Drivhusklubben/Greenhouse Forum/Gewächshausclub

26 Feb 2024 09:13

Guide: Succeed with cucumbers

If you are a greenhouse owner, you have probably already cultivated cucumbers. For some, they achieve success the first time, while other get it wrong and may lose the confidence to cultivate cucumbers. You are not alone; it is not necessarily easy to cultivate cucumbers.

Cucumbers, in contrast to cultivating tomatoes, are a more delicate crop with higher demands on you as the main person responsible for their well-being. Overall, you might say that cucumbers place higher demands on the climate, cultivation practices, and the greenhouse environment. Furthermore, cucumbers require good quality soil with plenty of air in it.

Over the next couple of months, we will teach you all about how to cultivate cucumbers. By following along, you will be able to cultivate cucumbers without much difficulty. In fact, you only need to learn a few key things to easily overcome the typical challenges in cucumber cultivation.


March marks the beginning of spring officially. You can finally feel the light returning, which is quite refreshing. Since December, the length of the day has slowly been increasing, and in March, we can appreciate much longer days. In the past, March used to be a month where the snow melted, and not much else happened. However, due to climate changes, March has become much warmer and drier, tempting growers to start pre-sprouting earlier and earlier. Yet, our best advice is still to wait before planting cucumber seeds in the soil. Instead, research the type of cucumber you wish to grow, as not all cucumbers are the same.

March – Different types of cucumber.

In March, we'll discuss the different types of cucumbers. Cucumber (Cucumis) is a plant with about 50 species, and Cucumis sativus, as we know it, is one of these species. Cucumbers are related to melons because they have similar cultivation requirements, and both thrive in the warmest spots in the greenhouse.

Cucumbers have been cultivated for over 3,000 years, originating from India. Their warm native origin is precisely what makes them heat-loving. It is believed that cucumbers were likely introduced to Europe by the Greeks or Romans in their time.

As a cucumber cultivator, it is important to know a little about the original wild cucumber. The wild cucumber is distinctively different from the ones we are familiar with today.  

It is almost round and originally had large spikes outside the fruit, making it bitter and unpleasant to eat. Modern F1 varieties have mostly eliminated spines and bitterness. However, on some varieties, you may still see hints of spikes, and they can become slightly bitter if left on the plant for too long. To avoid a bitter taste and unusual-looking fruit, it is recommended to purchase F1 seeds.

In March, it is crucial to decide which type of cucumber you wish to cultivate and consider the cucumber's intended use.

General Division of a Cucumber

  • Gherkin
  • Japanese cucumber
  • Mexican sour cucumber
  • Crystal Lemon
  • Snake cucumber
  • Mini cucumbers
March – Different uses

Here at Spirekassen, we often experience our customers not knowing the difference in cucumbers. Therefore, som often ends up cultivating a type of cucumber for a wrong kind of purpose. Here is the easy and simple explanation on which seeds you should use for what kind of purpose. 

Gherkin and Japanese cucumber

They are two types that aren’t very heat demanding. Gherkin and Japanese cucumbers have a crawling growth and are not cultivated in the greenhouse. It is cultivated in the kitchen garden. The leaves are a little course in the texture and might poke a little. The fruit has a little rougher texture, and several still has a hint of spikes, just like on wild cucumbers. If the fruit become too big on the plant, they develop a bitter taste. The bitter taste hasn’t got any significant meaning as both cucumber types are used for pickling.



Crystal Lemon

This is a round and yellow cucumber. It has a sour taste. The crystal lemon does it all. Some eat it fresh without preparation. It can also be pickled and fermented. Allegedly they could both be cultivated outdoors and in the greenhouse. At Spirekassen we have not tested if this was true or not.

Mexican Sour Cucumber

It is actually not a cucumber. The Latin name Melothria scaba gives away that the cucumber doesn’t belong with the Cucumis line.

