Hydroponics & the Tourism Industry

We grow European bibb lettuce in a raft culture system. The ponds are about one foot deep. We use a DeRuiter variety “Rex” that we have found tolerates the high light and temperatures better than many others. The nutrient solution of the lettuce ponds is cooled to 70 F or less to delay bolting and reduce fungal (Pythium) infection. The air temperatures are between 85 and 90 F, well beyond optimum for lettuce. We cannot cool the greenhouse anymore due to the high relative humidity (RH) outside. We have two chiller units (Photo 29) in each lettuce pond to adequately lower the temperatures of the solution in these ponds.

The large pond holds 4600 U.S. gallons and the smaller one 2300 U.S. gallons. The solution is mixed by pumping it around the perimeter of the ponds with a 2-inch diameter PVC pipe with gate valves every 2 feet.

The Styrofoam boards measure 1-inch thick by 4 ft by 4 ft. Each board contains 64 head of lettuce. During harvest we lift a board up and place it on a cart where we remove each head cutting it at the crown (Photo 30).

We place one board of lettuce in a plastic bin for transport to the kitchen. Since we harvest two boards daily and do not allow more than several days of product to remain in the kitchen refrigerator at a time, the lettuce will keep fresh.

Our market is only a short distance, so we can deliver the lettuce in this way. When we have excess lettuce we give it to the staff of the resort, schools, churches and the hospital.

Two boards of lettuce (128 head) must be harvested, seeded and transplanted every day of the week. Regardless of the hotel occupancy, this schedule must be closely followed so that we always have sufficient lettuce during high occupancy periods.

When we complete the harvest of a pond, we remove all of the boards from the pond, drain and sterilize the pond with a 10% chlorine solution. This takes several hours to complete. During this time we stack the boards with the immature lettuce plants root-to-root and keep them watered every 10 to 15 minutes to prevent their drying. This procedure is carried out about every 7 to 8 weeks in order to change the nutrient solution. While we test the EC and pH during the growing cycle, we have found it best to make up a new solution on a consistent period of time. The drain water is all directed to the landscape cistern, so we are not wasting the spent solution.

The raised beds are approximately 16 inches deep. They are lined with a 20-mil vinyl and 2-inch diameter drain pipes placed at the bottom. Igneous sand was brought in from Dominica with the intention of growing in sand culture, but the sand was not screened or washed, causing it to compact. It filled the beds to about 10 inches after compaction, so a 4-inch layer of Promix was placed on top of the sand to become the principal growing medium. The plants are fed by a drip irrigation system every other day for 10 minutes. Drip emitters are placed 3 inches apart within the drip lines, which are 1 foot apart.

Lettuce and arugula grow very well (Photo 31). We also grow the basils in another smaller bed.

All the vine crops (tomatoes, European cucumbers, eggplants and peppers) grow in perlite substrate with the bato buckets. We have had very good success with this system. We did some trials using rockwool, but the plants grew much better under our conditions using the bato buckets. The bato buckets are constructed of rigid plastic measuring approximately 10 inches (25 cm) square. Their volume is 4 gallons (16 liters). There is a small ½ inch diameter siphon pipe at the bottom of the pot to keep about 1 inch (3 cm) of solution in the pot at all times. The pots are set on top of a 1 ½ inch PVC drain pipe and are spaced 16 inches (40 cm) apart alternating from one side to the other (Photo 32).

We found for our conditions it was necessary to change the drain pipes to 2 inches (5 cm) in diameter to prevent root growth plugging them.

With tomatoes and peppers two plants are grown in each bato bucket. Each plant receives one drip line from a ½ gallon per hour Netafim emitter. Our irrigation cycles are 10 to 12 minutes every 1 ½ to 2 hours depending upon season and plant growth. The cucumbers receive three drip lines per plant and only one plant is in each bato bucket. Our spacing works out to almost 4 square feet per plant for the tomatoes and peppers and 10 square feet for the cucumbers. Rows are spaced 6 feet center-to-center of the drainpipes. Each two rows of plants is a station operated by a 1 inch solenoid valve.

Since our greenhouse has only 10 ft (3 m) gutters to be sufficiently strong to withstand the hurricanes, we must lower our tomatoes and peppers more often than in a taller greenhouse.

