Cucumber Conundrums

By Lynette Morgan
Published: December 1, 2016 | Last updated: March 28, 2017 02:23:08
Key Takeaways

While hydroponic cucumbers are a great choice for their quick growth and impressive yields, the crop is not without its problems. Thankfully, Dr. Lynette Morgan is here with some simple solutions to help easily solve many of these issues.

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For a fast-growing, high-yielding crop, you can’t go wrong with hydroponic cucumbers. These slender, green salad essentials produce a greater weight of fruit per unit area than capsicums, tomatoes, or just about any other hydroponic crop when grown under optimal conditions.


For those of us who are a little impatient, cucumbers are a satisfying plant that can fill an indoor garden within a short period and have young fruit hanging from the vine long before other plants have even managed to start flowering.

While the crop’s growth and size may be rapid and impressive, cucumbers are not without their baffling problems. Thankfully, though, many of these issues are easily solved or prevented with a few basic steps and an understanding of the sensitivities of this popular crop.


Cucumber Genetics

Many of the common cucumber problems experienced by indoor gardeners can be prevented with the correct genetics. There are a range of different cultivars, but those most suited to indoor gardens are greenhouse hybrids that have been bred for productivity, fruit quality, and disease resistance.

A resistance to common diseases like powdery mildew is particularly important. That’s because older, open-pollinated types of cucumber varieties are still extremely susceptible to developing mildew, even at a relatively early stage of growth.

EC Levels of Cucumber Gardens

Due to their rapid growth rate and large leaf surface area, cucumbers place a high demand on their hydroponic system, nutrient solution, and growing environment. They also have specific sensitivities that require a little more control and monitoring of the nutrient solution and root zone than many other crops.


An extremely common problem with first-time cucumber growers is the development of leaf yellow halo, which is a thin band of yellow coloring around the leaf margin. Typically, this will develop on cucumbers while all the other plants in the same hydroponic system are uniformly green and healthy looking.

The leaf halo effect is caused by EC levels that have become too high for the plant in the recirculating nutrient solution or in the substrate. These high levels may also cause cucumbers to develop slightly drooping leaves.


High EC levels can be caused by nutrient accumulation and salt buildup in some growing mediums, such as coco. These problems commonly occur under warm growing conditions where moisture evaporates from the surface of the substrate and a plant’s uptake of water exceeds its absorption of nutrient ions.

High EC levels are also found in mixed cropping systems where EC levels are typically run higher to boost fruit flavor and compositional quality in salt-tolerant plants, such as tomatoes.

Dropping EC levels back and leaching excess salts from the growing medium usually alleviates they symptoms of yellow halo and drooping leaves. The ideal EC levels for cucumbers are 1.7-2.0 during fruiting.

When checking levels, however, keep in mind that EC should always be measured in the leachate (solution draining form the root zone); in substrates, the EC in the root zone may be higher than that in the feed nutrient solution.

Chloride Levels in Cucumber Plants

A similar, yet often misdiagnosed, issue stems from the cucumber plant’s sensitivity to chloride. Chloride uptake by cucumber plants accumulates in the foliage, producing a band of pale green tissue around the leaf margin, followed by scorching and browning of the edges.

Cucumbers need a good-quality water supply that doesn’t contain high sodium chloride (common salt) levels.

Sodium chloride can be a common problem in many water supplies and where domestic water softener units are being used, and recirculating systems are particularly more prone to chloride buildup when sodium chloride is present in the water supply.

The use of an alternative water source or reverse osmosis is a good option to clear things up. In addition, avoid using fertilizers and nutrient products that contain potassium chloride and calcium chloride.

Cucumber Nutrition

Cucumber nutrition is like that of other heavily fruiting crops, with an emphasis on the use of vegetative and fruiting formulations as the plant moves through the different growth stages.

Due to the rapid and prolific nature of fruit production, cucumbers have a particularly high requirement for potassium, much of which ends up in the fruit tissue and plays a role in flavor, firmness, shelf life, and overall compositional quality.

Potassium also affects stress resistance, plant turgor, and osmotic control. However, deficiencies can be common in recirculating systems with heavily fruiting cucumber crops.

The recommended potassium levels during fruiting are 200-350 ppm. Using a potassium booster or aggressive bloom nutrient products usually provides these levels. Calcium levels are also essential during this phase and should be boosted as the first fruit are set.

Boron is another common nutritional problem with cucumbers. Plants deficient in this nutrient develop fruit that has cavities or is hollow in the center. Young fruit may also drop from the plant, have corky markings or yellow streaks on the skin, or develop a curved shape, while newer leaves may become distorted. Recommended levels of boron in the nutrient solution are 0.5-0.8 ppm.

Hydroponic System Sharing

While it’s common for an indoor garden to have a number of different plant species sharing the same recirculating nutrient solution and system, cucumbers can place a particularly high demand on oxygen and certain nutrients.

They also produce large amounts of organic root exudates. This may require a higher frequency of solution replacement, additional methods of oxygenation, and regular monitoring, as well as keeping a check on potassium levels during the fruit phase.

Increasing Cucumber Flavor and Fruit Quality

Some fruit quality issues are caused by maturity stage or genetics. Older, open-pollinated varieties are particularly likely to develop bitter flavors when harvested over mature. Modern greenhouse hybrids, on the other hand, have largely had this tendency bred out.