The Mexican sour cucumber is funny because it looks like a mini flattened watermelon and somewhat taste like a cucumber. A Mexican sour cucumber is primarily used for pickling, but some also eat it fresh in a salad. Yet you still get surprised when you chew on a fresh Mexican sour cucumber. The shell is thick and a little leathery. The Mexican sour cucumber likes the warmth of the greenhouse where it has a climbing and fast growth.

Snake and Mini Cucumber.

They are very good in salads as they do not need any preparation. The bitter taste has been bred out of out of the modern sorts that are used today. The school cucumber is suited for a lunch box as it is small and chubby. The snake cucumber is the classic cucumber we know from the supermarket. It can be up to 25-30 cm long. Salad cucumbers are heat demanding and need the warmth of the greenhouse to optimally thrive. The growth is climbing and the plants demand staking in the greenhouse.

NOTE. This article will go in depth with cucumbers that are suited for cultivation in the greenhouse.

The best advice for March
  • Choose the correct type of cucumber.
  • Familiarize yourself with climate conditions.
  • Wait to sow until the month of April.

April is one of the busiest months in the gardening calendar. During this time, the arrival of spring is palpable, and the sun regains its strength. However, it's a well-known fact that April can be capricious. Some days may feel like summer, tempting you to sow or place small plants in the greenhouse. Resist that temptation.

In April, we can still expect frosty nights and even more snow. Transplanting pre-germinated plants may go well, but the growth is not necessarily faster just because you have transplanted earlier. A tough start to outdoor life in the greenhouse can influence a plant for the rest of the growing season.

April - Ready for Pre-Germination Equipment

As you prepare for cucumber pre-germination in the greenhouse, ensure you've chosen the right type for your cultivation, such as a school cucumber or snake cucumber. While a large sprouting tray with ample soil volume is recommended for many plants, it's unnecessary for cucumbers.

Cucumbers are delicate plants with vulnerable root necks prone to infections. Whenever possible, avoid replanting small plants. Choose pre-germination in a 12 cm pot right from the start. Cucumber seeds, being relatively large, will sprout quickly, preferably within seven days, given the right sowing technique and temperature.

Always use seeding soil, a mild medium with less fertilizer, fine and porous, tailored to sprouting seeds and delicate plants. Consider adding perlite, a light and airy lava material, to enhance the soil. Some pre-mixed seeding soils already contain perlite.

April – Sowing and Placement

If you're running late with pre-germination, mid-April is still an opportune time. Moisten the soil and create a hole for the seed. Cover the seed with soil at a maximum of three times its thickness. Sow vertically to prevent water-related issues.

A crucial guideline is to sow 3-4 weeks before planting. When the first cucumber seed sprouts, the plant develops rapidly. Maintain a temperature of 22-24 degrees during sprouting. Avoid placing pre-germinated plants too far from the window glass to prevent them from becoming fragile and lanky.

Best Advice for April:
  • Initiate pre-germination in mid-April.
  • Use a sprouting tray or sow directly in a 12 cm pot.
  • Always use seeding soil with added perlite.
  • Avoid compost-based seeding soils.
  • Place plants close to the living room window.

In recent years, May has shown unpredictable weather. While it can be cold and rainy, it might also surprise with full sun and 20 degrees. Don't assume May will be mild; check weather reports, as nights can remain cold, especially during a full moon.

Clear nights result in high heat radiation, leading to cold nights in the greenhouse. If you've already planted in the greenhouse, cover plants with a layer of fibre cloth if the nights are cold and clear.

May – Climate in the Greenhouse

Despite appearances, it's easy to be misled. Markets and stores may offer small, finished cucumber plants, but avoid purchasing them. Cucumber plants dislike temperatures below 15 degrees, particularly when small. Such plants from supermarkets are often climate-stressed and not worth buying.

