Genetics: There are several names for the greenhouse cucumber (Cucumis sativus) including European, Dutch, English or burpless cucumbers (Fig. 1). Aside from generally being long and slender, there are several ways in which greenhouse cucumber varieties differ from field-grown cultivars. First, they are generally considered to have a milder flavor and less bitterness. Second, the skin on fruits is very thin and, as a result, peeling is not required prior to eating. Third, fruits are also parthenocarpic, or seedless. Beit Alpha is another type of greenhouse cucumber that is popular. They are similar to the European/Dutch cultivars in that they are mild-tasting and seedless and, though the skin is thicker than standard European varieties, it is still thin enough not to require peeling. However, they are smaller than the European/Dutch cultivars when mature and harvested, when fruits are 6 to 8 inches long.
Production systems: The most common production systems used for cucumbers are those systems used for other vining or high-wire crops, including Dutch buckets or a gutter system with slabs or bags of substrate.
Propagation and young plant production: Greenhouse cucumbers are propagated by seed. The seed for greenhouse cucumber varieties is expensive, approaching $1 per seed, due to the fact that the genetics are gynoecious and parthenocarpic (seedless). Seeds may be sown directly into 4-inch blocks of stone wool, coconut coir, or other substrate that will be transplanted into production systems. The air temperature should be maintained at 84° F for the first few days for optimal germination. After the radicle has emerged, lower temperatures to mid-70s° F. Supplemental lighting can be used to keep seedlings compact. Once transplants have three to four true leaves, they are ready to transplant.
Nutrient solution: Greenhouse cucumbers are a heavy-feeding crop. Complete nutrient solutions with an electrical conductivity (EC) between 2.0 and 3.0 mS/cm and a target pH between 5.5 and 6.0 should be used. The ratio of nitrogen to potassium (K) should be around 1:1.5; elevated levels of K are used for this heavy-fruiting crop. Avoid elevated nutrient solution and root-zone EC, as the high concentration of salts can damage roots and cause root rots to occur.
Temperature: Cucumbers are warm-growing compared to other greenhouse and hydroponically grown crops. Daytime air temperatures should be maintained between 75 and 80° F, but should not exceed 85 to 90° F. Temperatures during the night should not drop below 65° F.
Light: Cucumbers are, generally speaking, a high-light crop. There are some varieties that will perform better under lower-light conditions, but all will respond positively to increased light intensities. Supplemental light can be used to enhance growth and productivity, but only if the investments in lights and operating costs are economical.
CO2: Cucumbers benefit from carbon dioxide (CO2) enrichment in the greenhouse. Supplemental CO2 from liquid injection or burners up to 1,000 ppm can boost productivity, especially during periods where ventilation is reduced.
Pollination: Since nearly all the cucumber cultivars grown hydroponically are parthenocarpic and form fruits without ovules being fertilized, no effort to pollinate flowers is required.
Pruning and training: As with other vine crops, cucumbers require pruning and training. As with tomatoes, “suckers,” the shoots arising from leaf axils, are removed to maintain a single, primary stem. Cucumbers also produce tendrils, which should be removed. If left on the plant, they will wrap around other plants and infrastructure, causing problems. Fruit thinning should also occur for standard-sized cucumbers. If more than one fruit is allowed to develop at each leaf axil, the fruit load will be too great and the quality of individual fruits will be diminished.
Cucumbers need trellising since they are a vine crop, and there are several methods that may be used.
The drape method is where the stem is trellised on twine up the suspended wire, then allowed to drape down over the wire and grow toward the ground.
The umbrella method is similar to the drape method in that the stems are trained up to the wire. However, once stems grow above or over the wire, they are pruned back to the send leaf after the wire and two main stems are allowed to grow and drape over support wires, growing down toward the ground.
The V-cordon method is somewhat similar to the drape method, in that the vine is trellised up to the wire and then grows back down toward the ground. However, the high-wire is not directly above the plants in a V-cordon. Instead, the wire runs a few feet out from the plant into the aisle, requiring stems to be trellised at an angle. With one row leaning into an aisle and the other row running toward the other aisle in a double aisle, it allows more light to be intercepted by each individual plant by reducing the shading from neighboring plants.
Some growers are also staring to use the lean-and-lower method (Fig. 2), similar to what is used for tomato. As plants near the support wire, the trellis each stem is trained to is lowered so the growing point is below the support wire, and then moved down the wire several inches.
Pests: Spider mites, thrips, whiteflies and aphids are some of the most common insect pests of greenhouse cucumbers. One of the most distinctive signs of an infestation is the mottled, chlorotic cucumber leaves resulting from spider mites. At first glance, it could appear like high-light damage or a nutrient deficiency, but this occurs when spider mites feed on leaves and cause stippling all over them.
Diseases: Cucumber mosaic virus can have detrimental impacts on crops. Since aphids can transmit this disease, keep aphids under control and your plants virus-free to avoid outbreaks. Powdery mildew is the most common disease in cucumber production. Look for those varieties that have some resistance to powdery mildew and start control measures early to avoid the development of hot spots.
Physiological disorders: Fruit abortion occurs when there are too many fruits on the plant at any one time and there is insufficient vegetative growth to maintain the crop load. Fruits may also abort when there is a sudden change in crop culture or the environment.
Harvesting: Growers can harvest from cucumber plants for approximately 12 weeks, though this will vary with cultivar, environment and culture. For standard-sized European cucumbers, fruits are usually harvested when they are between 12 and 14 inches, with the USDA requiring a minimum fruit length of 11 inches. Frequent harvests up to three to four times per week are necessary to keep plants balanced between vegetative and reproductive growth, as well as to maintain fruit quality.
Postharvest care: While the thin skin of greenhouse cucumbers is a positive attribute with respect to flavor and convenience, it creates challenges for postharvest; the thin skins allows more water loss from fruits compared to thicker-skinned field cultivars. To minimize water loss and extend postharvest quality, individual fruits should be shrink wrapped. For Beit Alpha cucumbers with the slightly thicker skin, multiple fruits may be placed on a polystyrene tray that is then shrink-wrapped, or they can be placed inside a plastic bag with ventilation holes. While fruits can be stored at room temperature, they can also be cooled to extend product life, but temperature should not get below 50 to 55° F to avoid chilling injury. Cucumbers may yellow in response to ethylene, so exposure should be minimized throughout the postharvest chain.