A Greenhouse Grower’s Guide to Cucumbers
How to Grow Cucumbers in a Greenhouse: The Ultimate UK Guide
Key Takeaways: Greenhouse Cucumber Essentials
- ✅ Temperature Matters: Seeds need a constant 21°C to germinate. In unheated greenhouses, wait until late April.
- ✅ Vertical Training: Use string or canes to train vines upward to maximize space in smaller structures.
- ✅ Watering Rule: Water little and often, focusing on the soil—never the stem—to avoid rot. Inconsistent watering is the #1 cause of bitter fruit.
- ✅ The "Male Flower" Mistake: Unless you are growing a ridge variety, remove all male flowers immediately to prevent bitter crops.
- ✅ Succession Sowing: Don't stop in May! Sow a second batch in July for harvests that last until October.
Growing your own cucumbers is one of the most rewarding experiences for a UK gardener. The difference between a store-bought cucumber—often wrapped in plastic and slightly rubbery—and one picked fresh from your own greenhouse is night and day. Home-grown cucumbers are crisp, intensely flavourful, and can be harvested at the perfect size for your needs, whether that's tiny snacking gherkins or impressive long slicers.
In our 16+ years helping UK gardeners, we’ve found that while tomatoes often get the glory, cucumbers are actually more productive per square foot if treated right. However, they are a bit more temperamental. They hate the cold, they resent having their roots disturbed, and they can turn bitter if stressed.
If you are currently looking at greenhouses for sale or planning your winter maintenance, you are already ahead of the game. Successful cucumber growing starts with choosing the right variety and preparing your environment before a single seed is sown.
Getting Started: Choosing Your Varieties
The first step to success is understanding that not all cucumbers are the same. In the UK, we generally divide them into two categories: Greenhouse (All-Female) and Ridge (Outdoor). For a greenhouse environment, you almost always want the former.
Greenhouse vs. Ridge Cucumbers
| Feature | Greenhouse (All-Female F1) | Ridge (Outdoor) |
|---|---|---|
| Skin Texture | Smooth, thin skin | Rough, prickly, thick skin |
| Pollination | Do NOT require pollination | Require insect pollination |
| Male Flowers | Remove immediately (causes bitterness) | Keep them (needed for fruit) |
| Yield | Higher yield, faster growth | Lower yield, hardier |
| Best Varieties | Bella F1, Carmen F1, Passandra | Marketmore, Burpless Tasty Green |
Matt's Top Tip: If you are a beginner, stick to an "All-Female" F1 Hybrid variety like Bella. These have been bred to produce only female flowers, saving you the hassle of identifying and removing male blooms.
Sowing Seeds: Timing is Everything
One of the biggest mistakes we see is sowing too early. Cucumbers are tropical plants; they sulk in cold compost.
The "Heat vs. Light" Balance
According to the Royal Horticultural Society (RHS), cucumber seeds require a minimum temperature of 21°C (70°F) to germinate successfully.
- Heated Greenhouse / Propagator: You can sow as early as mid-February to March. However, you must be able to maintain that heat 24/7.
- Unheated Greenhouse: Wait until mid-April. If you sow earlier without heat, the seeds will likely rot in the cold compost.
How to Sow
- Pot Size: Use small 7-9cm pots filled with high-quality seed compost.
- Sowing Depth: Plant seeds on their side (not flat) about 1-2cm deep. This prevents water from sitting on the flat seed coat and causing rot.
- Germination: Place in a propagator or on a warm, sunny windowsill. Keep moist but not sodden.
Troubleshooting "Leggy" Seedlings:
If you started early and your seedlings have shot up with long, thin, wobbly stems (leggy) due to low light, don't panic. When you transplant them into their final pots, bury the stem deeper than it was before—right up to the first set of leaves (cotyledons). The buried stem will sprout new roots, anchoring the plant and making it stronger.
📅 Pro Tip: The "Succession" Secret
Most growers stop sowing in May, but for a harvest that lasts until late October, sow a second batch of seeds in early July. These plants will take over just as your first batch starts to tire and succumb to mildew in late summer, giving you fresh cucumbers well into autumn.
Preparing Your Greenhouse Environment
Before you move your precious seedlings into their final home, your greenhouse needs to be ready. Cucumbers thrive in high humidity and consistent warmth—conditions that can be tricky to manage in a standard 8x6 Greenhouse.
Soil vs. Containers
While you can grow cucumbers in greenhouse borders, we highly recommend growing them in containers or grow bags.
- Disease Control: It avoids soil-borne pests like nematodes.
- Temperature: Pots warm up faster than the ground in spring.
- Drainage: It is easier to ensure the "moist but not wet" soil conditions cucumbers love.
Use pots that are at least 30cm (12 inches) wide. If using grow bags, plant two cucumbers per bag, not three, to ensure they have enough root space.
Positioning
Cucumbers need light, but scorching direct mid-summer sun can damage the leaves. Position them where they get plenty of light but perhaps are shielded from the fiercest midday rays. If you are unsure about the best layout for your structure, our Greenhouse Buyers Guide has excellent tips on positioning for light and shade.
Also, if you are growing tomatoes in the same space, read our guide on When to plant Tomatoes in an unheated Greenhouse to coordinate your planting schedule effectively.
The "Male Flower" Rule (Crucial!)
This is the content gap that trips up many beginners. Most modern greenhouse cucumbers are "parthenocarpic," meaning they produce fruit without pollination.
If a bee pollinates a greenhouse cucumber flower, the resulting fruit will be bitter and filled with hard seeds. To prevent this, you must identify and remove male flowers unless you are growing an all-female F1 variety.
How to Spot the Difference
- Female Flower: Has a tiny, miniature cucumber visible behind the yellow bloom. KEEP THIS.