If you've bought ready plants or are considering planting while nights are still cold, your plants may stop growing or die. For safety, refrain from planting cucumbers before June 5th, unless the weather forecast promises at least a week of warm weather. Pay attention to night temperatures; consider using a thermometer to monitor temperature fluctuations in your greenhouse day and night. Find our Min-Max Thermometer here.

Temperature and Placement in the Greenhouse.

Cucumbers grow fastest at temperatures of 28-30 degrees Celsius. However, you will yield more cucumbers if the daytime temperature ranges between 20-22 degrees and the nighttime temperature 18-19 degrees is. This typically occurs around the end of May and in June. Ensure that the soil temperature does not fall below 15 degrees, as growth will completely stop at this point.

Cucumbers are heat-loving plants, so you can place them in an area where your tomato plants may not thrive – the hottest and sunniest spot in the greenhouse.

May - Collar Rot 

Cucumbers are highly susceptible to collar rot, a fungal attack affecting the lower part of the stem, right above the root. The responsible fungus is Fusarium oxysporum, which overwinters in the soil.

Planting out too early is the main cause of collar rot, as the root collar becomes too cold or moist. Several factors in April and May can exacerbate the plant's vulnerability to collar rot, such as large temperature swings, a low pH value resulting from exclusive watering with rainwater, excessive fertilizer concentration, and, most importantly, planting the cucumber too deep.

While we are accustomed to planting tomatoes deep if they have grown too long during pre-cultivation, this is not suitable for cucumbers. The root collar must be free and should NEVER be covered with mold. Keep the root neck as dry, airy, and mold-free as possible. To prevent a moist root collar, consider purchasing high-quality mold or plant bags, characterized by a high number of airho

Airholes facilitate quick water drainage when covering your plants. When using drip irrigation, avoid placing the drip near the root collar. Always ensure the root collar remains dry.

Best advice for May
  • Buy a min/max thermometer.
  • Never buy plants from the supermarket
  • Plant out late in late May/early June.
  • Always plant during a warm period
  • Cucumbers should have the warmest spot in the greenhouse.
  • Cover the plants with fleece if the night is cold and clear.
  • Never plant cucumbers deep in the soil
  • Always keep the root neck free, warm, and dry
June Month

Summer officially begins in June, bringing warm days on the terrace and long summer evenings in the greenhouse. It's the perfect time for cucumber planting, especially in early June. The average temperature for June in England is 18 °C. The greenhouse experiences significant growth during June, with tomatoes, peppers, chili, and cucumbers thriving rapidly. While growing tomatoes is relatively easy, cucumbers can be more challenging due to their susceptibility to various diseases and pests.

June - Diseases and Pests

The primary disease affecting cucumbers is powdery mildew, a fungal infection that gives the plant's leaves a powdery appearance, causing them to yellow and eventually wither. Powdery mildew can only survive on a living plant, making it an obligate parasite. Preventing and reducing the growth of this fungus can be achieved by maintaining high humidity. Opting for mildew-resistant cucumber varieties helps but does not eliminate the problem entirely.

The main pest attacking cucumbers is spider mites. While spider mites can infest various plants, they particularly favor cucumbers. These tiny mites suck leaf cells, causing yellow-brown discoloration on leaves and shoots, ultimately turning the leaves completely pale. Spider mites always reside on the undersides of leaves. They get their name from the web they create, aiding their movement from leaf to leaf and plant to plant.

Some late-generation mites in the fall may go into hibernation in the greenhouse, reinitiating the problem the following year. The persistent nature of this pest has rendered it resistant to many chemical substances after years of using pesticide solutions.

All insects thrive in dry conditions, so the solution is to do the opposite. Combat spider mites with tap water! Use a sprayer to give the undersides of the leaves a good soak, where spider mites typically reside. Use clean water and give your cucumber plants a water spray bath every time you pass by the greenhouse.

It's also possible to purchase beneficial insects against spider mites online. If you buy plants from organic nurseries, you might be lucky to receive beneficial insects, as they are commonly used for pest control in organic gardening.

