Top of The Crops - Basil

Growing Basil In A Polytunnel

Basil is an aromatic herb with a delicious flavour and is used in many Mediterranean dishes. There are several different basil varieties well worth growing in a polytunnel or elsewhere – not only for their use as herbs, but also as beneficial companion plants for a number of other crops. 

Key Information

Many of the basil varieties that are most commonly grown are cultivars of Ocimum basilicum – common basil. 

There are varieties of common basil that have larger leaves or smaller ones, red or purple-hued basil varieties, stronger and more delicate-flavoured basils, and ones with different flavours, such as lemon basil, for example...

There are also other Ocimum species offering other basils to grow. Holy basil, or tulsi, Thai basil, cinnamon basil, lime basil, and clove basil are just a few examples...

Different basils do vary somewhat in their environmental requirements and the care that they require but for most, the details provided below for their preferred conditions, growth and care will hold true. 

Sweet basil and other related basils are nutritionally beneficial and have a long history of culinary and medicinal use. As useful in the kitchen and around the home as they are in the garden, basil can be a great choice for any polytunnel grower. 

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The Preferred Conditions for Basil

Basil will like a position in full sun for most or all of the time. These are plants that typically require at least 6 hrs of sunlight each day. 

This is a plant that will do best in a warm and sheltered spot, where it will be protected from chilly conditions. Temperatures above 21 degrees Celsius through the summer and no lower than 10 degrees Celsius throughout the winter months are required. 

When growing basil, whether this is in the ground, raised beds or containers,  you also need to make sure that you provide a moist yet free draining soil or potting mix, ideally with a pH of 6-7, though basil can typically tolerate a pH anywhere between 5 and 8. 

How to Grow Basil

If you would like to grow some basil at home then of course you will need to decide which type and variety of basil to grow. You will find some discussion of specific varieties a little later in this guide. 

You will also need to determine where you will grow your basil. Basil can be grown:

  • Indoors, on a sunny windowsill. 

  • In a polytunnel or greenhouse bed or border.

  • In a dedicated herb garden outdoors. 

  • In mixed planting schemes alongside edible and ornamental flowering plants.

  • In containers of many kinds, or even in water rather than soil in a hydroponic or aquaponic system. 

Typically, basil is grown either from purchased potted plants, or from seed. 

How to grow Supermarket Basil

The basil that is sold in pots in supermarkets is usually a number of young plants with poorly developed root systems all crammed together into one pot. They will not usually thrive if you try to grow them on as they are in their original pot. However, it is possible to save them if you separate each young seedling and grow those on in their own individual pots. 

Sow Basil Seeds

Basil seeds should be sown indoors in March or April. As a general rule of thumb, it is best to sow around 4-6 weeks before the last frost date in your area. Sow seeds shallowly and make sure that the young seedlings get plenty of light. 

Plant the seeds into seed trays or some small pots, around 6mm deep. Use a propagator or ensure that temperatures remain around 21 degrees C. for the best germination rates. The seeds should germinate within a week or two. 

Seedlings sown in a seed tray can be potted up into individual containers once they develop a few sets of true leaves. 

The containers should be filled with a suitable, peat-free potting mix or a homemade equivalent. 

Seedlings can then be hardened off and planted out in your polytunnel as soon as the weather warms up properly. You may wish to plant out the basil seedlings at the same time as the tomatoes – they are said to be good companions both on and off the plate.

Harvesting Basil

You can prolong the harvest period of the basil by regularly pinching the growing tips for use. Harvesting regularly by taking the tips will encourage the plants to become bushier and more productive. 

You can start harvesting basil as soon as the plants reach around 15-20cm tall. Handle the leaves carefully when harvesting as they bruise easily. Typically, you can expect to harvest individual leaves from your plants and to nip out the growing tips between June and September, perhaps even beyond when you are growing in a polytunnel, under cover. 

Harvest from basil plants early in the morning when their essential oil production should be high and the plants should be less stressed for the best flavour. 

