How To Grow Spinach In A Greenhouse

How To Grow Spinach In a Greenhouse | Ultimate Guide

Growing spinach in your greenhouse is incredibly easy.  What’s more, spinach is an extremely healthy crop to eat as it’s tasty green leaves contain high levels of iron, calcium and vitamin K.

There are two varieties of spinach, annual and perpetual.  Annual is the most common variety that you find in supermarkets.  This guide focuses on the growing and harvesting of the annual variety.

We’ve put together the most comprehensive guide on the Internet to show you how easy it is to grow this super tasty plant.  So read on and we’ll have you growing spinach in no time!

SowTransplantSpacePlant careHarvestSeason
Feb-Apr
Aug-Oct
Sow in seed trays. Transplant seedling to small pots.15cm – 20cmFull sun. Water regularly. Feed every 7 weeks.50 days or regularly as a “cut and come again” crop.Mar-May
Sept-Mar

Before we discuss how to sow spinach, it’s best to start by discussing how much spinach should be sowed at any point.  

Spinach is best eaten when it is young and fresh.  Leaving it to develop to a mature plant will cause it to develop a bitter taste and the plant will be more likely to bolt (grow a flower spike) and become inedible.

What’s more, with a greenhouse, you can grow spinach all year long.

With this in mind, it’s best to sow your spinach in smaller numbers but in regular intervals.  This is known as successional sowing.  This will give you a constant supply of fresh tasty spinach throughout the year. 

So how do you sow spinach?  Follow these simple steps.

  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. Make a series of holes in the compost, no deeper than 1.5cm and 5cm apart.
  4. Pop one seed into the hole.  How many you sow is up to you however sow a few more than what you need just in case some seeds don’t germinate into seedlings.
  5. 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.
  6. Gently press down the top of the compost to ensure that the seeds are making good contact with the soil.
  7. Lightly water the tray once again.
  8. Place the tray in a warm, sunny spot and try to maintain a temperature of 65F or 18C.  A heated propagator or mat might make this easier.

Keep the compost moist while the seeds germinate.  Your seedlings will germinate within 7 to 10 days.

Consider sowing spinach every 3 to 4 weeks for successional sowing.

Buy new spinach seeds each year and don’t be tempted to use an older packet.  This is because spinach seeds don’t age well.  Older seeds are less likely to germinate so it’s not worth trying to save a few dollars by using older seed.  Just buy a new packet and you will be much happier with the outcome!

Transplanting spinach

Transplant seedlings 3 to 4 weeks after germination.  These can be transplanted into larger containers or pots as well as directly into a greenhouse bed.

  1. Fill some 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 spinach plant.
  4. Use the pen or pencil to tease out the seedling.
  5. Place the seedling into the hole of the pot so that the seedling leaf is just above the soil surface.
  6. Water the pot thoroughly.

Soil & Location in a Greenhouse

Spinach is a very hungry plant.  It will require a nutrient rich, fertile soil to grow well in.

If you are growing spinach in a greenhouse bed, consider adding a large amount of well-rotted compost or manure to the bed first.

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 don’t have any manure or compost, add in a general slow release fertilizer to the soil.

If you are growing spinach in pots, fill the pot with a general garden compost available at any retailer.  Do not reuse old compost as most of the nutrients necessary to grow healthy plants will have been removed.

Ideally spinach likes to be grown in a soil with a Ph level between 6.5 and 7.5.

You should place the spinach in a part of the greenhouse that is exposed to as much sun as possible as they prefer a full sun position.  They can also tolerate partial shade too.  

As spinach doesn’t like to get too hot, it might be worth placing them in the cooler part of the greenhouse such as near a vent or door where there is better ventilation.

It probably won’t be possible to grow spinach during the hottest months of the year in your greenhouse.  If possible, move the containers to outside of the greenhouse during June and July.

Spacing spinach

Spinach requires a particular amount of space to grow a healthy plant, however depending on which variety you are growing dictates on how much spacing you need to give to each plant.

For baby leaf spinach, you should ensure that the plant is placed no closer than 8cm from any other plant.

For normal annual spinach, you can increase the spacing to 15cm to 20 cm.

Please note, if you are harvesting the crops quickly, in a “cut and come again” scenario, you can space these plants much closer together.

