Growing cucumbers in a greenhouse

Cucumbers are a fantastic summer treat to have, particularly, when you grow them yourself in your own garden. However, if you live in a relatively cool climate it may be tempting to grow the cucumbers in the greenhouse. So how do you do this?

Growing cucumbers in the greenhouse is a little bit trickier than first might think primarily because you need to either hand pollinate the flowers to ensure you get fruit or be very selective about the type of cucumber seeds that you plant as not all cucumber seeds are created equal.

Selecting the variety of cucumber to grow in a greenhouse 

There are three types of cucumber plants which can be put into the garden;

  1.  Monoecious
  2.  Gynoecious
  3.  Parthenocarpic

Monoecious types of cucumbers are varieties that produce both male and female flowers on the plant. This means that this type of cucumber is self pollinating. Typically, most heirloom or open pollinated varieties of seed monoecious. With these types of plants it is common to see the cucumbers produce the male flowers first followed by the female flowers. 

As there is a timing difference of up to weeks between the two types of flowers appearing pollination can be a little bit hit-and-miss if you only have one plant present.

The second type of cucumber is a gynoecious type which is designed to maximise production by only producing female flowers on the plant, which increase number of flowers and is designed to create a higher yield for plant. It is commonly used in commercial applications for this reason.

The third type of cucumber is a parthenocarpic type which produces fruit without the need for pollination. The primary benefit of growing this type is that it is ideally suited for growing in greenhouses because the lack of pollinators will be a problem with the other two types of plants.   However, the downside of growing these types of plants is that they do not produce seeds which means you need to buy seed every single year.

If you choose to grow a monoecious or gynoecious variety, then you will need to hand pollinate the flowers as they appear in the greenhouse, which can be done either by hand-pollinating the flowers or using a hormone spray that is available commercially.

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To hand pollinate the flowers, it is easiest to do it by getting a fine artist’s brush and gently place it into the center of the male flower and rotating the brush gently to pick up the pollen before transferring it to the female flowers. This process ideally needs to be done a couple of times to ensure a high rate of pollination.

When and how to sow cucumber seeds

Cucumber seeds intended to be grown in a greenhouse can be sown slightly earlier and those that are grown outside in the garden. As a result of this we generally recommend that you plant the cucumbers approximately 6 weeks prior to the last frost of the season, as this will ensure that you can take advantage of the full growing season.

When planting cucumber seedlings it is recommended that the cucumber seeds are planted into modular seed trays first to ensure that the amount of space required in the greenhouse is minimised. 

Prepare the seed trays by using a high-quality seed-raising.mix that is designed specifically for the purpose, as using garden soil is not ideal, as you run the risk of introducing pathogens and weeds. 

When filling seed trays, ensure that the mixture is pressed firmly into the individual modules as this will help to create plugs that will be easier to transplant them later on. Place two seeds per cell into the soil mixture at a depth of 1 cm to ensure that you get germination in each and every cell.

When determining the number of seeds to grow it is best to aim for 50 to 100% more seeds than you think you will need. This will make 100% sure that you have all of the seedlings that you will need for growing in the greenhouse.

Once planted in the seed trays the plants will typically take between 7 to 14 days to appear, however, this is greatly dependent on the conditions in which you keep the seed tray,  as colder conditions will reduce the rate of growth.

Overall, the seedling will take around 6 weeks to reach a sufficiently large size enable them to be transplanted into their final position within the greenhouse. During this period it is important that you keep the seedlings moist by watering them regularly to ensure that they developed properly.

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Transplanting seedlings into the greenhouse

Before planting the seedlings in the greenhouse it is important to prepare the soil carefully by adding large amounts of organic matter in the form of compost. Ideally, if you’re using pots to grow your cucumbers in, it is best to use larger pots that are at least 14 to 15 inches in diameter, as this will reduce the chances of the cucumbers drying out to quickly.

Seedlings should ideally be spaced around 30 to 40 cm apart, depending upon the nature of the support structure that you are going to use to support growth. If you are using trellising structures to lift the plants off the ground the seedlings can generally be planted slightly closer together. However, if you are going to allow the cucumber Vine to spraw across the ground then planting further apart is preferred to maximise the yield.

The seedling should ideally be planted at a depth approximately equal to the depth the seedlings were growing in the seed tray which can be done by ensuring that the top of the soil plug is approximately level with the height of the soil in the greenhouse.

Building support structures for growing cucumbers

As mentioned in the previous section cucumbers can be grown on the ground or they can be grown on a support structure. Generally, it is recommended that you use a support structure particularly in a greenhouse primarily because it will reduce the amount of space required for the plants and therefore allow you to fit additional plants in the greenhouse.

The other benefit of growing cucumbers on a trellis system is that it keeps the fruit off the ground and reduces the chance of the fruit rotting before you have an opportunity to pick it. If you choose to grow cucumbers along the ground and are worried about the fruit rotting, you can either place straw underneath the fruit or even cardboard.

When building a support structure for cucumbers it is recommended that you use a single guide string rather than putting in a trellis system as it is simpler to maintain the fruit as it develops and takes up less space.

To do this place of one end of the string underneath the seedling when it is planted into the ground and tie the other end to the top of the greenhouse, creating a single line of support.

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The string can be wound around the plant every week or so without requiring the need to tie in the plant which makes it easier to look after.

Looking after cucumbers as they develop

Cucumbers are relatively easy to look after once the plant becomes established. The main consideration is ensuring that you protect them from slugs and snails and water the plants  regularly to ensure proper development of the fruit.

To protect the plant from slugs and snails is important to sprinkle snail bait around the base the plant when it is put into the ground. It is not safe to assume that because the plant is growing a greenhouse that snails will not be able to get to them. 

The second key thing is to ensure the plant receives adequate water, as the fruit is over 90% water. So a lack of water will result in poor fruit development and also affect the flavour, as plants that do not receive enough water will produce bitter fruit that doesn’t taste very good.

The last critical thing to do is to ensure that if you are growing varieties that do require pollination that you actually do so on a regular basis once flowers start to appear to ensure that you get a decent pollination rate.

Picking cucumbers at the right time

Cucumbers, unlike many other fruit do not start change colour or show indications that they are ripe and ready to pick until the  fruit is already overripe and starting to deteriorate rapidly. The most common sign of this is the yellowing of fruit.

Thankfully, cucumbers are extremely forgiving in terms of when you pick them; they can pretty much be picked at any stage once the fruit has reached a sufficiently large size. It is important to note that it is not necessarily to wait until they reach a size that is similar to what you could buy commercially.

In fact we recommend that you pick them when they are smaller as this reduces the chances of having the fruit deteriorate. Additionally, picking fruit will encourage the ongoing formation of flowers which will increase the overall yield that you can get from the plant.

Cucumbers will typically continue to produce fruit throughout the summer months until the last frost of the season when the weather will become too cold. 

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