Basil is a versatile herb and there is plenty to do with the tips and leaves you have taken. Fresh basil works especially well in a range of Italian recipes. A pesto is wonderful if you have a lot of basil to use. If you keep up with the tip pinching then you can be harvesting basil right through until the autumn.

Storing Basil

Basil is best eaten or used fresh from the garden, as soon after harvesting as possible. 

However, you can also freeze basil chopped and with olive oil, or in the form of a pesto, in ice cube trays, so that you can use these later in a range of recipes. Or you can potentially use it in a range of bottling or canning recipes. 

Care Tips for Basil

Basil is not too challenging to care for as long as you have placed your plant or plants in a suitable location. 

Watering is one of the most important things to get right when you are growing basil. You need to keep the soil or growing medium in containers moist but not allow waterlogging to take place. 

Water below plants, try not to wet the foliage where possible. When watering, try to do so early in the morning as basil abhors having wet roots overnight.


You can help ensure that basil gets the nutrients that it needs by mulching around your plants with a good quality homemade compost or well-rotted mature, or another organic mulch. 

Basil growing in pots especially, but any basil that needs a boost, will also appreciate being fed with a nitrogen-rich organic liquid plant feed every couple of weeks through the growing season. A nettle tea could be ideal for this purpose, for example. 


Weed well around basil plants to reduce competition. Reasonably close spacing plus the use of an organic mulch around your plants will help keep weed growth to a minimum. 

Removing Flowers

The main reason to nip off flowering shoots when these form so that the harvesting period can continue. Basil will not taste as good after the plants' flower. 

We also remove any flowers that start to develop as soon as possible, unless you are ready to let the plant go to seed to allow the plant to focus on producing new leaves rather than on producing flowers and setting seed. 


Most basils are grown as annuals indoors in a temperate climate, and seeds are sown fresh each year. They can survive over multiple years but won't thrive, so growing replacement plants each year is generally the best idea. 

Varieties of Basil

Sweet Basil varieties that have received an award of garden merit from the RHS are:

  • ‘Aroma 2’ - a vigorous F1 hybrid with some bolting and disease resistance. 

  • ‘Lemonade’ - a vibrant-leaved basil with lemon flavoured foliage. 

  • ‘Mrs Burns' Lemon’ - another lemon basil with lemony flavoured leaves. 

  • ‘Pluto’ - bush-forming, small-leaved type forming small mounds around 20cm high. 

  • 'Red-leaved' – AKA 'Basilico Rosso', with deep purple stems and leaves. 

  • ‘Salvo’ - a variety with good flavour, also noted for its weather and disease resistance. 

These may be a good place to start, though of course there are many more varieties of sweet basil, and also other basil varieties to consider, including holy basil, or tulsi, thai basil, lemon basil, lime basil, cinnamon basil, clove basil etc...

Common Problems for Basil

Basil does not usually encounter too many serious issues as long as the growing conditions and care are correct. But remember that a range of problems can arise if, for example, the temperatures are too low, there is not enough sun, the plants dry out entirely or waterlogging occurs... 

When growing basil you should also look out for pests like slugs and snails, and aphids. Make sure you have boosted biodiversity and done all you can to attract natural predators of these pests to your garden and to create a balanced ecosystem where you live. 

If growing undercover, make sure you ensure good ventilation throughout the summer. This will reduce the chances of a fungal issue like powdery mildew, grey mould or damping off in seedings from taking hold. Watering correctly and avoiding wetting the leaves where possible will also reduce the chances of this type of problem. 

Top Tips for Growing Basil in a Polytunnel

While you can grow basil all year round on a bright, light and warm windowsill inside your home, basil in a unheated polytunnel should be considered a summer crop. 

The best place for basil is next to your tomatoes – some even say that having basil planted nearby will improve the flavour of the fruits. As well as tomatoes, peppers and other members of the same family may also benefit from proximity to this herb. Planting chamomile or anise near basil may help to increase the essential oils in the plant.


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McCulloch, M., (2023) Basil: Nutrition, Health Benefits, Uses and More. Healthline. [online] Available at: 

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growing basil in a polytunnel