Lettuce (Lactuca sativa) is the fourth most important vegetable crop grown hydroponically in greenhouses proceeded by tomatoes, European cucumbers and peppers. Although greenhouse production of lettuce is very small in comparison to field grown, it has a specific market niche as a gourmet, high-quality item. Being clean and free of soil, it isparticularly popular in restaurants. It is generally packaged in plastic bags or rigid plastic clamshell containers to display it as an attractive product on the supermarket shelves. The choice of packaging depends upon the marketplace. For the restaurant trade it is better to use plastic bags for lettuce without roots. With clamshell containers that present the product as “living lettuce” in asupermarket keep the roots in their growing cubes attached. When leaving the roots on use a special clamshell container with a depression in the bottom to contain the roots and growing cube (photo 1). The majority of hydroponic lettuce is bibb or European buttercrunch types. It can be marketed as “Hydro-Bibb” lettuce. Bibb lettuce has a soft head, not the dense compact head of “iceberg” or head lettuces.Some novelty leaf lettuces are grown hydroponically, but accounts for only a small portion of greenhouse lettuce. Varieties include: “Romaine” (“Parris Island”-green and “Freckles”-red), “Oakleaf” (“Cocarde”, “Berenice”- green; “Oscarde”, “Dano”- red), red curly varieties such as “Lolla Rosa”, “Ruby”, “Red Sails”, “New Red Fire”, “Brunia”, to mention a few (photos 2-3). During the longer days ofsummer months these lettuce will mature within 40 to 48 days. Seedlings are grown for 12- to 18-days as transplants and then 28 to 32 days to maturity.
Since the majority of greenhouse hydroponic lettuce is of the bibb type, culture of this lettuce is specifically discussed. Some of the most suitable European bibb varieties include: “Deci-minor”, “Ostinata”, “Salina”, “Vegas”,and “Rex.” Choose your variety according to climatic conditions, especially light and temperature, potential disease infection and market. Ostinata, Vegas and Rex are suited to higher temperatures and are resistant to tip-burn. Lettuce is a cool season crop, so prefers cooler temperatures. It bolts rapidly at higher temperatures, especially if they exceed 80 to 85 F. “Bolting” is the lettuce shootsup to go to seed (photo 4). Bolting makes the lettuce non marketable. Salina is more tolerant to Pythium fungal root-rot infection, but is not resistant to it. It is often used during summer months, especially in warmer regions such as Florida. There are many different varieties available through different seed companies, so you should carry out a number of varietal trials to determine those thatgrow best under your specific conditions. We have found at Cuisinart Resort & Spa Hydroponic Farm that “Rex” is by far the most resistant to high temperatures of our tropical conditions and resists “bolting” under stress. However, we must harvest about 26 to 30 days after transplanting 18-day-old seedlings to prevent bolting. Any extended time after that causes rapid bolting. Temperature mustalso be taken into consideration when choosing a hydroponic system for growing lettuce. The raft culture system is generally better for higher temperatures as the root temperatures may be chilled below 70 F to slow bolting under high air temperatures of 85 to 90 F.
STARTING THE PLANTS:
Seedling Production: The growing system to which the lettuce seedlings will be transplanted determines whatmethod of sowing is best. Substrates used include: rockwool cubes, Oasis cubes, and multipacks (celled trays) with a peatlite or vermiculite medium. The rockwool and Oasis cubes are available in dimensions of 1” x 1” x 1 1⁄2” and are held together in multicubes to fit a standard 10 1⁄2” x 21” flat or mesh tray (photo 5). The rockwool cubes come as
200 per sheet and the Oasis cubes as 162 per...