June - Pruning Cucumbers

If it's your first-time growing cucumbers in the greenhouse, you'll likely be surprised by their massive climbing growth. Once your cucumber plants have settled in the greenhouse, they reward you with vigorous growth. The plant's lushness explodes in your greenhouse. It's now crucial to find a balance between shoot growth and fruit setting. Allowing plants to produce many cucumbers will divert energy from further growth, causing stagnation.

Some cucumber varieties produce male and female flowers. Male flowers should be removed immediately, while female flowers are identified by the small cucumber already forming beneath the flower. Female flowers should stay on the plant, and male flowers should be removed. Many modern F1 hybrids only produce female flowers, making it much easier for you, as no flowers need to be removed.

The First Fruits and Pruning

The initial cucumbers form on the main stem, and it's tempting to leave them be, but they should be removed. The most common method is to remove all flowers on the main stem, leaving every second lateral shoot, but prune it beyond the second leaf pair. Remove the main stem's top shoot when the plant reaches a height of about 2 meters. Removing the top shoot prompts the plant to produce numerous new lateral shoots. These lateral shoots, in turn, generate even more shoots, resulting in a somewhat chaotic situation but a method that yields many cucumbers.

Remember that, like tomatoes, cucumbers need to be trellised in the greenhouse. When summer is in full swing, it's time to remove all leaves and shoots on the lower half (closest to the ground) of the main stem. This is done, among other things, to improve air circulation throughout the greenhouse.


July brings the "Dog Days," the period from July 23rd to August 23rd, traditionally the hottest days of the year in the Northern Hemisphere. The Dog Days also signal the arrival of the harvest, especially for greenhouse crops like cucumbers.

July - Cucumber Care:

When the cucumber plant's root system is well-established, growth truly begins. Pruning, as discussed earlier, should continue throughout July. It's also crucial to provide support for the cucumber plant. Use jute or hemp twine for tying your plants in the greenhouse. Ensure the twine is doubled or tripled if it's thin.

Binding DIY:

  • Create a loose loop around the plant's main stem.
  • Do these two leaf pair up the stem to avoid damaging the root.
  • The loop should be loose as the stem increases in diameter during the summer.
  • Then, wind the twine around the plant in loose twists clockwise.
  • Secure the end of the twine to the greenhouse roof.

Remember, as you tie, consider that the stem will expand as the plant grows, so avoid binding too tightly. All binding where you twist the plant around a string vertically should be clockwise due to our location in the Northern Hemisphere. Adequate support is essential as fully developed cucumbers can weigh a lot!

Even though cucumbers love warmth, it's essential to maintain ventilation 24/7 in July. This creates a greenhouse environment with just the right amount of moisture. Open windows also help keep the temperature down on hot summer days. The open windows are also beneficial for pollinating insects.

July - Watering and Fertilizing:

Cucumber plants have a massive leaf surface, leading to high evaporation. Therefore, cucumbers need daily watering in the greenhouse if you're not using self-watering containers. In July, with the growth in full swing, there is a need for fertilization. The type of fertilizer is somewhat a matter of preference; a solid pellet fertilizer is convenient for fixed soil beds. Both organic and inorganic options are available.

If you use self-watering containers, fill them exclusively with tap water. If you want to use rainwater, mix it with tap water in a 1:1 ratio. Always follow the dosage of liquid fertilizer indicated on the product. It's crucial not to overdose liquid fertilizer in a self-watering container.

Note: Plant nutrition and plant fertilizer are NOT the same. When the term "nutrition" is on a product, it's not very concentrated. When "fertilizer" is on the product, it's concentrated and goes a long way.

Best advice in July:
  • Always bind clockwise.
  • Ventilate around the clock.
  • Use tap water in self-watering containers.
  • Know your fertilizer type well.

