Growing Cucumbers in a Greenhouse: Sowing, Care, and Harvesting!

By the AtmoSerre team

🥒 Introduction

Cucumber is one of the most popular vegetables among French people, whether in salads, gazpacho, marinated, or plain. But did you know that growing cucumbers in a greenhouse allows you to get harvests that are twice as abundant as in the open ground, with more regular, less bitter fruits available much earlier in the season? With a polycarbonate greenhouse, cucumbers reveal their full potential.

Demanding in terms of heat (they don't like temperatures below 15°C), cucumbers are one of the vegetables that benefit most from cultivation under cover. In a greenhouse, you can start sowing as early as March and harvest until November, which is a season 2 to 3 times longer than outdoors.

In this comprehensive guide, we will walk you step-by-step through growing cucumbers in a greenhouse successfully: from choosing varieties to pruning techniques, including watering, fertilizing, and disease prevention. Whether you have a small 6 m² greenhouse or a large 20 m² structure, you will find all the keys here for an abundant and tasty harvest.

🌟 Why Grow Cucumbers in a Greenhouse?

✅ Major advantages of greenhouses for cucumbers

🌡️ Constant and controlled heat Cucumbers are tropical plants that need heat to grow properly. Below 15°C, growth stops. Below 10°C, plants can be severely damaged. In a greenhouse, you maintain an optimal temperature of 20 to 28°C, even when nights are cool outside.

📅 Significantly extended season In open ground in France, the cucumber season lasts little more than 3 months (July to September in most regions). In a greenhouse, you can sow from March and harvest until November, which means 6 to 8 months of production!

🥒 Better quality fruits Greenhouse cucumbers are generally longer, more regular, and less bitter than those grown in open ground. Protected from bad weather, they develop a thin skin and do not show deformities related to sunburn or thermal shock.

💧 Fewer diseases Downy mildew and anthracnose, two major cucumber diseases, are transmitted by splashing rain. In a greenhouse, your plants remain protected, which significantly reduces the risk of infection.

🏆 Yields up to 3 times higher A cucumber grown in a greenhouse can produce 15 to 25 fruits per plant compared to 5 to 10 in open ground. With 3 to 4 plants in a 6 m² greenhouse, you will have plenty to feed your family all summer!

Discover our polycarbonate garden greenhouses specially designed to create the ideal environment for your vegetable crops.

🌱 Choosing the Best Cucumber Varieties for the Greenhouse

The choice of variety is crucial for the success of your cucumber cultivation in a greenhouse. There are two main categories of cucumbers suitable for cultivation under cover.

🥒 "Long" greenhouse cucumbers

Marketmore A very productive, disease-resistant variety with long fruits measuring 20 to 25 cm. Excellent for beginners, it forgives many mistakes and produces regularly throughout the season.

Tanja The reference variety for greenhouse cultivation! Fruits are 30 to 35 cm long, with thin, smooth skin, crisp flesh, and few seeds. Very productive and resistant to downy mildew.

Picolino F1 A mini-cucumber variety perfect for the greenhouse. Fruits are 10 to 12 cm, very crisp, with no bitterness. Ideal if you have little space as the plants are compact.

Long White An original variety with ivory white fruits, very crisp and without bitterness. It will be a surprising addition to your salads and on your tables!

🥒 "Lebanese" and "Persian" cucumbers

Shorter (15-18 cm), with thin and edible skin, these cucumbers are highly appreciated for their delicate taste and low seed content. They grow very well in a greenhouse and produce abundantly.

🥒 "Gherkin" cucumbers

If you like homemade pickles, grow varieties like Vert Petit de Paris or Calypso in a greenhouse. You will get a production of small cucumbers perfect for canning, much earlier than in open ground.

📊 Comparison table

🌿 Sowing Cucumbers in a Greenhouse

🗓️ Sowing calendar

Cucumbers are sensitive plants that do not tolerate frost or cold. In a greenhouse, you can anticipate outdoor sowing:

• Early March: sow in heated pots (germination temperature: 25°C minimum)
• Mid-March to April: sow in pots in an unheated greenhouse
• May-June: direct sowing in the greenhouse
• July: sowing for autumn production

🌱 Sowing technique

Required materials:

• Pots 8-10 cm in diameter
• Seed starting mix
• Cucumber seeds
• Plastic film or cloche

Steps:

1. Fill the pots with slightly moistened seed starting mix
2. Place 1 to 2 seeds per pot, 1-2 cm deep (cucumbers do not tolerate transplanting, one pot = one final plant)
3. Cover with a thin layer of seed starting mix and gently tamp down
4. Water gently with a sprayer
5. Cover with plastic film or place under a cloche to maintain warmth and humidity
6. Place in the warmest area of your greenhouse (or on a radiator): germination requires a minimum of 25°C
7. Germination occurs in 3 to 7 days at the correct temperature

💡 Tip: soak your cucumber seeds in lukewarm water for 12 hours before sowing. This softens the seed coat and accelerates germination by 1 to 2 days.

