained timely, soil humidity becomes too high, and tender stems easily grow excessively, resulting in “waterlogging-induced excessive growth”; under soil drought and high temperature conditions, tender stems grow thin and weak, easily leading to “drought-induced excessive growth”.
Countermeasures: Therefore, watering in lettuce cultivation should be moderate, preventing both “waterlogging-induced excessive growth” and “drought-induced excessive growth”. Before rosette leaf formation and row closing, appropriately control watering, keeping the bed surface alternately dry and moist; after row closing, increase water supply, keep the bed surface moist, to ensure rapid growth and expansion of tender stems.
8) Insufficient fertility or excessive nitrogen fertilizer. Insufficient base fertilizer or delayed topdressing, low soil fertility, and inhibited vegetative growth can accelerate reproductive growth and easily cause premature bolting. Excessive nitrogen fertilizer can cause leaf excessive growth, low nutrient accumulation, and often the young stem grows tall before developing.
Countermeasures: Therefore, base fertilizer should be sufficient, applying 4000 kg of decomposed manure and 25 kg of compound fertilizer per mu, combining organic fertilizer with compound fertilizer. In addition, timely topdressing of seedling recovery fertilizer, cluster-stage fertilizer, and stem-promoting fertilizer should be done, with each topdressing applying 15-20 kg of ammonium sulfate or urea per mu, or the first 2 times applying 1000 kg of 50% dilute manure water each time.
9) Unsuitable climatic conditions. When lettuce is grown late or planted in summer and autumn, due to high temperature and long daylight, leaves tend to differentiate less, flower buds differentiate early, and flower organs develop quickly; slight mismanagement can easily cause excessive growth and bolting, affecting quality and yield.
Countermeasures: In addition to strengthening water and fertilizer management, regular spraying of chlormequat is an effective measure to control bolting in lettuce cultivation during high-temperature seasons. Spraying chlormequat can delay lettuce bolting, promote leaf growth, increase plant nutrient area, and thicken tender stems. Spraying usually starts at the rosette stage, once every 7-10d (days), for a total of 2-3 times, with a concentration of 350 mg/kg.
10) Delayed harvest. The suitable harvest time for lettuce is when the heart leaves and outer leaves are “level”. Especially for late spring lettuce, summer lettuce, and autumn lettuce grown in high-temperature seasons, flower buds often form before harvest; if not harvested timely, tender stems will rapidly grow tall and bolt.
Countermeasures: If delayed harvest for supply is desired, pinching off the tip and removing buds can effectively control tender stem elongation and bolting, that is, during the “level” period on a sunny day, timely pinch off the top growth point or flower buds of lettuce. After topping, the plant can inhibit top growth, increase nutrient return to the stem, prevent tender stem hollowing, promote stem enlargement, allow delayed harvest by 5-7d (days), and increase yield by 10%-20%.