a reprint of an article in the American Vegetable Grower
by P.A. Schippers
who at the time was Senior Research Associate
Long Island Hosticultural Research Lab in Reverhead, NY

In the article "The Americanization of Dutch Lettuce" (AVG Dec. 1977), the potential of Dutch greenhouse lettuce of the butterhead type for growing under Long Island conditions was explored. In the May 1978 issue of AVG, some results were reported on experiments with butterhead lettuce to increase yields by space saving growing methods ("A Vertical Hydroponics System"). Since then, interesting new results have been obtained concerning both these aspects of greenhouse lettuce growing which may be of practical use to greenhouse growers. These results have been extensively reported in V.C. Mimeo 213 of the Vegetable Crops Department of Cornell University which is titled Greenhouse Lettuce; NFT Trials with Butterhead Lettuce in 1978 and 1979, and is available from the author.

One of the most important factors in obtaining satisfactory growth of butterhead lettuce in the greenhouse is choosing the right variety. This is not a simple matter, since the specific light requirements of each variety make it suitable for only part of the year. For instance, varieties growing well in spring and summer will usually not perform well under the poor light conditions of winter, while typical winter varieties will bolt or in other respect "misbehave" under longer days and higher temperatures. Finding the right variety is further complicated by the fact that most American butterhead varieties are not well adapted to greenhouse growing. This was for us the reason to try out a large number of Dutch varieties bred especially for greenhouse conditions.

Recommendations regarding the most suitable time of the year for growing of these varieties are based on Dutch light conditions which are quite different from those in most parts of the U.S. Therefore, a great number established and well known varieties were tried over a period of three years during different parts of the year. On the basis of these experiments a number of varieties was found to be suitable for certain periods and they are (were) listed, with their recommended harvesting periods.

Since these results are valid only for Long Island and areas with similar quality and quantity of daylight, they have to be viewed with caution, and growers in areas with different light conditions would have to do some experimentation to find the most suitable varieties. This is also the reason for the inclusion of some varieties which are not our first choice, but might be excellent for other areas.

Some of the illustrations show the type and size of a couple of these varieties. In general, the average head weight of the "right" variety should vary from 7-9 ounces, although some varieties may reach weights of 12 ounces or more under the right growing conditions. Under less favorable conditions one may have to be content with a weight of 5-7 ounces.

The most satisfactory variety under our circumstances was Salina, mainly because of its very reliable performance year round. During certain periods of the year some other varieties had a larger or better shaped head, but none performed as evenly over the whole year. In the middle of the winter we found L 7105 quite exceptional, with large heads of a dark green color. This is a welcome addition to Salina and Ravel which also performed well under our conditions. In the spring and early summer Bellona formed heads of very high quality. Some good new summer varieties are Karma and Brenado, although we need some more experience with them. Alora and Marmer look quite different from the usual butterhead type and are more crisp. They needed a couple of more weeks to grow to sufficient size and were often heavier than the others.

For those planning to grow butterhead lettuce year round --- which is quite feasible --- not only the choice of varieties for the various growing periods is important, but also knowledge regarding the duration of growth of each crop during these periods. Fig. 1 (not included on this web page), which is valid for conditions similar to those on Long Island, shows that the periods from seeding to harvest varied from six weeks in the summer to 13 weeks during the winter. Therefore, it is possible to grow up to 10 successive crops in one year under our growing conditions, provided that each seeding is made sufficiently early to get the seedlings ready for transplanting immediately after the previous crop has been harvested.

We found during previous experiments that growing a number of consecutive lettuce crops in peat-lite mix resulted in a continuously increasing percentage of plants suffering from fungus diseases, particularly those attacking the stem of the plant at ground level. This was one of the main reasons to change to the hydroponic system which we called the nutrient flow technique (AVG May, 1977).

PERITE USED

Although we started our variety trials in channels without any substrate, it became evident through other experiments that better results could be obtained by growing the lettuce plants, usually seeded in Jiffy 7's, on a layer of 2-inch of perlite in the channels. This often resulted in larger and better shaped heads and nearly always in a lower incidence of tipburn. Therefore, we recommend the use of perlite as a substrate, and most of our variety trials were done with perlite in the growing beds.

We found as important advantages of the nutrient flow technique that fungus diseases were only rarely encountered; that a very clean product is harvested; and that each crop could be planted immediately after the previous one had been harvested. In spite of omitting any disinfect ion, diseases have been no problem at all, even though many successive crops were grown on the same perlite.

In order to increase the profitability of greenhouse growing, as little growing space as possible should be wasted. Since the nutrient flow technique enables us to do so, we thought initially that it would be possible to utilize space better by growing lettuce in vertical pipes. However, time and time again it appeared that growth was very uneven because of differences in light conditions at various heights. Therefore, a different approach was tried by eliminating as much as possible the large amount of space which is wasted when the plants are young.

In a conventional crop planted at, say, 7.5 inches x 9 inches the young plants occupy only a small fraction of the growing area and they will gradually fill in the open spaces. The nutrient flow technique, however, offers the possibility to gradually move the plants apart, at least in one direction, by the use of movable growing channels. This is (was) explained in various illustrations.

The greenhouse used in this experiment was about 25 feet X 48 feet and was divided into four sections about 9, 11, 12, and 15 feet long. Lettuce seedlings were planted at 9-inch distances in growing channels which consisted of aluminum downspouts cut lengthwise through the narrow sides and lined with black polyethylene. These channels were placed next to each other in the first section which had a manifold with outlets at 3.5 inch distance. When the open space was filled up by the young plants --- after one or two weeks --- these channels were moved to section two where the outlets of the manifold were 4.75 inches apart. The first section was occupied by a group of channels in which seedlings were planted which had been seeded one or two weeks after the first batch of plants.

Another one or two weeks later the first batch of plants was moved to the third section where manifold outlets were 6 inches apart, the second batch was moved to the second section and the first section was again occupied by young seedlings.

This procedure was repeated a fourth time, which filled up all four sections in the greenhouse with successive age groups with spacings of 3.5, 4.75, 6, and 7.5 inches between channels and 9 inches within each channel. By doing it this way instead of having all plants at distances of 7.5 X 9 inches from the start, the number of plants in the greenhouse was increased by more than 35%. By modifying the spacings it may even be possible to achieve somewhat larger increases. Essential for this method is that seeding and harvesting take place periodically, starting the crops out at one end of the greenhouse and keeping them moving at intervals to the other side where they are harvested. Once this procedure has been established, it can continue year-round.

By utilizing the right varieties at each time of the year, by continuous cropping, and by high density culture considerable quantities of butterhead lettuce can be harvested from a certain area. By making use of movable growing channels, as outlined above, the number of plants per square foot can be increased from 2.2 to 3.3 in the actual growing area. In practice, when paths are included, the numbers of lettuce heads per square foot are more likely to be from 2.0-2.75.

If four successive seedings which fill up the four sections of the greenhouse are considered to constitute one crop, 8-10 consecutive crops per year can be grown under our conditions. This would mean that 20-25 lettuce plants per year can be harvested per square foot. Although in this way, a staggering number of plants per acre would be harvested over the whole year - close to a million - there is no reason to believe that this cannot be achieved.

(Note: the late Dr. Schippers eventually established Hydro-Harvest in Ashley, MA to grow hydroponic lettuce very successfully. His son, Skip, now runs the company and is growing cut flowers in the same perlite hydroponic systems.)