How to grow cucumbers in a greenhouse: tips & advantages

Cucumbers are heat-loving plants and since our UK summers can be unreliable, they are often grown undercover in a greenhouse, both at home and by commercial growers. Find out how to grow cucumbers in a greenhouse and what varieties are best.

Cucumbers (Cucumis sativus) are popular with home-growers and for good reason, as their delicious fruits can be produced for months on end if given the right conditions. With cucumber varieties available for growing outdoors or undercover this article will cover which varieties are suitable for growing in a greenhouse and how to look after them to get the best harvest.

Benefits of growing cucumbers in a greenhouse

With our variable summers, many gardeners choose to grow their cucumbers and tomatoes (Solanum lycopersicum) in a greenhouse. Providing protection from the elements, especially at night, a greenhouse can either be warmed by just the sun or with a supplementary heat source as well. Even an unheated greenhouse can provide precious warmth early on in the season, meaning that cucumber plants can get a 2 to 3 weeks head-start over those planted outside. With additional heating or when using a hotbed, a heated greenhouse can mean cucumbers can be planted even earlier in April, giving months of growing time ahead. However, greenhouses do not just provide the opportunity for earlier plantings but can also extend the season into October as the temperatures begin to dip outside.

Grown vertically in a greenhouse, cucumbers require strings or canes to be tied into as they grow. Not only is growing cucumbers vertically a space-saving method, but it can increase air circulation, which can help prevent disease and make plant care and harvesting the fruits easier. For more information on how to support cucumbers, check out our article on cucumber trellis.

Even with good ventilation, at the height of the summer, greenhouses can become too hot and reach temperatures over 45 °C. To avoid heat stress, sunburn and scorching of the foliage, damping down and shading the cucumber plants with white fabric and even the whole greenhouse, may become necessary.

When grown in a greenhouse or polytunnel, cucumbers are protected from wet weather, which when prolonged, can lead to fungal diseases occurring. However, other cucumber diseases can still be a problem and preventative measures may need to be taken. Pests can also be an issue when growing undercover, especially aphids (Aphidoidea), which can transmit diseases, but can be dealt with by introducing predatory ladybirds (Coccinella septempunctata) and lacewing larvae (Chrysoperla carnea).

Benefits at a glance:

• Longer growing season, due to earlier planting
• Protection against wind, rain and hail
• Space-saving method of growing vertically
• Controllable lighting and temperature by shading and damping down
• Pest control by introducing beneficial insects

Cucumbers do not have to be grown in a greenhouse, as you can successfully grow cucumbers in a raised bed in a sunny and sheltered location. Furthermore, if you are short of growing space you can even grow cucumbers in pots.

The best cucumber varieties for greenhouses

There are many cucumber varieties available to grow that can differ in habit, fruit size, colour and even shape. However, when growing cucumbers in a greenhouse or polytunnel it is worth growing cultivars bred for growing under cover, as they tend to be more prolific and have increased resistance to the most common greenhouse diseases. Here are some of the best cucumber varieties for growing in a greenhouse:

• ‘Burpless Tasty Green F1’: forming long ridge-type fruits up to 30cm in length, this cultivar produces delicious non-bitter fruits that can be harvested around 60 days after sowing.
• ‘Carmen F1’: long, slender and straight fruits up to 40cm in length. Good tolerance against mildew and cold weather. Awarded the RHS Award of Garden Merit.
• ‘Emilie F1’: producing heavy yields of green fruits harvested at 20cm long, this hybrid cultivar has been awarded the RHS Award of Garden Merit. Good disease resistance and superbly flavoured fruits. 
• ‘Mini Munch F1’: heavy yields throughout the season of small fruits harvested when 8cm or longer. Superb flavour and crunchy. Awarded the RHS Award of Garden Merit.
• ‘Socrates F1’: prolific small fruits harvested when 15cm long. Bitter-free taste and an excellent flavour. Awarded the RHS Award of Garden Merit.
• ‘Telegraph Improved’: an improved version of the long-grown heirloom variety. Long and smooth-skinned green fruits up to 30 – 40cm, with a mild and crisp flavour.

