Growing cucumbers in your garden feels like a pretty straightforward endeavor until their leaves turn yellow. This can be disheartening for novice gardeners especially if you don't know what's causing it.
Those leaves aren't just for a show, yellowing leaves are the first sign that something is off so it's important to take steps to prevent the underlying problem from getting worse and undermining a good harvest. Here are common reasons and solutions for cucumber leaves turning yellow:
It's crucial to take action if you see yellowing leaves in your cucumber patch. However, not all leafs yellowing is a bad sign as it could simply be the old leaves showing their age. As part of the cucumber plant's natural aging process, older leaves turn yellow and fall off to give way to new growth.
Hence, even if we notice a sudden change in leaf color, not all of them warrant a hostile response so understanding the context in which they appear is important.
What To Do:
Observe your cucumber plant leaves closely over a period of days. If only the oldest leaves are yellowing then there is no concern to be had, but if the yellowing spreads from one part of the plant to another, then it's time to identify the possible issues.
Look for patterns in the yellowing, such as uniform discoloration or spots which may indicate disease or webbing for fungus and in some cases pests. If it is associated with symptoms like stunted growth, it might be a nutritional deficiency.
2. Moist Leaves
In most plants, allowing their foliage to get wet whether through improper air circulation or having moist foliage through improper irrigation techniques invites diseases and cucumbers are no exception.
Avoid getting water on the leaves because it attracts airborne fungal diseases such as powdery mildew. High-moisture environment on the leaves can also encourage mealybugs, gnats, ladybugs, etc, and result in less production with your plants.
What To Do:
When watering, water as close to the soil. If possible, utilize drip irrigation which prevents the issue of water splashing onto the leaves and also saves you from having to water the plants entirely.
If these aren’t the problem then lack of proper spacing could be the culprit. Only the bottom leaves being humid is enough to attract pests and fungus. Space cucumbers 36-60 inches (90-150cm) or utilize trellis and space them 12 inches (30cm) apart. A thick layer of mulch also helps prevent humid leaves.
If humid leaves aren't the cause then it may be due to poor sunlight. Cucumber plants thrive in heat and sunlight with them requiring a minimum of six to eight hours of bright, direct sunlight daily, and do not tolerate shade.
As a vegetable grown for its fruit, the full sun helps produce sufficient energy to create the abundance of fruits as well as the nutrients to be poured into the fruits so the cucumbers taste crispy crunch, and fresh instead of bland or worse, bitter.
What To Do:
Cucumbers prefer warmth and lots of light, so if you aren't already farming them outdoors, place them on south-facing windows with a warm golden glow that bathes them in direct sunlight.
It is still best to grow cucumbers outdoors in the garden with suitable soil. If the garden soil isn't suitable for it, utilize raised beds with the appropriate soil. Raised beds also have the advantage of being well-draining and being isolated from the rest of the garden in terms of spreading disease through the soil.
4. Excessive Sunlight
While the cucumbers do prefer full 6-8 hours of sunlight, no plant can survive in the extremes. The variation in climates means some regions can be too hot even for our sun-loving cucumbers.
If your garden gets extended periods of temperatures exceeding 85°F (30°C), too much direct sunlight causes leaf burns, browning, and crinkly leaf edges on cucumbers. Eventually, the leaves turn completely yellow and die while exposing the fruits. The leaves are necessary for protecting the developing fruit from overpressure which reduces flavor and yield.
What To Do:
To reduce the chances of sunburn on the leaves, since cucumbers do still require sunlight for the best yield and quality fruits, provide some afternoon shade to reduce possible yellowing from excessive sunlight in hotter regions.
Furthermore, you can also perform trellising which is a practical and space-saving farming technique to grow cucumbers while providing a good balance of direct sun for the vines and healthy foliage. Companion planting with a sister plant that provides shade is also a good alternative.
If irrigation and temperature aren't causing an issue then the next common cause would be having inadequate nutrients and micronutrients such as nitrogen, phosphorus and potassium in the soil. Especially with Nitrogen deficiency which is responsible for the production of chlorophyll, your leaves are highly likely to turn yellow.
Iron deficiency causes yellow leaves except the veins, phosphorous causes stunted growth with gray-green leaves, and potassium deficiency causes scorched yellowing which moves inwards before the leaves fall off and magnesium deficiency causes interveinal chlorosis.
What To Do:
The first thing to do when you suspect nutritional deficiency is to test the amount of nutrients available in the soil by either hiring an arborist or using a purchased DIY kit. Do not ever add additional nutrients or fertilizer without proper confirmation.
It is best to talk with a certified arborist before supplementing with nutrients. Our experts recommend using an NPK fertilizer of 2-4-6 ratio for these heavy-feeding plants. It can be beneficial to apply liquid fertilizer every two weeks to cucumber plants.
6. Over-Fertilizing
Over-fertilizing cucumbers is equally bad which is why we do not recommend supplementing nutrients unless the soil isn't fertile enough and organic fertilizers are unavailable, and then again this should not be done without proper confirmation of nutritional deficiency in the soil.
Over-fertilizing the soil can lead to root burn which causes the plant distress, yellowing their leaves, and in extreme cases even death. In most cases, the plant will develop lush foliage instead of flowers or fruits. This increase in foilage also increases the chances of attracting pests.
What To Do:
When dealing with over-fertilizing, nitrogen is the common culprit but any nutrient in excess will cause similar problems. When using multiple fertilizers and soil amendments, understand everything that they will introduce to the soil as adding several different types containing the same nutrient can easily result in one nutrient being over the limit.