Plant stress, unfavorable growing conditions, and pests and diseases can also create flavor issues. In particular, slow-growing plants under less-than-ideal temperatures and humidity levels often develop stronger, unpleasant flavors.

Light levels also affect cucumber fruit quality. Light produces assimilate for fruit growth and insufficient light will result in poorly flavored and under-sized fruit. Under severe light deficiency, young fruitlets will often abscise and fall from the plant.

Cucumbers particularly require high light levels, similar to those given to tomatoes. Canopy shading can be an issue, however, as cucumbers take up a lot of space (ideally, plant density is one to 1.5 plants per square meter).

Finally, excessive heat, high EC, and low humidity can also result in plant wilting and fruit that lacks crunch and has a short storage life.

Cucumber Fruit Loading, Drop and Shrivelling

Commonly reported issues with some cucumber crops are the abscission and drop of small fruitlets, or the shrivelling and dying back of young fruit on the vine.

This can simply be a result of heavy crop loading; the plant will limit the development of new fruitlets if there are already a number of larger fruit dominating the supply of assimilate produced in the leaves.

Other common reasons are a lack of sufficient light for fruit growth, nutrient deficiencies, or sub-optimal growing conditions (particularly, cool night temperatures). If this is the case, most healthy plants will eventually start to set fruit once growing conditions are suitable.

Pests and Diseases of Cucumber Plants

Cucumbers are prone to root rot diseases under certain growing conditions, especially those that restrict oxygenation around the root zone or oversaturate the growing medium. Temperatures that are either too high or too low can also stress cucumbers and make them more susceptible to pathogen attack.

Cucumber seedlings can develop Pythium if overwatered, so a sterile growing medium and clean water source are recommended during the early propagation phase. Larger plants are less susceptible to root rot disease unless the root system is damaged in some way through water or EC stress or, more commonly, when plants are over irrigated and suffer root zone suffocation.

When it comes to pests, the most common and problematic for cucumbers are mites. These pests thrive in warm, dry indoor gardens and can be difficult to eradicate. Inexperienced growers also often misdiagnose the early sings of mite infestations on cucumber plants as the pests are too small to see easily and the foliage symptoms are easily missed in the early stages.

Mite damage starts as a slight bronzing, speckling, or loss of color on the foliage. Later on, superfine webbing can be seen in the leaf axils and mites, which look like tiny red dots, appear on the undersides of leaves. The use of a magnifying glass is helpful when identifying mites and early and repeated control is essential. Also, increasing humidity and damaging down with water sprays can help reduce mite populations.

Whitefly, aphids, and thrips can also cause problems with cucumber crops, though they produce symptoms many growers don’t associate with these pests. Thrips feeding on young cucumber fruit results in fruit deformities that become apparent long after the insect has moved on. Thus, early monitoring for these pests is vital. Blue sticky traps can be used to check for the presence of thrips infestations.

Aphids and other sucking insect pests can carry a wide range of cucumber viruses that can cause a number of strange and serious symptoms such as leaf mottling, leaf and fruit deformities, plant stunting, and lack of fruiting.

The most common, however, is cucumber mosaic virus, which causes ‘shoestring’ growth. This is when new leaves grow out thin, twisted, thickened, and alien-looking, and the entire plant becomes extremely stunted.

As such, selecting cultivars with some degree of resistance to cucumber mosaic virus is recommended if aphids and other pests are an issue. Many modern greenhouse hybrids also have genetic resistance to other significant disease issues such as cucumber vein yellow virus, powdery mildew, target spot, scab, and Fusarium root rot, making crop production much more reliable.

Are Your Cucumber Plants Wilting?

Under warm conditions, cucumbers may naturally wilt slightly during the warmest part of the day, but regain turgor at night. However, long term, severe wilting is a sign that root zone or environmental conditions are causing plant stress and need adjustment.

Cucumbers seem to be more prone to wilting than any other hydroponic crop. This is largely due to the size of the leaves, which have a high rate of transpirational water loss, particularly in low humidity conditions.

This transpiration can result in a rise in EC in the root zone or nutrient system, which can lead to even more progressive wilting. Other conditions such as certain nutrient deficiencies, root rot disease, and excess temperature may also be the cause. Under hot growing conditions, the EC can be dropped back slightly, particularly if the plants are wilting under overhead lights.

Wilting can also point to issues in the root zone, though it is not necessarily a sign of a lack of moisture. In hydroponics, wilting it is more commonly caused by overwatering, which results in a lack of oxygen in the root zone.

Despite what may seem like a long list of checks and precautions when contemplating cucumber cultivation, most crops are usually trouble-free, high-yielding and thoroughly rewarding. Still, as with all hydroponic plants, it pays to know what to look out for with specific species, and a little background knowledge always goes a long way in preventing any major problems.


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Written by Lynette Morgan | Author, Partner at SUNTEC International Hydroponic Consultants

Profile Picture of Lynette Morgan

Dr. Lynette Morgan holds a B. Hort. Tech. degree and a PhD in hydroponic greenhouse production from Massey University, New Zealand. A partner with SUNTEC International Hydroponic Consultants, Lynette is involved in remote and on-site consultancy services for new and existing commercial greenhouse growers worldwide as well as research trials and product development for manufacturers of hydroponic products. Lynette has authored five hydroponic technical books and is working on her sixth.

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