August is the last month of summer, signifying the harvest month. Normally, grain ripens in August, but in recent years, it has been harvested as early as July in Denmark. We can expect an average temperature of 16.8 degrees Celsius, an average rainfall of 86.1 mm, and around 199.5 hours of sunshine. August brings plenty of warmth and light for cucumber cultivation.

August - Cucumber Time - How do I use all those cucumbers?

You are likely fortunate to harvest cucumbers almost every day now, maybe even struggling to keep up. A cucumber plant produces many cucumbers. Some of us may not have much knowledge of how to store harvested cucumbers best. Remember to harvest fruits in time, as overly large fruits can develop bitterness.

Many store their fresh cucumbers too cold. Cucumbers cannot tolerate temperatures below 7 degrees Celsius; if placed under these temperatures, they become glassy and lose flavour. The glassy texture ruins the cucumber, and it needs to be discarded. For the same reason, cucumbers cannot be stored long-term in the freezer as the texture is destroyed.

Place your fresh cucumbers in a bag to prevent evaporation, as high evaporation can make the cucumber limp. In the refrigerator, make sure your vegetable drawer does not get colder than 7 degrees Celsius. If it does, a cool spot in the kitchen is better.

Another good trick for fresh cucumbers is to keep them away from tomatoes, apples, pears, and bananas. These fruits release large amounts of the ripening hormone ethylene, which is a gas. Fresh cucumbers are very sensitive to ethylene. If your newly harvested cucumbers are exposed to ethylene, they will overripen rapidly.

Ideas for using cucumbers 
  • Make a cucumber sandwich 
  • Make remoulade which is a Danish cold dressing made with mayonnaise, fresh cucumbers, onions, and green tomatoes.
  • Pickled cucumbers
  • Use cucumbers in juice 

Autumn has begun, and it's time to say goodbye to cucumber cultivation for this season.

Climate changes have generally made September milder, wetter, and sunnier. In Denmark, this month brings an average temperature of 13.6 degrees Celsius and an average of 147.5 hours of sunshine. Day length has decreased by five hours since the summer solstice in June. We can expect an average of 12.1 days of rainfall in September. The time for cucumber growth is almost over.

September - Harvesting Seeds:

Many people want to harvest seeds for the next year, especially because cucumber seeds are often quite expensive. Cucumbers have undergone significant improvements over time. Male flowers have disappeared in most varieties through breeding. The spines on the fruits are gone, as is the bitterness in taste. Some varieties are resistant to the mildew fungus.

Behind all this is years of breeding work. Most modern cucumber varieties today are what we call F1 hybrids. With F1 hybrids, we have intervened in nature's course and created completely unique properties. Creating an F1 variety involves initially self-pollinating two varieties, like cucumbers, for many generations until they degenerate. This is what we popularly call inbreeding.

Then, you cross two inbred plants. Now you get entirely new, good properties. However, this ONLY happens in the first generation, the F1 generation. You'll often see the term "F1" on seed packets, especially for cucumber seeds. If you take seeds from your F1 plants again, you end up with significantly inferior properties from the earlier inbred parent generation.

Therefore, you cannot take seeds from most modern cucumber varieties.

Greenhouse Cleanup:

Before winter begins, it's essential to maintain good hygiene in the greenhouse. Remove all old plants, roots, and especially the cucumber's root neck. This is to prevent harmful fungi and bacteria from surviving in old plant parts and soil for the next growing season.

Top 10 tips:
  • Start late with pre-growing in April.
  • Choose F1 varieties to avoid bitterness and spines.
  • Transplant late, around June 1st or in very warm periods.
  • Never plant too deep; keep the root neck dry, warm, and free from soil.
  • Mist daily with water on the underside of the leaves.
  • Remove the top shoot when the plant is about 2 meters high.
  • Remember proper support.
  • Don't let fruits get too large on the plant.
  • Store fruits at 7 degrees Celsius, away from cucumbers, apples, pears, and bananas.
  • Avoid harvesting seeds for new sprouting next year.