If you realize that your plant has been overfertilized, act immediately. Pour a lot of water into the soil to dilute and drain the fertilizer mixture and always follow the instructions on the packages regarding proper disposal of run-off fertilizer.
Overwatering the cucumbers causes equal problems as underwatering them causes. When water is logged in the soil, the cucumber plants are vulnerable to root rot which causes wilting of leaves, yellowing, and finally dropping off.
It defoliates the plants until it finally dies. An easy telltale sign of overwatering is seeing yellow leaves at the base of the plant. A more reliable method of checking is to either use a moisture meter near the root regions in the soil or simply press your fingers to check the soil oversaturation level.
What To Do:
If high soil moisture is caused due to heavy or clay soil attempt improvements through the loosening of the soil with a broad fork, then add compost, peat moss, or vermiculite to make it well-draining.
If the soil quality is unsalvageable, consider utilizing raised beds with store-bought soil. Overwatered soil can also lead to a leaching effect, which causes essentials such as nitrogen to wash away causing the same problems as low nutrients in the soil in tandem to root rot.
8. Under-Irrigation
To check if the cucumbers are underwatered, an easy telltale sign is seeing their leaves dry, yellowing with brown and crispy edges. When they get extremely dry, it causes curled leaves that then turn brown at the edges and fade from green to yellow thus causing dry yellow leaves.
The cucumbers are composed of 96% percent water, so it makes complete sense that they would need more water. At the end of it, the yellowed cucumber leaves fall off which ultimately kills the plant.
What To Do:
Provide them with an inch of water each week and irrigate them at the soil level. You can consider adding a thick layer of mulch to the cucumber which not only aids in water and moisture retention in the soil and thus prevents under-irrigation, they also isolate the soil which prevents the diseases present in the soil from spreading to the plants.
Another thing you can consider is utilizing a timer to help water regularly or using a drip irrigation system so you don't have to water the plants manually.
It is an absolute truth that temperature and the weather can either support or thwart our gardening efforts. In hotter areas, despite providing plenty of afternoon shade from the sub to cucumbers, an extreme heatwave spell can cause the temperature around the plants to be hot enough to cause yellowing.
Ideally, the temperature should be below 82°F (28°C) or at worst below 95°F (35°C). The extreme heat triggers a survival mode which restricts the nutrient flow causing a sickly yellow leaf color as it attempts to conserve energy.
What To Do:
If you already have some form of protection against the heatwave such as providing shade or mulching, double down on it by adding additional cloth for shade and increasing the thickness of the mulch to retain the moisture in the soil for much longer.
Cucumber plants have a thick tap root system that can go 28 inches (70cm) deep. Use deep watering techniques to ensure that cucumbers still have access to water despite the topsoil drying out.
10. Insufficient Temperature
Conversely, having unseasonably cold temperatures can also stall the growth cycle of the warmth-loving cucumbers and result in a similar yellowing discoloration. Cucumbers are sensitive to cold with the possibility of chilling injury occurring if air temperature drops below 65ºF (18ºC) and complete loss below 55ºF (13ºC).
Chilling and sudden cold snap injuries include upward leaf rolling, browning, and bleaching of the leaf veins and finally, decay and pitting of cucumbers.
What To Do:
If you live in regions that are capable of dipping temperatures, avoid open or low spots in the garden where cold air can flow collect. Grow the cucumbers in sheltered areas of the garden, fences, or boulders for protection.
If a sudden cold snap is forecast, cover the plants and cucumbers with anything available on hand such as plastic, newspapers, bed sheets, etc. Open the covers daily to allow the accumulated condensation to evaporate in the morning and close before midday to trap the heat.
Even when the weather and climate are perfectly perfect, your cucumbers could still succumb to yellowing due to the presence of pest infestations. Luckily however, pests are the easiest bunch to detect and diagnose when your cucumber leaves are yellowing.
These pests turn the leaves to yellow and die off by sucking the sap which drains the vines out of all their moisture while also encouraging and spreading diseases. Some of the most common culprits are aphids, cucumber beetles, spider mites, and white flies.
What To Do:
First, identify the type of pests involved. If you cannot identify with confidence, hire a professional arborist or ask for help in online gardening forums. Once you have identified the correct pest, take appropriate measures by consulting with professionals.
For mild infestations, use a garden hose to kill and knock them off the leaves. Remember to check the undersides of the leaves and use insecticidal soap, neem oil, or beneficial predators for serious infestations. Spider mites leave webbings on the undersides of leaves and cucumber beetles cause damage by directly chewing the cucumbers.
12. Disease
If you ensure good basic gardening practices, the chances of diseases occurring in the garden are low but not impossible. If you have tried diagnosing the cause of yellowing from temperature, light, pests, and nutrients then perhaps it's time to consider testing for diseases.
Cucumber Mosiac Virus and fusarium wilt can be spread by aphids and cause yellowish patches, molting on the leaves or yellowing with brownish-red spots. Cucumber Vein Yellow Virus causes yellow veins and downy mildew causes discoloration and white fluffy growth.
What To Do:
Early downy mildew can be treated with neem oil. Yellow Mosaic and Fusarium Wilt can be prevented through strict aphid control while Cucumber Vein Virus by controlling whiteflies. Excluding downy mildew, most diseases are incurable and the plants must be disposed of. Do not put infected plants in your compost bin!
Ensure proper spacing and airflow to prevent fungal diseases and use fungicides if necessary. You can also plant crops in different parts of your garden every year to prevent pest and disease build-up.
