How To Grow Tomatoes In a Greenhouse | Ultimate Guide

How To Grow Tomatoes In a Greenhouse | Ultimate Guide

Greenhouses offer the perfect micro-climate to grow the tastiest tomatoes.  By growing just a couple of plants, you and your family will be rewarded with plenty of fruits during the summer and autumn.

By adding a handful more tomato plants to your greenhouse will have enough tomatoes to start creating your own preserves to enjoy for the rest of year.

Read our ultimate guide on how to grow tomatoes in your greenhouse.

SowTransplantSpacePlant careHarvestSeason
Late Jan - mid MarSow in seed trays. Transplant seedling to small pots.45cmTie in stem and pinch-out side shoots every week.20-22 weeksJul - Nov
  1. Fill a seed tray with potting compost.
  2. Water the tray with a watering can with a fine rose.  Watering at this stage will mean that you won’t run the risk of washing the seeds to the corners of the tray later.
  3. Evenly sprinkle the seeds across the tray.  
  4. Lightly sprinkle a thin layer of potting compost over the seed tray (a few millimetres is fine).  This task will be easier if you use a garden sieve.
  5. Gently press down the top of the compost to ensure that the seeds are making good contact with the soil.
  6. Lightly water the tray once again.

“Prick out” i.e. remove any seedlings that might grow so close to each other that their leaves are touching.  If their leaves touch for any length of time, the seedlings growth will be hindered and will ultimately weaken the plant.  Remove the weaker of the two tomato seedlings.

The optimum greenhouse temperature for germination is 21C.  Germination should occur between 7 – 14 days.

Don’t let the tray dry out, however don’t water the tray so that it is constantly sodden as this will encourage “damping-off” disease with will cause your seedlings to rot.

Transplanting Tomatoes

Once your tomato seedlings have reached the height of 7.5cm to 10cm, it’s time to transplant the seedlings.

  1. Fill some 9cm pots with a general purpose compost.
  2. Use a pen or pencil to create a small hole in compost ready for the new seedling.
  3. Gently remove a seedling from the seed tray taking care not to damage the root of the tomato plant. Use the pen or pencil to tease out the seedling.
  4. Place the seedling into the hole of the 9cm pot so that the seedling leaf is just above the soil surface.
  5. Water the pot thoroughly.

Planting Tomatoes in a Greenhouse

Allow the seedlings to grow in their pots until the roots of the plant have established. 

You will know when the tomato plant has established strong roots as they begin to turn slightly yellow when they have used the majority of the nutrients found in the compost.

Tomato plants generally need planting up in May.

Tomatoes can be planted directly into beds or containers.

Tomatoes can also be grown in grow-bags however if you do this, don’t add any more than one plant per grow-bag so that the plants aren’t over-competing for nutrients.

Tomatoes are very hungry plants so they will require a lot of nutrients to be present in the soil or compost that they are growing in.  If you are planting into a bed within the greenhouse, ensure that you have added some additional organic material such as garden compost or manure.  

You might want to add approximately a wheelbarrow full for each 3 – 4 metre squared of bed.  If the bed looks like it is overflowing, don’t worry as the compost/manure will rot down further over the course of the season.

If you are growing in containers, it is important to make sure that you aren’t using old compost.  Fill the container full of a general purpose compost instead so that the plant has plenty of nutrients to feed on.

Tomatoes can grow to a height of 2 metres, so remember to locate them in a part of your greenhouse that will allow them to freely grow to their natural height.  If the plant reaches the top of your greenhouse, don’t worry about trimming the top of it to prevent it touching the glass.  It’s more likely for mold to build-up if the leaves touch the glass.

Spacing tomatoes

Tomato plants should be given at least 45cm – 50cm of space between each plant.  Maintaining these spacing’s will increase the yield of the plant as well as it’s health generally.

How To Care For Your Tomato Plants in a Greenhouse

In order to care for your tomato plants properly, there are four main tasks for the greenhouse gardener.  They are:

  1. Pinching side-shoots.
  2. Support the stem.
  3. Watering.
  4. Feeding.
  5. Leaf removal.
  6. Stopping/Cutting stem.

How to pinch out tomato side-shoots

It is important to remove the side shoots growing from the main stem.  These can be identified as a shoot that is growing between the main vertical stem and the horizontal leaf stem.  The side-shoot will be at an approximate 45 degree angle to these. Pinching out side shoots on a tomato

When the side-shoot is young it can literally be pinched off the plant by squeezing it with your fingers.