- Male Flower: Has a thin, plain stalk behind the bloom. REMOVE THIS.
Action: Check your plants weekly. Pinch off male flowers as soon as you see them. If you are growing F1 All-Female varieties, you can skip this step, but keep an eye out just in case a rogue male flower appears due to plant stress.
Vertical Training & Pruning
To get the most out of Small Greenhouses, vertical training is non-negotiable. It keeps fruit straight, clean, and saves massive amounts of floor space.
The Single Stem Method
- Support: Run a vertical string from the roof of your greenhouse down to the base of the plant. Loosely tie it to the base of the main stem.
- Twist: As the plant grows, gently twist the main stem around the string.
- Stop: Once the main stem reaches the roof, pinch out the growing tip.
- Side Shoots: Pinch out the growing tip of side shoots two leaves beyond a fruit. This focuses the plant's energy on filling the fruit rather than growing more leaves.
This method improves airflow significantly, which is your best defence against fungal diseases in humid Aluminium Greenhouses.
Care & Maintenance: Water and Feed
Watering: The Golden Rules
Cucumbers are thirsty plants, but they are prone to stem rot.
- Target: Water the soil, never the base of the stem. A good trick is to sink a small plastic pot into the soil next to the plant and water into that.
- Consistency: Inconsistent watering causes moisture stress, which leads to bitter fruit. Try to keep the soil evenly moist.
- Humidity: They love humid air. On hot days, "damp down" the greenhouse floor by watering the path.
Feeding
Start feeding with a high-potash liquid fertiliser (tomato feed is fine) once the first fruits begin to swell. Feed once every 10-14 days.
Companion Planting for Cucumbers
Competitors often overlook what you can grow alongside cucumbers. Smart companion planting can help deter pests and improve growth. If you are new to this concept, our What To Grow In A Greenhouse Beginners Guide covers the basics of mixed planting.
| Companion Plant | Benefit to Cucumber | Planting Tip |
|---|---|---|
| Dill | Attracts hoverflies (which eat aphids) and improves growth rate. | Plant close to cucumbers but avoid letting it flower too early. |
| Nasturtiums | Acts as a "trap crop." Aphids love them and will attack them instead of your vines. | Plant near the greenhouse door or vents. |
| Marigolds | Strong scent confuses and deters Whitefly. | Pot them up and place them on the staging between your cucumber pots. |
| ❌ Potatoes | AVOID. Releases substances that inhibit cucumber growth. | Keep in a separate area of the garden. |
| ❌ Sage | AVOID. Aromatic oils can stunt cucumber vine development. | Keep in the herb garden, not the greenhouse. |
Troubleshooting Common Problems
Even in the most beautiful Wooden Greenhouse, issues can arise.
| Problem | Symptom | Cause | Solution |
|---|---|---|---|
| Powdery Mildew | White, dusty coating on leaves | Poor airflow / Dry roots | Remove affected leaves, increase ventilation, keep roots moist. |
| Red Spider Mite | Yellow mottling on leaves, fine webbing | Hot, dry air | Increase humidity by misting daily. Introduce biological controls. |
| Bitter Fruit | Unpleasant taste | Water stress / Pollination | Water more consistently. Remove male flowers immediately. |
| Yellowing Leaves | Lower leaves turning yellow | Magnesium deficiency / Age | Feed with Epsom salts (foliar spray) or simply remove old leaves. |
Harvesting: The Size Guide
Harvesting is the best part! But timing is key. Letting cucumbers grow huge is a rookie mistake; they become bitter and the plant thinks its job is done, stopping production of new fruit.
Harvest at these specific lengths:
- Standard Greenhouse (e.g., Bella): 25–30cm (Supermarket size).
- Mini / Snack (e.g., Passandra, Mini Munch): 10–15cm (Do not let them get larger!).
- Ridge / Outdoor: 15–20cm (Before they turn yellow).
How to Harvest:
Always use sharp secateurs or a knife. Pulling the fruit can damage the brittle vines. Store them in the fridge crisper drawer where they should last 1-2 weeks.
Summary and Next Steps
Growing cucumbers in a greenhouse is a fantastic way to utilize your space. By choosing the right variety, managing humidity, and keeping on top of your watering, you can enjoy a bumper harvest from June right through to October.
Once your cucumber season is winding down in late autumn, you might be wondering what to do with the space. Check out our guide on how to grow garlic in a greenhouse for a perfect winter crop that makes use of the empty pots.
Ready for dessert?
If you have mastered cucumbers, why not try something sweet? Read our guide on how to grow strawberries in a greenhouse for your next challenge.
Frequently Asked Questions
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When should I plant cucumber seeds in a greenhouse in the UK?
For heated greenhouses, sow in late February to March. For unheated greenhouses, wait until mid-to-late April. Seeds need 21°C to germinate, so early sowing without heat usually results in failure.
-
Do you pinch out side shoots on greenhouse cucumbers?
Yes, absolutely. Pinch out side shoots two leaves beyond any fruit. This "stopping" technique ensures the plant puts energy into the cucumber rather than unnecessary foliage, and keeps the greenhouse tidy.
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What is the difference between male and female cucumber flowers?
Female flowers have a miniature cucumber visible behind the petals; male flowers just have a thin stalk. Unless you have an "All-Female" variety, you must remove male flowers to prevent bitter fruit.
-
Why are my greenhouse cucumbers bitter?
Bitterness is caused by high levels of cucurbitacin, usually triggered by stress (irregular watering or heat fluctuations) or accidental pollination by male flowers. Keep soil consistently moist to prevent this.
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What can I plant next to cucumbers in a greenhouse?
Dill, marigolds, and nasturtiums are excellent companions. They help deter pests like whitefly and aphids. Avoid planting potatoes or strong aromatic herbs like sage nearby.
