Welcome to Top of the Crops - today, you will learn how to grow aubergines in a polytunnel. For more gardening insights, be sure to check out our blog Polytunnel Gardening too!
Aubergines can be difficult if not impossible to grow outside in much of the UK. A polytunnel, however, makes it possible, even easy, to grow this heat-loving summer crop – even when the summer here leaves something to be desired.
Aubergines come from the plant with the Latin name Solanum melongena. They are related to several other common crops often grown in UK polytunnel gardens, including tomatoes and peppers. They are related to potatoes too, though they need rather different conditions to grow well.
Like tomatoes and peppers, aubergines are summer crops that need plenty of sun and warmth to grow and ripen successfully. Since our summers can often leave much to be desired for warm season crops, a polytunnel or other undercover growing area will make things a whole lot easier.
A polytunnel can give aubergines the extra warmth and the humidity that they need to grow and set fruit successfully. They can be grown in the ground, in grow bags or in containers within the polytunnel.
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While aubergines can grow happily in a polytunnel, the seeds are best sown inside as they will need to be started early in the year – in either February or March – and require a high temperature for germination.
Either a warm location indoors or a heated propagator will be required since seeds need temperatures of at least 21 degrees C. for germination to take place. In optimal conditions, seeds will germinate in around 2-3 weeks.
When precisely it will be best to sow will depend on where you plan on placing the plants during the growing season. If you plan on planting them outside then you should typically hold off sowing until at least early March, but those to be grown in a heated greenhouse or conservatory can be sown much sooner – perhaps as early as January.
Once germination has taken place, and seedlings emerge, place them in a warm, bright location – maintaining temperatures between 16 and 18 degrees C. Water sparingly but be sure to keep the soil moist during this time.
Grow your aubergines in 9cm pots or similar initially, pricking them out and potting them into these containers, and then when the roots fill those pots, transfer to larger containers as required until the time comes for planting them into their final growing positions.
You can transfer aubergines to a heated polytunnel in April or to an unheated polytunnel in May, after all risk of frost has passed and the polytunnel has warmed up considerably.
When transplanting aubergines outdoors you need to harden off plants well before they are placed into their outdoor growing positions. This involves gradually acclimatizing the plants to outside growing conditions so that change does not come as too much of a shock.
Make sure that you plant them to the same depth that they were in their previous containers.
If you are planting in a grow bag then you can place two- three aubergine plants per bag, or one plant per container of around 30cm in diameter.
Choose a suitable potting mix that is peat free, and moderately moisture retentive yet free draining.
In the soil, a spacing of 60cm between plants is usually recommended. Aubergines prefer a moist yet free-draining soil rich in organic matter. Ensuring good, healthy soil conditions is key.
If growing outdoors, it is a good idea to warm the soil for a couple of weeks with cloches or row covers before you plant out your aubergines in the area. This, plus a top dressing of good quality garden compost, can help make sure your aubergines get off to the best possible start.
Not all aubergines will ripen successfully in the UK climate. To stand the bet chance of getting these fruits to ripen successfully, you should choose varieties with smaller fruits, which will mature in time within our shorter growing season.
Aubergines are rather fussy plants and will not fruit well unless you give them some TLC.
It is important to make sure the plants get plenty of warmth and light, and consistent water, as well as the nutrients they need.
You should also mist the foliage with tepid water at least twice a day to discourage red spider mite and to help encourage the fruits to set when growing undercover.
Of course, in order to fruit well aubergines also need to be pollinated. So make sure the polytunnel is open and insects can reach the plants once flowers emerge.
Misting the flowers will also help with self-pollination, or, where pollinators may be in short supply, you can also hand-pollinate with a small paintbrush.
To make sure fruits ripen towards the end of the season, remove further flowers on cultivars with larger fruits once 5 or 6 have started to ripen. If you are growing aubergines with smaller fruits, however, or those with round fruits, many more can be supported on each plant.
After late summer, on any type, it is best to remove flowers as these will not ripen in time, and take energy from fruits already ripening. If fruits are not ripe by late September, you will likely have to give them extra protection to allow them to do so.
Check out the care tips below for more detail to help you successfully grow aubergines in a polytunnel where you live.
Caring for an aubergine plant in your polytunnel or elsewhere in your garden is not rocket science. But you do need to make sure that you deliver the right care in order to get good results and obtain a good yield towards the end of the growing season.
You should be sure to water aubergines regularly, especially when growing undercover and when you cannot rely on natural rainfall at all.
Try to water early in the morning if possible, ideally with rainwater that you have harvested.
And aim for the soil or growing medium below the plant and not the plant itself, so that you use less water and so that the water ends up where it can be accessed by the roots of the plant. Getting water on the foliage, flowers and fruit can also make it more likely for fungal issues to arise.
It is important to provide moist yet free-draining conditions and not to let the soil or growing medium dry out entirely. You should also be sure not to create waterlogged conditions, however, as that will also cause problems for your plant.
When planting aubergines, it is a good idea to mulch around the base of the newly planted plants with organic matter. This might be a good quality homemade compost, for example, or a well-rotted manure.
It is also a good idea to consider adding a leafy mulch of comfrey leaves or other material rich in potassium once the plant begins to flower, as this will help to meet the plant's nutritional needs while also covering the soil and reducing moisture losses from it.
Feed aubergines with a high potassium organic feed every couple of weeks once the first fruit has set. (Banana skins placed beneath the plants can also help with potassium needs.)
When the plant has reached around 30cm in height, nip off the growing tip of the main stem. This is done to encourage the plant to get bushier and branch out. It focusses the plant on flowering and setting fruit rather than on foliage growth.
It is a good idea to tie in growing aubergine plants to a support as they grow. Otherwise the plants may bend and break under the weight of their fruits as these mature and ripen. This is not always needed with varietals with smaller fruit but it can often be beneficial.
It is important not to leave it too long before harvesting your aubergines. Make sure you cut them when they are still shiny, around 15cm in length. Once fruits have dulled they become bitter. The harvest should come between late July and September.
If necessary, you can store aubergines in your fridge for a few days. But it is best to use up your fresh aubergines as quickly as you can. One of the great things about growing them yourself is that you can enjoy them much fresher than those you can buy from the shops.
You can use your aubergines in a wide range of recipes from Mediterranean and Asian cuisines and so should find it easy to find ways to bring your harvest to your table.
Aubergine varieties which are good for UK growing, and which have received an Award of Garden Merit and/or which are recommended by the RHS are:
‘Bonica’
‘Clara’
‘Galine’
‘Kaberi’
'Moneymaker'
So these recommended varieties could be a good place to begin when selecting the right variety to grow in a UK garden.
The biggest challenges when growing aubergines are our short growing season and summers that can be poor, Remember, these are plants that need lots of sunshine and warmth over at least five months and often longer from sowing to harvesting.
Certain pests can also pose a problem, such as whitefly, aphids, and red spider mites. So remain vigilant and nip any problems of this kind in the bud before you have a serious infestation on your hands. Biological controls are available where these things become a serious concern.
A polytunnel can give aubergines the extra warmth and the humidity that they need to grow and set fruit successfully.
They can be grown in the ground, in grow bags or in containers within the polytunnel and will do well in the same conditions as peppers in the polytunnel and can be placed in rotation with other members of the nightshade family such as tomatoes or potatoes.
Cucumbers also like similar conditions. French beans and peas can also be good companions for aubergines, as can herbs such as thyme and basil.
BBC Good Foods. (2020) Aubergine Recipes. [online] Available at: https://www.bbcgoodfood.com/recipes/collection/aubergine-recipes
