I’ve planted iceberg lettuce; however, they have bolted with long stems and have curled up. Is this normal?

By Lynette Morgan | Last updated: December 15, 2021

Bolted lettuce

Bolting in lettuce is the premature formation of a flower head which causes the plant to elongate and creates a looseness of leaf formation on the stem. Bolting can have a number of causes, however, the main issue is usually temperature (for many iceberg cultivars, temperatures over 75°F can be a factor with bolting).

Lettuce is a cool season crop and is prone to bolting under summer conditions, particularly where long day length is combined with warm temperatures. Other environmental factors which can also induce bolting are low light, overcrowding of seedlings or plants, being pot-bound (highly restricted root system), and incorrect cultivar selection. To prevent bolting, select varieties which are bred specifically for warm season or summer production (slow bolt varieties). Iceberg lettuce has a large number of cultivars, with both summer and winter types, which are more suited to different growing conditions. For indoor or greenhouse production where temperatures are generally warm, growing summer varieties is recommended. Also, make sure the plants are well spaced, receive sufficient light (iceberg produces well under full sunlight) and do not suffer from root restriction.

Iceberg lettuce has a high nutrient requirement, much greater than other types of smaller hydroponic lettuce, so the EC should be from 2.0–2.4 to ensure sufficient nutrition as low nutrient levels can also be a factor in bolting. A high EC or salinity, or lack of moisture are other known triggers for bolting as are other plant stresses and general plant health. To help prevent bolting in hydroponics, the nutrient solution can be chilled to below the ambient air temperature (57-64oF) which helps the plant grow under warm conditions. Keeping temperatures down with evaporative cooling of the growing environment and maintaining a good rate of air flow over the plants can also assist with keeping lettuce cool and prevention of bolting.

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Written by Lynette Morgan | Author, Partner at SUNTEC International Hydroponic Consultants

Profile Picture of Lynette Morgan

Dr. Lynette Morgan holds a B. Hort. Tech. degree and a PhD in hydroponic greenhouse production from Massey University, New Zealand. A partner with SUNTEC International Hydroponic Consultants, Lynette is involved in remote and on-site consultancy services for new and existing commercial greenhouse growers worldwide as well as research trials and product development for manufacturers of hydroponic products. Lynette has authored five hydroponic technical books and is working on her sixth.

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