By Glenn I. Teves, County Extension Agent, UH CTAHR
Lettuce is one of the oldest garden plants and is always the essence of a summer salad, but finding high quality lettuce during our hot Molokai summers can be a challenge due to less than ideal growing conditions. There are between 50 and 75 lettuce species, and was first cultivated by the Egyptians for oil from its seeds. There are several lettuce types, but the three most common include leaf, crisphead and romaine. These can be crossed with each other to create an array of leaf types, shapes, and textures.
Often described as a low-nutrient vegetable, crisp-leaf lettuce is one of the richest vitamin sources of choline, as essential brain nutrient related to focus. Its Latin name Lactuca or Lac means “milk” due to its characteristic milky sap. This milk is believed to contain sedatives, including Lithium, and some cultures consume them at the end of the meal to make them sleepy. Of the lettuce varieties, loose-leaf types are reputed be the most nutritious, as they receive lighter and are often more fully-pigmented with green, yellow and red antioxidants. Lettuce is also a good source of Vitamin A and potassium.
Native to a large area from the Mediterranean to Siberia, most lettuce evolved in areas with hot days and cool nights. The Hawaiian summer climate can be quite variable, but hot days and warm nights are usually the norm with a wide range of humidity. These warm night conditions in particular do not allow plants to rest and recover from hot days, resulting in a breakdown of the plants internal system. These conditions create undue stress on lettuce such as tip burn on leaf edges, bolting or early flowering, and the accumulation of milk causing bitterness. Tip burn is caused by a combination of stress conditions resulting in a calcium deficiency causing tips to leaves to die, and is especially prevalent in crisp head types.
Minimizing the stresses of summer include planting in shady areas, installing a screen over plants, and misting plants to slow their metabolic rate, or harvesting when young and planting heat-tolerant varieties. Many lettuce varieties will purport to be heat-tolerant, but may not make the cut in Molokai’s brutal tropical summers. Heat-tolerance implies that the variety can withstand early bolting and tip burn, and still have a good taste. Adequate water and essential nutrients are also important in keeping plants healthy and thriving.
Batavian types, also known as Summer Crisp or French Crisp are among the most heat tolerant, have thickened leaves, are large-framed, and also well-flavored. Batavian varieties include Sierra, Nevada, Mottistone, Magenta, Concept, Cimarron, Muir, Cherokee, Tahoe, Teide and others. Other heat-tolerant lettuces include Rex, Ostinata, Jericho, Helvius, Salvius, Buttercrunch, Anuenue, Manoa, Kauwela, and Tropicana. More screening of varieties in the dog days of our Hawaiian summer may reveal more heat-tolerant lettuce varieties adapted to our diverse climate. We export more than 85 percent of the lettuce we consume, mostly from California and Arizona, and growing our own lettuce can ensure freshness and high quality, and is another step to food security. Plus, you cannot beat freshly picked sweet lettuce, chilled for a while, and eaten at the peak of perfection!