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Cultivar Report 12.2- Varied Leaf Trial

Posted by Dr. Natalie Bumgarner on 12/26/2012
Variety Trial Report- Fall/Winter 2012
CropKing Research Greenhouse- Lodi, OH
Hydroponic lettuce production in the United States now encompasses a wide spectrum of lettuce types and cultivars. While Bibb cultivars still occupy a large percentage of the market, many growers are also seeking attractive and distinctive lettuce cultivars to meet consumer demand. Due to these factors, leafy cultivars, including looseleaf and Lollo Rossa types, are becoming more common in hydroponic greenhouses. However, many of these cultivars have been more often grown in soil based systems, and there is a need to better understand their performance in the greenhouse. Consistency in both productivity and timing is important for greenhouse growers, and seasonal conditions can have a large impact on cultivar performance. Trialing of available cultivars under differing environmental conditions as influenced by seasons is important in informing grower decisions. Important points of evaluation are germination and seedling quality as well as growth rate, yield and visual coloration. The goal of this set of trials was to evaluate a selection of leafy lettuce cultivars through a range of late fall, winter, and early spring conditions to evaluate their potential for greenhouse growers in the Midwest and northeast. Cultivars were obtained from a variety of seed suppliers to represent a broad selection of cultivars available to lettuce producers.

Materials and Methods


Plant Management-

All plants were grown for the entirety of the crop in the CropKing research greenhouse in Lodi, OH. Primed and pelleted seeds were seeded by hand in pre-moistened (with pH adjusted water) 1” x 1” x 1 ½” rockwool cubes. Seeds were germinated uncovered in clear water in seeding trays in the CropKing nursery area. Nutrient solution was added in the nursery 7 days after seeding. Seedlings were produced in flowing nutrient solution as described below for an additional two weeks prior to transplanting. For the final week before transplanting to the channels, plants were grown in the nursery under supplemental lighting (6 bulb, 4 ft., T5 fluorescent light) on 16-hr days. After transplanting, lettuce plants were grown out in the channel for five weeks prior to harvest. Fourteen plants of each cultivar were produced. Total production time was approximately eight weeks reflecting low light conditions typical of many Ohio winter production schedules.

Figure 1. Seedlings at transplant (3 weeks after seeding)

Growing System-

After transplanting, lettuce was produced to harvest in CropKing NFT channels. These 4” wide growing channels are food-grade, UV resistant PVC with matching top caps punched to fit rockwool or other similar growing mediums. Spacing for plant production is 8” within and across channels. All channels are fed by nutrient feed lines supplied a continually recirculating nutrient solution. Each channel drains into a completely closed drain line which returns the nutrient solution to the reservoir. Galvanized steel frames support the channels and the drain line. Solution is also continually cycled through the CropKing Fertroller where automatic pH and EC adjustment is carried out to meet programmed solution set points. The pH was maintained at 5.8 by the addition of dilute sulfuric acid. EC was maintained at 1.8 by the addition of concentrated fertilizer solution and source water.

Greenhouse Conditions-


Air temp

average (°F)

Relative Humidity

average (%)

solar radiation

average (W/m2)*

Carbon dioxide



to 12/19


66.4 70.5 57.2 449.5
*Solar radiation reflects ambient greenhouse conditions and does not include any supplemental light applied during the seedling stage.

Nutrient Solution Formulation-

Nutrient solution was supplied by the Fertroller system discussed above. Stock solutions #1 and #2 were prepared using greenhouse grade calcium nitrate, potassium nitrate, magnesium sulfate, monopotassium phosphate, DTPA iron chelate, and CropKing MicroMix. Nutrient solution formulation was based on laboratory results from on-site samples of source was and were intended to reach macro and micronutrient targets specific for leafy crops in NFT systems.


Germination %
(Fully emerged and viable seedlings 6 days after seeding)
Cultivar Individual Head Wt. (g ± SD)






New Red Fire


Ruby Sky




Figure 2. Mature lettuce plants immediately prior to harvest.


This initial evaluation of leaf lettuce cultivars demonstrated growth rate differences as well as potential variation in suitability for winter production in northern areas. For most cultivars, germination was similar and seedlings were all of appropriate size at transplant (Figure 1). However, by harvest, differences in size and plant form were apparent (Figure 2). Light levels in this production season were greatly reduced from summer and early fall conditions. This reduction in light potentially impacted this lettuce crop in three important ways. First, reduced solar radiation slowed down the growth of the crop and lengthened the production time from approximately 6 total weeks in the summer to 8 weeks in this winter study. Secondly, plant appearance was impacted as these low light levels contributed to plant stretching or undesirable stem elongation (some stretching is even apparent in Figure 1 at the seedling stage). Cultivars, such as Locarno, Tropicana, Ruby Sky, and New Red Fire, all exhibited some degree of stretching in some or all of the lettuce heads. Additionally, reduced leaf coloration in both red and green cultivars also may have been a result of lower solar radiation in this study. These results indicate that in some climates, certain cultivars may require supplemental lighting to be most marketable for year-round production.   

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