Older stems will likely need to be cut off with a sharp knife or garden secateurs.

Remove the side-shoot even if it is bearing flowers.

How to support the tomato stem

Tomato plants grow tall quickly and it is essential to ensure that the main stem/vine is supported.  This is usually achieved by training the stem up a support.  Some growers wrap the stem around a support such as string or a bamboo cane, that runs the length of the plant.  Other growers tie the stem onto the support. 

If you use the tie method, remember to leave enough slack on the knot to allow the stem to swell naturally as it grows through the rest of the growing season.  Failure to do so will cause the string to cut into the plant and make it susceptible to disease.

How to water your tomatoes

Water your tomatoes regularly.  The frequency of watering will depend on many factors including how large the growing space is for the roots of the tomato plant as well as how hot the greenhouse is.  

The key is not to allow the soil to dry out.  The fruits of the tomato plant require a constant supply of water otherwise they are susceptible to blossom end rot as well as splitting of the fruit itself.

How to feed your tomatoes

Once your plants are established and have begun forming trusses (the stems that the fruit grow on), begin to feed your tomatoes with a tomato fertilizer.  There are plenty of these specific fertilizers on the market but all of them perform the same task.

General purpose fertilizers aren’t recommended as it will cause a growth in leaf production whereas what you want is for the plant to focus it’s energy on the tomato fruits.

Tomato fertilizers are higher in their potassium content rather than nitrogen.  The higher amounts of potassium create larger, healthier tomato fruits.

As always, follow the instructions on the container so that you apply the correct measure of tomato feed to the plant.

As a general rule of thumb, you should apply the tomato feed on alternate waterings, so that you don’t over-feed your tomatoes.

It’s also possible to create your own homemade tomato fertilizer by using comfrey plants.  It’s a little bit more work and quite a lot smellier!

Leaf removal

As the growing season progresses you will notice that some leaves begin to turn yellow.  Remove these leafs with a pair of sharp secateurs.

This will improve the air circulation around the plant and prevent mold and diseases.  It will also improve the light levels reaching the fruits to help them ripen and turn a beautiful red color.

That said, don’t remove too many leaves as the tomato plant will not be able to function correctly.

Stopping/Cutting off stem

In the late summer, it is wise to have the main stem “stopped” i.e. cut off near the top.  This is to prevent the plant from creating new fruit trusses that will have little to no opportunity of ripening the tomato fruits in time for harvesting.

This will waste the plants energy.  Instead, allow the last four main trusses to stay on the plant and then cut off the top of the stem.

This will refocus the plants energy into developing the remaining tomato fruits.

When To Harvest Tomatoes

The time for harvest depends on a number of factors including the variety used as well as when they were planted.

Generally, you should be able to harvest tomatoes between July to November.

Tomatoes are fruit that can ripen further after they have been picked.  This is due to a gas that they emit called ethylene.  The tomato will emit more of the ethylene gas as it begins to go through the ripening process.

Ethylene reduces the amount of chlorophyll which gives the tomato it’s initial green color and increases the amount of carotenoids which give the fruit it’s orange and red color.

When you notice that the green is beginning to change to yellow and orange, it’s time to harvest the tomato.

To do this hold the stem of the truss in one hand to support it, and in the other hand gently pull the tomato fruit away.

The tomato will ripen itself further once it has been picked.

Don’t be tempted to leave red fruits on the tomato plant as they are likely to split and burst before you harvest them.

Store the plants at room temperature.  If you wish to slow down the ripening process, reduce their temperature by placing them in the fridge.  If you would like to decrease the time it takes for them to ripen, place them in a paper bag in a slightly warmer environment.

Common Tomato Problems in a Greenhouse

Fruit splitting

Tomato splitting
Courtesy of Quinn Dombrowski- Creative Commons BY-SA 2.0

Fruit splitting is where the flesh of the tomato splits and bursts.  

This is usually caused by irregular watering patterns.  The fruit may receive a sudden burst of water after being relatively dry for a sustained period of a few days. 

Try to ensure that the roots of your tomato plants don’t dry out.

