September Growing Guide

Premier team

September will be a busy month in your polytunnel, with many of your summer crops past their prime and new ones just emerging.

You will notice the inevitable drop in temperature and the days drawing in. However, if you are lucky enough to own a polytunnel, the adverse weather is no problem. You will find the extra protection your polytunnel provides to be extremely helpful in allowing your crops to grow to their potential.

To make the most of your polytunnel before the winter starts creeping up on you, we have made a guide to help you get organised and decide what to plant in September in the UK.

Top growing tips for September

Have a clear-out

Start clearing summer crops to make space for the veg to plant in September. Be ruthless and pull up any plants that are no longer at their best, thus making valuable space for winter crops.

In wet weather, use spare space inside the polytunnel for drying outdoor crops. Preserve tender herbs, such as basil, lemon verbena and sweet marjoram, which start to deteriorate in damper, cooler weather. This will help capture the last of their delicious summer flavour.

A top gardening tip for this September is to pull up any weeds and remove debris, such as pots, canes and rubbish that could be hiding diseases and pests, such as red spider mites, aphids and whiteflies that could be carried over into your winter crops.

Keep the weather out

Shut doors and vents on cold nights which may start to become frosty before the month is out. However, ensure ventilation is good during warm days and also cooler ones. Good ventilation in the polytunnel aids winter crops and helps minimise condensation which can lead to an increased risk of fungal diseases.

As the weather gets cooler and growth slows down, reduce the amount of watering inside your polytunnel so your crops are not left to sit in waterlogged, cold soil.

Plants such as tomatoes, cucumbers and melons may still have lots of fruit, but their growth is slowing down and they are using less water. If you keep giving them the same amount of water, they will quickly become waterlogged and it could spoil the rest of your crop. You must take this into account when allotment planting this September.

Protect your crops

Top growing tips for in a Polytunnel in SeptemberDon't let cucumbers get too cold as the temperature drops. During colder days and every night, try to keep them as warm as possible. Give them a weekly dose of liquid tomato feed and they should remain productive right up to the first frosts.

Remove lower leaves from tomatoes as they turn yellow, allowing more sunlight to reach the ripening tomatoes. September is the time to harvest tomatoes, but some tomato plants can continue cropping into October. Don’t hesitate to pull out any sickly plants though.

Now is also the time to remove any summer shading to ensure your plants can make the most of the sunlight as the days start getting shorter and the sun gets lower. By doing this, you will open up your options of what veg you can plant this September.

Wash off any dirt from the polytunnel cover to allow more sunlight in as daylight hours get shorter during the winter months. You should also repair any tears in your polytunnel cover before the winds get higher.

What to plant in a polytunnel in September

As a polytunnel gardener, you will have to decide whether to leave your summer plants in place in the hope you can harvest them at another time or replace them with winter crops. It can be difficult to decide, but the choice is yours.

However, if you decide to keep those summer crops, keep an eye on their progress as one week or even a day can be the difference between perfect and overripe.


Summer sowings of beetroot, lettuce, spring cabbage, swiss chard and kohl rabi will be ready for planting out as soon as there is space in your polytunnel. Winter crops need to get as much growing done as they can while the soil is still warm.

Broccoli and kale are also crops that you can plant in your polytunnel this September as these crops will grow quickly with the extra protection and benefit from the plant feed left behind from the previous crop.


When trying to decide on what seeds to sow in September, we would suggest cauliflower, coriander, lettuce, peas, radish, rocket, mizuna, spring onions and, of course, strawberries.

Here you will start to build your collection of scrummy winter vegetables for soups and roast dinners while still enjoying the lovely summer taste of berries.


It is important to know what seeds to plant in September, but it is just as important to know what to harvest.

The crops found in your polytunnel this September will be a mix of yummy fruits and some hearty autumn vegetables. Whether you want to create a tasty salad, make a satisfying crumble or begin to tuck into some winter vegetables – enjoy this time of the year and all your polytunnel has to offer.

Some of the crops you will be harvesting include cucumber, tomato, peppers, lettuce, radish, rocket, spinach, spring onion, carrots, cauliflower, aubergine, sweet potato, strawberries, melon, melons, french beans and so much more.

Want a top gardening tip for September in the UK? After harvesting your cucumbers, don’t put them in the fridge as this will make them go soggy. Instead, try wrapping them in cling film and keeping them somewhere dry and dark. Hopefully, your cucumbers will last you through the autumn or even the winter months.

Why choose Premier Polytunnels?

We have over 30 years of experience in polytunnel design, catering for domestic and commercial growers, so you can trust us when it comes to tips on what to plant in September.

Whether you want to sell your produce or keep it for your enjoyment, we can advise you on the best polytunnel for your needs. In addition, we provide tools and extras to help you make the most of your growing seasons. We offer raised beds, irrigation tools and other accessories designed to help you grow the healthiest crops.

You can join our mailing list for more information. But if you have any questions, please contact us on 01282 811250 or email and we’ll be happy to help you.