Tip: for something unusual, why not try growing the round and yellow ‘Lemon Cucumber’, which is best suited to growing in a greenhouse. Learn more about growing this exotic-looking lemon cucumber in our other article.

How to plant cucumbers in a greenhouse

When grown in a heated greenhouse, cucumber seeds are sown 1 to 2cm deep in individual pots from late February to March. For an unheated greenhouse, it is best to wait until April to begin sowing or alternatively sow indoors. Requiring a temperature of 21 °C to germinate, cucumber seeds are often sown in a propagator to provide this warmth. Find out more on planting cucumbers in our expert article.

Once the young cucumber plants are large enough, they can be potted on or planted out into a trench in a heated greenhouse from late March, or in an unheated greenhouse in late May. Cucumbers prefer a rich and fertile soil and benefit from being fed during the growing season. On planting, a slow-release granular fertiliser can be added to the soil, which will help provide nutrients to encourage strong and healthy growth. Our Plantura Tomato Food contains the necessary nitrogen, potassium and phosphorus to feed the cucumber plants for up to 3 months.

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When growing cucumbers in a greenhouse, it is advisable to provide good air circulation between the plants, which can help prevent disease. Planting at a minimum spacing of 45 to 50cm and if space allows, even more, is recommended. On planting, canes can be inserted into the soil or strings hung from the roof to train the vines upwards. Water thoroughly and keep the soil moist but not wet.

Good companion plants

Companion plants are those that can be grown together for mutual benefit and not compete against each other. Companion plants for cucumbers include those that are light feeders and not part of the same Cucurbitaceae family of plants.

Tomatoes are often grown in a greenhouse, as they prefer similar growing conditions to cucumbers. They can be grown together in a greenhouse, but to stop them competing for each other’s nutrients and space, it is advisable to plant them in separate areas away from each other.

Companion plants for cucumbers can include:

• Borage (Borago officinalis)
• Calendula (Calendula)
• Lettuce (Lactuca sativa)
• Fennel (Foeniculum vulgare) 
• Dill (Anethum graveolens)
• Onion (Allium cepa)
• Garlic (Allium sativum)

Polyculture or mixed culture gardening does not just work outside in the garden, it can be beneficial inside a greenhouse as well.

Cucumber plant care in the greenhouse

Whether grown outdoors or in a greenhouse, cucumbers need to be watered and fed regularly throughout the growing season. Along with providing enough moisture and nutrients, pruning cucumbers can also help encourage a good harvest and prevent disease.

Watering and fertilising cucumber

Cucumbers prefer a moist but well-drained soil, so regular watering is a must to prevent the soil from drying out. Where possible, harvested rainwater is recommended as well as adding a layer of mulch to help conserve moisture and prevent weeds.

In late summer, once the slow-release fertiliser applied at the time of planting has run out, cucumber plants can be fed with a liquid tomato feed. Easier to apply, since it does not have to be worked into the soil around the plant’s roots, the plants can be fed at the same time as watering. A liquid tomato fertiliser such as our Plantura Liquid Tomato Food can be used weekly until the end of the season.

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Cucumber diseases that can affect cucumbers in a greenhouse

Good ventilation is key for growing in any greenhouse, as if unventilated, the humid warm air can quickly become stagnant and encourage fungal diseases. Here are some of the most common cucumber diseases that can affect plants when grown in a greenhouse:

• Cucumber Mosaic Virus
• Cucumber wilt caused by Fusarium and Verticillium fungi
• Cucumber downy mildew (Pseudoperonospora cubensis)
• Cucumber powdery mildew (Podosphaera xanthii and Erysiphe cichoracearum)
• Grey mould (Botrytis cinerea)
• Sclerotinia disease (Phomopsis sclerotioides)

If cared for correctly, cucumber plants can produce impressive yields for weeks on end. Learn more about how to harvest and store your cucumbers.