How To Care For Your spinach Plants in a Greenhouse

Spinach is a super easy crop to grow.  That said, that are still a few things that you will need to keep in mind when growing this tasty crop.  They are:

  1. Temperature control.
  2. Watering.
  3. Feeding.

Temperature control

Spinach doesn’t like excessive heat.  If your greenhouse is regularly exceeding 80F or 26C, fit some shading to your greenhouse to reduce the temperature.  You might also want to increase the cooler airflow by opening the greenhouse doors and windows.

If spinach gets too hot, it will bolt (start to grow a flower spike).  It produces the flower spike as the plant thinks that it might die due to the heat and therefore it decides to produce seed so that it can live on by growing a new plant!  Although the flower spike might look pretty, the taste of the plants leaves’ will suffer by turning bitter. 

Watering

Keep your spinach well watered.  How much water you give it will always depend on your particular greenhouse environment however, just remember to keep the soil moist but don’t constantly saturate the soil.  

Don’t let the spinach roots dry out as this will also cause the plant to bolt.

Feeding

Spinach is a hungry plant.  Feed it a general fertilizer after it has reached 45 days old.  Keep it fertilized every month following that too.  Always remember to follow the instructions on your fertilizer bottle/packet.

When to harvest spinach

Spinach harvestedAs a general rule of thumb, harvest the plant when the leaves are 2 to 3 inches tall.

When to harvest spinach depends on the variety of spinach that you are growing.  Baby leaf spinach should be harvested when the plant is 40 days old whereas normal annual spinach should be harvested after 50 days since germination.

If you are growing spinach as a “cut and come again” crop, you can probably harvest this after 40 days.

Spinach can tolerate a light frost.  Therefore it is possible to grow this crop in a cold greenhouse that is in a sunny position during the winter months.  This is great as it extends your overall greenhouse growing season.

Common spinach Problems in a Greenhouse

Spinach is an easy plant to grow however it can suffer from the following problems on some occassions.

Greenhouse whitefly

Whitefly is an aphid that feeds of the sap of some indoor plants such as spinach. You will know if your spinach is 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 moldy. 

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.

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 aphid destroying wasps can be purchased online. The timing of deploying this control is critical. The best time to introduce encarsia formosa is just as the first whiteflies are appearing on the plants.

Slugs

Slugs like the taste of young fresh spinach leaves just as much as we humans do!  Who could blame them?

One way of safely controlling slugs within a greenhouse without adding potentially harmful chemicals to your soil is to use nematodes.  Nematodes a microscopic organism that kills slugs by the slug ingesting them through the soil.  The nematode then kills the slug by poisoning it from inside. 

Nematodes can be purchased from online retailers.  It’s best deploy these earlier in the growing season just as the slugs begin to become a nuisance to the greenhouse gardener.

Downy mildew

Downy mildew is a fungal disease that kills plant leaves.  It is spread through the air via fungal spores.  

The best ways of preventing downy mildew is by adjusting your watering regime.

Water your greenhouse plants during the morning rather than in the evening.  Watering in the evening will increase the humidity of the environment just as temperatures begin to drop in the greenhouse.  This is the perfect growing conditions for fungal diseases such as downy mildew.

Also, if possible, water the soil rather than the leaves of the plant.

If a plant becomes severely affected by downy mildew, remove it from the greenhouse altogether.  Do not add this to your compost heap as you might spread the disease throughout your garden.  Ideally, the plant should be burnt so that the fungal spores die.

Best Varieties of Spinach for the greenhouse

These are some of the best varieties of spinach for greenhouse growing.

Atlanta – Atlanta is a hardy crop that is more resistant to frost and colder conditions.  This makes it the ideal variety to grow during the winter months.

Palco – Palco is a tasty, slow to bolt variety.  As it is slower to bolt it makes it a better variety to grow during the warmer months of the year.

Melody –  Melody is a variety that has developed a strong resistance to downy mildew.  This makes it ideal for late spring and late summer planting.  It’s also super tasty too!

How Many Spinach Plants Should I Grow in a Greenhouse?

Four plants at any one time is going to produce enough spinach leaves to feed yourself and your family.

Sow additional plants every 3 to 4 weeks to ensure that you have a constant supply throughout the year.

Using spinach as a cut and come again plant will allow you to harvest a crop more regularly, as and when you want to cook with the plant or add it to some delicious salads.

Spinach Greenhouse Growers Kit List

We have selected a kit list of key equipment and tools to get you growing tasty spinach in your greenhouse in no time.    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

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