Greenhouse whitefly

Greenhouse whitefly is an aphid that feeds of the sap of some indoor plants such as tomatoes.  You will know if your tomatoes are suffering from a whitefly infestation as you will notice a sticky substance known as honeydew forming on the lower leaves of the plant.  Some leaves will begin to turn mouldy.  You might also notice small white flies when you disturb the leaves.

The best method of controlling whitefly is to introduce a biological control called Encarsia formosa.  These are a tiny parasitic wasp that kills the whitefly.  The eggs of these wasps can be purchased online.  The timing of deploying this control is critical.  The most effective time of introducing encarsia formosa is just as the first whiteflies are appearing.

Whiteflies are spread on the surface of other plants.  Therefore a greenhouse full of plants that have been grown from seed are less likely to be affected by greenhouse whitefly.

Blossom end rot

Blossom end rot

Blossom end rot is a condition that causes the base of the tomato fruit to turn black.  This is caused by a calcium deficiency.

Ensuring that the tomato plants are regularly watered will prevent blossom end rot from occurring.

Grey mold

Grey mold can easily be spotted as it leaves large white/grey patch of mold on the leaves, stems and tomato fruits of the plant. 

This is usually caused by poor ventilation within the greenhouse, especially during cold damp periods of the growing season.

To prevent grey mold from establishing itself, try to improve ventilation by using louvre vents, ceiling windows as well removing any dying or yellowing tomato leaves.

If grey mold has already taken hold of a plant, remove any affected leaves or fruits.  Then create a milk and water spray by mixing one part milk to 5 parts water.  Spray this mixture onto the affected tomato plants for several consecutive days, until the mold stops spreading.

Best Varieties of Tomatoes

There are two kinds of tomato plants.

  1. Cordon/Indeterminate tomatoes
  2. Bush/determinate tomatoes.

Cordon tomato plants grow up a single main stem/vine with tomatoes that grow on trusses. 

Bush tomatoes grow several stems that end with a truss of fruits.  These are often grown in hanging baskets.

Both cordon and bush tomatoes come in an additional four further types.  These are:

Cherry tomatoes – these are the smallest, sweetest varieties.

Medium tomatoes – these are often referred to as salad tomatoes and have a good balance between size and taste.

Plum tomatoes – these are oval in shape and are often fleshier than medium or cherry tomatoes.  They are ideal for making cooking sauces.

Beef tomatoes – these are the largest variety.  They have the meatiest taste.  They are often sliced and added to sandwiches or grilled and added to burgers.  Yummy!

Best Cherry tomatoes

Gardeners Delight – an excellent cropper with a good all round taste.  This is a cordon tomato.

Tumbling Tom Red –  produces an abundance of sweet, juicy tomatoes.  This is a bush variety which is ideal for hanging baskets.  Suspend these from your greenhouse roof!

Best Medium tomatoes

Tigrella – it gets its name from it’s stripy flesh.  It’s interesting skin is matched by an equally interesting taste.  Quite unique.  This is a cordon tomato.

The amateur –  This is a good bush variety that is classed as a medium/salad tomato.

Best Plum tomatoes

Astro Ibrido – a great tasting plum tomato of a cordon variety.  Ideal for cooking sauces.

Rio Grande – this is an excellent cropper with a wonderfully deep taste.  A bush tomato.

Best Beef tomatoes

Marmanade – produces large tomatoes with a strong flavour.  In fact, it’s that good, the Royal Horticultural Society gave it an Award of Garden Merit.  A cordon variety.

Country taste – a meaty tomato that should set large ripe tomatoes quite early in the season.  A cordon variety.

How Many Tomato Plants Should I Grow in a Greenhouse?

Five tomato plants should be enough to provide a sufficient amount of tomatoes for your family between July and November.

If you want to make some tomato preserves, jams and chutneys, then grow an additional ten plants.

Tomato Greenhouse Growers Kit List

We have selected a kit list of key equipment and tools to get you growing tasty tomatoes in your greenhouse in no time.  The tomato varieties listed are the ones selected in our Best Varieties section above.  All of the items below are links to products available on Amazon giving you the added convenience of home delivery.

Tools

Seed propagation tray

Garden sieve

Seedling sower dibber

Greenhouse grow bags

 

Tomato Seeds

Gardeners delight

Tumbling Tom Red

Tigrella

The Amateur

Rio Grande

Marmande

Country taste

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