🌱 Caring for young plants

• Remove the plastic film as soon as the first leaves appear
• Water very sparingly to avoid damping-off (a fungus that kills young plants)
• Maintain a minimum temperature of 20-22°C
• If two plants have germinated in the same pot, remove the less vigorous one by cutting it with scissors (do not pull it out to avoid damaging the roots)

🏡 Planting and Trellising

📐 Spacing and arrangement

Cucumbers are voluminous plants that need space. Plan for:

• 1 plant every 50 to 60 cm in the row
• 80 cm to 1 m between rows
• In a 6 m² greenhouse, you can therefore comfortably plant 3 to 4 plants

To maximize your space, use wooden raised garden beds or galvanized steel raised beds, which allow for better drainage and easier work.

🔧 Trellising: essential!

Cucumbers are climbing plants that absolutely need support. In a greenhouse, vertical trellising is the ideal solution because it allows for:

• Maximizing available space
• Improving ventilation between plants (reducing diseases)
• Facilitating harvesting
• Obtaining straighter and more presentable fruits

How to trellis cucumbers in a greenhouse?

1. Install a strong wire or string stretched horizontally at 2 m height under the greenhouse structure
2. For each plant, tie a string to this horizontal wire and stretch it down to the base of the plant
3. Gently wrap the main stem of the cucumber around the string as it grows (about 1 turn per week)
4. The cucumber's tendrils will naturally cling to the string

Our AtmoSerre greenhouses are equipped with solid structures perfectly suited for this type of vertical trellising. An automatic opening system is also very useful for regulating temperature without having to be constantly present.

💧 Watering and Fertilization

💧 Watering cucumbers in a greenhouse

Cucumbers are very thirsty plants; a cucumber fruit is composed of 96% water! Insufficient or irregular watering results in bitter, deformed, or hollow fruits.

Frequency:

• In summer: water every day or every other day
• In spring and autumn: every 2-3 days
• Never let the substrate dry out completely

Method:

• Water at the base of the plant, never on the leaves (risk of fungal diseases)
• Water preferably in the morning so that the leaves dry quickly
• Use room temperature water (cold water shocks the roots)

For hassle-free cucumber cultivation, an automatic irrigation system is an investment that changes lives. It maintains constant humidity and frees you from daily watering constraints.

🌿 Fertilization

Cucumbers are one of the most demanding vegetables in terms of nutrients. Without regular inputs, production quickly collapses.

Before planting: Generously incorporate compost (5 kg/m²) into the substrate. If using containers, prepare a mixture of 60% potting soil + 30% compost + 10% sand.

During growth:

• Vegetative phase (before flowering): fertilizer rich in nitrogen (N) every 2 weeks
• Flowering-fruiting phase: fertilizer rich in potassium (K) and phosphorus (P) every week
• Calcium deficiency causes blossom end rot (blackening fruit tips): water with hard water or add agricultural lime to the substrate

In organic farming:

• Diluted nettle manure (1:10) for nitrogen
• Diluted comfrey manure (1:15) for potassium
• Crushed horn for a slow and long-lasting supply

✂️ Pruning and Training Cucumbers in a Greenhouse

Pruning is the most important and most misunderstood operation in greenhouse cucumber cultivation. Without pruning, your plants quickly become exhausted and production drops.

🌿 Single-stem pruning (recommended in a greenhouse)

This is the most suitable method for greenhouse cultivation as it maximizes production while maintaining good ventilation.

Principle: only one main stem is kept, by removing all lateral stems.

How to do it:

1. Below the 5th node: remove all flowers and lateral shoots. This forces the plant to develop a strong root system before fruiting.
2. From the 5th to the 10th node: leave 1 fruit per node, remove lateral stems.
3. Above the 10th node: leave 2 fruits per node and pinch lateral stems after 2 leaves.
4. When the main stem reaches the training wire: pinch the tip and let 2 stems fall back on each side.

✂️ Pruning frequency

Prune your cucumber once or twice a week throughout the season. Cucumbers grow very quickly (sometimes 10 cm per day in mid-summer) and lateral stems develop rapidly.

Equipment: always use clean and disinfected scissors or secateurs (70% alcohol) to avoid transmitting diseases from one plant to another.

🐛 Cucumber Diseases and Pests in a Greenhouse

🦠 Main diseases

Powdery mildew Most common in greenhouses! It manifests as a white felt on the leaves, first in spots then invading the entire foliage. It is favored by heat and significant temperature fluctuations. Prevention: good ventilation, avoid excess nitrogen. Organic treatment: baking soda (1 tablespoon/liter), wettable sulfur.

Downy mildew Yellow spots on the top of the leaves, grey-purple felt underneath. Develops in humid and cool weather. Prevention: watering at the base, good ventilation. Treatment: Bordeaux mixture (approved for organic farming).

