Cucumber production 201

Cucumbers are an increasingly popular fruiting vine crop to grow hydroponically in greenhouses. The fruit quality of greenhouse cucumbers is fairly different from their field-grown counterparts due to their thin skin, seedless flesh and mild flavors. With a shorter crop time than tomato or pepper, maximizing productivity and quality — and avoiding problems during production — is important. While we covered the fundamentals of growing cucumbers in an early Hydroponic Production Primer, this article will focus on additional considerations for growing the best crop possible.

Fruit crooking or bowing can be a problematic physiological disorder for hydroponic cucumbers grown in controlled environments. When greenhouse cucumbers are graded using the standards the USDA has published for them, in order to receive the “U.S. Fancy” grade, they must be considered “well-formed.” In addition to minimal tapering at each end of the fruit, well-formed also means the 11-plus inch fruits must be fairly straight. Since greenhouse-grown cucumbers are considered a value-added crop, producers want as many fruits as possible to receive the best grading possible and cucumber crooking can reduce marketable fruit numbers. There are a few different causes of cucumbers forming with a curvature or crook.

First, insufficient pollination and subsequent ovary fertilization can be the cause of crooked cucumbers. However, greenhouse cucumbers are gynoecious (forming only female flowers) and are parthenocarpic, forming fruits without any pollination (thus the seedless fruits). For greenhouse cucumbers, crooking can be caused by the young, developing fruits coming into contact with a potential impediment to growth, such as the twine or string used to trellis fruits up, or the clips attached to the twine for supporting vines. Mindful placement of trellis clips and twine can help avoid this problem.

Crooking can also be caused by water uptake and pest infestations. Try to manage water uptake to occur as uniformly as possible throughout the day, avoiding long periods without water followed by rapid uptake; this is similar to managing irrigation in tomato crops to avoid fruit cracking. Additionally, watch aphid populations as they can contribute to fruit crooking.

In fruiting vine crops, we are commonly managing nutrient solutions carefully to ensure crops receive sufficient calcium (Ca). Insufficient Ca can lead to blossom end rot in tomatoes and peppers, making fruits unmarketable. However, although Ca is still an important nutrient for cucumbers, potassium (K) is more important for this crop compared to other vine crops. While K deficiencies are not as common for tomato and pepper, yellowing or chlorosis on cucumber leaf margins are a common sign of K deficiency in cucumbers.

Additionally, K is important for promoting rapid and uniform fruit growth, and is an important nutrient for cucumbers and their rapid growth. There are a few approaches to avoiding K deficiencies in cucumber crops.

First, ensure plants are getting adequately fertilized by monitoring the electrical conductivity (EC) of nutrient solutions provided to plants, as well as the EC of the leachate. However, ensuring adequate EC alone may not solve the problem. In its ionic form, K is positively charged (K+) and is a cation. Calcium (Ca2+) and magnesium (Mg2+) are also cations and carry a positive charge. Since K, Ca, and Mg are all cations, and are also taken up passively through bulk flow when water is taken up by roots, they can compete with one another for plant uptake. Too much of one of these nutrients can cause an antagonism with the other nutrients and diminish uptake.

For instance, if Ca concentrations are too high, less K will be taken up. A two-tank approach to fertilizing cucumbers is most common. One tank filled with calcium nitrate provides Ca (as well as the majority of nitrogen), while the second tank provides the other macronutrients (including K) and micronutrients and has an analysis of 5-12-26 or something fairly similar. The proportion that each fertilizer contributes to the overall fertilizer EC may need to be adjusted to ensure not only adequate K, but to avoid excessive Ca.

Another important aspect of managing cucumber is pruning or top-working to ensure consistent growth patterns. Cucumber crops have a fast rate of growth and development, and this includes stems, leaves, tendrils and fruits. The rapid rate of stem extension, up to several inches per day, requires frequent trellising, and the most common training methods for canopies were outlined in the earlier Hydroponic Production Primer.

Tendrils, a type of modified leaf, develop all along cucumber stems. While these help support cucumber vines, they are unnecessary and undesirable in greenhouse systems where trellis clips are used to help support vines. The tendrils can wrap around stems, leaves, fruits and trellis twine and cause problems if they are left to establish. No tool is needed to remove them, as they snap simply off the plant. The leaf unfolding and maturation rate can also be very fast in cucumber production, and plants can develop an excessive number of leaves on the stem.

While the total number of mature, fully expanded leaves to leave on a stem at any one time varies with the trellis height and training method used, it can be beneficial to remove the oldest leaves on the crop as new ones unfold. This can also help diminish pest and disease pressure.

It is also important to thin the number of fruits allowed to develop in each leaf axil. While multiple fruits may be left at a single leaf axil to develop for small “cocktail”-type cucumbers, only a single fruit should be allowed to develop in each leaf axil for the standard long-fruited greenhouse varieties. While it is best to thin excessive fruits as early as possible, be careful to still leave a fruit to develop at each node.

Regardless of what type of pruning you are performing — tendrils, leaves, or fruits — try to conduct pruning as frequently as possible (i.e. daily) so excessive numbers of tendrils, leaves, and fruits are not allowed to develop, only to be removed en masse at a later date. Cucumbers do not respond well to this approach. For example, if excessive fruits are allowed to develop and then thinned, fruits left to develop may abort.

Cucumbers are a great vining crop, and their fast growth and high yields can make them a greenhouse crop to consider. The tips and tricks outlined in this article are meant to provide you with some insight to fine-tune your cucumber production.

Christopher (ccurrey@iastate.edu) is an associate professor of horticulture at Iowa State University.