Tomatoes are sown in rockwool cubes and transplanted to rockwool blocks about 2 weeks later (Photo 33). They are then spaced and grown in the blocks until they are 5 to 6 weeks old from sowing. They are then transplanted to the Bato Bucket perlite system (Photo 34). Our growing cycle is from Sept. 1 to July 15th. The crop changeover is started in mid-July so that we may have renewed product by mid to late November. The resort closes for the months of September and October, so that gives us two months to get the plants established. One of the main challenges with growing tomatoes in the Caribbean is the high relative humidity combined with high temperatures. During the summer months RH of 85% or greater and temperatures of 90 F is common. With minimum night temperatures at 80 F or higher our tomatoes have difficulty in setting fruit. Even during the cooler months from December through March, night temperatures still remain in the mid-70 F range, so at no time are the temperatures optimum for fruit set. As a result, our annual production is between 25 and 30 lbs per plant.

Four rows of peppers are seeded on the first of August. As they take about 4 months from seeding to first production, harvesting begins by late November or early December (Photo 35).

Peppers suffer the higher temperatures better than the tomatoes. Towards the end of the season, we must be careful to keep the peppers from becoming too generative and slowing the growth and fruit size. We do this with extra irrigation cycles.

The cucumber crop is alternated on a 5-week basis. We have only two rows of cucumbers, so each row is 5 weeks apart in age. The cucumbers grow very rapidly from seed to first production in 5 weeks (Photo 36), hence the 5-week age difference to keep always some production. Peppers take about 3 ½ months and the tomatoes from 2 ½ to 3 months from seed to production.

All of the crops are scheduled in detail over the entire year to obtain sufficient product for the resort on a continuous basis from November until August. The resort closes for the months of September and October, giving us time to get the crops back into production by the resort’s opening in November. This two-month closure of resorts in the Caribbean is common practice in the area during the hurricane season when there are no tourists.

We have 66 plant towers growing herbs, watercress and bok choy. They are located along the perimeter of two sides of the greenhouse. These towers were purchased from Verti-Gro in Florida. They consist of 10 Styrofoam pots each rotated 45 degrees to enable them to sit one on top of the other (Photo 37).

They are filled to within 1 inch with perlite. Irrigation is by two drip lines in the top pot and one halfway down the tower. The solution percolates from one pot to the next to the bottom where the leachate is conducted into a drainpipe taking it to the gray-water cistern for the outside landscape.

The plant towers were designed to grow strawberries vertically in greenhouses. This method allows 6 to 8 times as many plants per unit area of the greenhouse in comparison with the use of horizontal beds. Each tower occupies 10 square feet (1 sqm) of surface area. Four clumps of herbs or one bok choy is placed or seeded into each pot as the square pots have 4 corners exposed when set one on top of the other. We are unable to grow strawberries due to the high night temperatures, but we grow herbs and bokchoy very successfully.

Herbs grown include thyme, oregano, mint, sweet marjoram, sage, tarragon, rosemary, lavender, chervil, chives, Italian parsley, moss curled parsley, dill and watercress (photo 38).

With the herbs we seed directly 8 to 10 seeds in each corner of the pots. Some of the towers are reseeded after 3 to 4 months, such as watercress, dill and chervil; the others will go for the entire season.

Bok choy grows rapidly in the plant towers (Photo 39). It is sown in Oasis cubes, then transplanted 18 days after to the plant towers. They can be harvested from 28 to 35 days after transplanting. The number of towers of each crop depends entirely on the demand of the resort’s restaurants. The bok choy is a miniature variety “Green Fortune” from Ornamental Edibles in California. The bok choy is cut in half and simply steamed for several minutes. In this way, the maximum of vitamins and minerals stay in the food. The guests are so pleased with the bok choy that the demand continues to increase.

Watercress was first grown outside in an NFT bed. However, armyworm larvae infestation could not be controlled, so we decided to grow it inside using the plant towers. Watercress, of course, prefers beds of flowing water, but as we do not presently have space for such a system, we continue to grow it in the towers. The only change we make in cultivation is that the crop must be re-seeded after 3 harvests.

The injector system designed by CropKing consists of two 300 gallon (1135 liter) stock tanks, one 195 gallon (738 liter) pH adjustment tank, the injector and a 40 gallon (155 liter) blending tank (Photo 40).

The stock solutions A and B are 75 times normal strength. They are diluted back in the ratio of 1:75 by the injector as the solutions enter the blending tank. From there they are pumped to each of the stations. The greenhouse has 9 stations controlling the rows of vine crops, 3 stations for the raised beds and 2 stations for the plant towers. An 18 station Superior Controls, Inc. controller operates all these.

Irrigation cycles are set according to the plant stage of growth and weather conditions. Most of the vine crops have a 10 to 12 minute period of irrigation every 2 hours, with a total of 6 cycles per day. The plant towers are irrigated for 4 minutes twice a day and the raised beds for 10 minutes every other day. We maintain between 25% and 30% leachate with the bato buckets. The pH for the vine crops is between 6.0 and 6.4, with an EC of from 2.3 to 2.5 mMho.