Collar rot The stem rots at ground level, the plant collapses. Caused by excess moisture and poor drainage. Prevention: do not bury too deeply, good drainage, avoid excessive watering.

🐛 Common pests

Aphids They colonize the tips of the stems and the underside of the leaves. In a greenhouse, they multiply very quickly without natural predators. Treatment: diluted black soap, ladybug release, nettle manure.

Whitefly (aleurode) Tiny white insects that live under the leaves and weaken plants by sucking sap. Treatment: yellow sticky traps, black soap, introduction of Encarsia formosa (natural parasitoid).

Red spider mite Common in greenhouses in hot, dry weather. Leaves turn yellow and are covered with fine silky threads. Prevention: maintain good ambient humidity, spray cool water under the leaves.

For a comprehensive approach to pest control, see our article How to protect your plants from diseases in a greenhouse.

🥒 Harvest and Storage

🗓️ When to harvest cucumbers?

Harvest generally occurs 40 to 60 days after sowing, or 10 to 15 days after flowering. Do not let your cucumbers over-ripen on the plant: an oversized cucumber becomes bitter and sends a signal to the plant to stop producing!

Signs of a cucumber ready for harvest:

• Characteristic length of the variety (25-35 cm for long varieties)
• Smooth and firm skin, uniform bright green
• Fruit detaches easily with a slight twist

Harvest frequency: in mid-summer, check your plants every 1 to 2 days. A cucumber can go from ideal size to too large in 48 hours in hot weather!

📦 Storage

• Refrigerator: cucumbers keep for 5 to 7 days in the crisper drawer. Wrap them in paper towels to absorb excess moisture.
• Room temperature: cucumbers store better at 12-15°C than in the intense cold of the refrigerator. Do not store them below 10°C (risk of cold damage).
• Freezing: raw cucumbers do not freeze well (they become soft when thawed). However, cucumber gazpacho freezes perfectly!

❓ FAQ - Your Questions about Growing Cucumbers in a Greenhouse

🥒 Why are my cucumbers bitter? Cucumber bitterness is caused by compounds called cucurbitacins, produced by the plant in response to stress: lack of water, excessively low temperatures, root stress, or lack of nutrients. In a greenhouse, maintain regular watering, stable temperatures between 20 and 28°C, and regularly apply potassium fertilizers. Modern F1 varieties like Tanja are selected to be less bitter.

🌸 Why do my flowers fall off without producing fruit? Two main causes: either pollination is insufficient (cucumbers need pollinating insects or manual pollination), or temperatures are too high (above 32°C, flowers abort). Manually pollinate with a brush and ventilate the greenhouse more. Note that some modern varieties are parthenocarpic (produce fruit without pollination), excellent for the greenhouse.

💧 How much water for a cucumber plant in a greenhouse? A mature cucumber plant in full production can consume 2 to 4 liters of water per day in summer! This is considerable. An automatic drip irrigation system is truly the ideal solution for maintaining a constant water supply without waste.

🌡️ What is the minimum temperature for cucumbers in a greenhouse? Cucumbers suffer below 12°C and can be killed by temperatures below 5°C. In practice, keep your greenhouse above 15°C to ensure continuous growth. If your greenhouse is not heated, wait until night temperatures stabilize above 15°C before planting, which is generally late April in most French regions.

🪴 How many plants in a 6 m² greenhouse? In a 6 m² greenhouse (e.g., 2m x 3m), you can comfortably install 3 to 4 plants of cucumbers, spacing them 50-60 cm apart. With vertical training, you optimize space and can potentially add other ground crops (lettuce, radishes) that appreciate the partial shade of cucumbers.

🔄 Can you get two cucumber harvests per season in a greenhouse? Yes, absolutely! Sow a first batch in March-April for a harvest from June to August, then a second batch in July for a harvest from September to November. Between the two batches, clean the greenhouse and enrich the substrate with compost. To learn more about organizing your greenhouse crops by season, read our article on "What to grow in a greenhouse in spring".

🎯 Conclusion

Growing cucumbers in a greenhouse is one of the most satisfying experiences in sheltered gardening. These generous vegetables, greedy for heat and water, reveal their full potential in the protected environment of a polycarbonate greenhouse: extended season, exceptional yields, superior quality fruits, and far fewer diseases.

The key to success? Choosing the right varieties, mastering single-stem pruning, maintaining regular watering, and taking advantage of appropriate accessories (automatic irrigation, trellising, ventilation) to create optimal conditions.

Ready to start growing cucumbers in a greenhouse? Discover our complete range of polycarbonate garden greenhouses and our greenhouse accessories to ideally equip your growing space. Happy harvesting! 🥒

Sources: INRAE, Interprofessional Technical Centre for Vegetables (CTIFL), Institute of Organic Agriculture and Food (ITAB)