September in the greenhouseSkip to content

September in the greenhouse

September brings cooler weather, changing garden colours and sunny glimpses of summer.

The calendar now says September, and the first clear signs of autumn are beginning to appear in the garden. The colours are slowly changing, with shades of gold, red and brown joining the green hues. The days are getting shorter, and the garden is gradually entering a new season as nature prepares for the cooler months.

We also notice a clear drop in temperatures, with the average temperature now around 13 degrees. At the same time, there are significantly fewer hours of sunshine than in the previous months, and rain becomes a more regular part of the weather. Although September marks the transition into autumn, the month can still bring some lovely surprises. The sun may suddenly break through, giving us mild days with a touch of summer before autumn truly takes over.

Check Your Greenhouse

Start by checking the entire greenhouse, inspecting the roof, walls and joints for cracks, holes or other signs of damage. Pay particular attention to glass and polycarbonate panels, as even minor damage can worsen and cause problems later.


A thorough check now makes it easier to spot and fix any issues before autumn really sets in.

Clean the Glass and Panels

After the summer, dirt and algae may have built up on the greenhouse glass or polycarbonate panels, reducing the amount of light coming in.


Give the surfaces a thorough clean using a soft brush and cleaning solution to avoid scratching the material. If you find any damaged glass panels, replace them as soon as possible to prevent the damage from getting worse.

Tighten and Lubricate Moving Parts

Finally, it’s a good idea to check all screws, bolts and joints, and tighten them if they have become loose during the season.


At the same time, check the hinges, locks and other moving parts on doors and windows, and apply a little oil or lubricant. This helps ensure that everything works smoothly and does not risk seizing up when the cold and damp of winter arrive.

Greenhouse Tasks

Sow New Crops for Autumn

September is an ideal time to sow new crops that thrive in cooler temperatures. Try winter salads, lamb’s lettuce, rocket and spinach, for example, which can provide fresh vegetables well into autumn. Choose varieties suited to autumn growing that can cope with shorter days and lower temperatures. Sow in several batches to ensure a continuous harvest.

Start Winter Growing

The cooler months are also a good time to plant crops that will overwinter and provide a harvest next year. Onions and garlic, for example, can be planted now, giving them time to establish before winter. Make sure the soil is well drained so the bulbs do not sit in water over winter. This way, you can start laying the foundations for next year’s vegetable garden now.

Keep an Eye Out for Pests

Although summer is coming to an end, pests may not have disappeared from the greenhouse. Inspect your plants thoroughly, paying particular attention to the undersides of the leaves, where aphids and other small pests can hide. Remove affected leaves and plants, and act quickly if you notice a larger infestation. The earlier you tackle the problem, the easier it is to keep your plants healthy.

Protect Against Slugs

Slugs can still be active in the greenhouse, especially when the weather is mild and damp. If you have problems with slugs, try burying a cup of beer in the soil as a simple trap. Place the trap close to areas where you have spotted signs of slugs. At the same time, remember to remove hiding places such as dead leaves and other organic material around the plants.

Bring Tender Plants Indoors

Plants and tender fruit trees that have enjoyed the summer outdoors should be moved into the greenhouse before temperatures drop too much. Give them a thorough inspection before moving them, so you do not bring pests in among the other plants. Remove dead leaves and lightly prune the plant if necessary, particularly if it is taking up too much space. This way, you can get the most out of your plants before the season comes to an end.

Ripen the Last Tomatoes​​​​​​​

Tomato plants can still provide a final harvest in September, but growth naturally begins to slow down. Pinch out the tops of the plants to limit new growth and encourage the plant to direct its energy towards the existing tomatoes. Once the plants have stopped developing, you can also cut back the stems and allow green tomatoes to ripen in a light, warm place. It is a great way to make the most of the season’s final tomato harvest.

Tidy Up After Summer​​​​​​​

Once your crops have finished, it is important to remove dead plants, spent crops and other plant material from the greenhouse. Plant debris can harbour pests and diseases that may otherwise survive and cause problems next season. At the same time, clean shelves, surfaces and pots, and wash your tools thoroughly. A good tidy-up now will leave you with a healthier and more organised greenhouse.​​​​​​​

Remember to Ventilate​​​​​​​

Although temperatures are falling, it is still important to ensure good ventilation in the greenhouse. Damp autumn days can quickly create a warm and humid environment where mould and fungal diseases thrive. Open windows or ventilation vents on milder days to allow the air to circulate. Just remember to close them again when the temperature drops, so the plants are not exposed to unnecessary cold.

Ripen the Last Tomatoes​​​​​​​

Tomato plants can still provide a final harvest in September, but growth naturally begins to slow down. Pinch out the tops of the plants to limit new growth and encourage the plant to direct its energy towards the existing tomatoes. Once the plants have stopped developing, you can also cut back the stems and allow green tomatoes to ripen in a light, warm place. It is a great way to make the most of the season’s final tomato harvest.

Bring Tender Plants Indoors

Plants and tender fruit trees that have enjoyed the summer outdoors should be moved into the greenhouse before temperatures drop too much. Give them a thorough inspection before moving them, so you do not bring pests in among the other plants. Remove dead leaves and lightly prune the plant if necessary, particularly if it is taking up too much space. This way, you can get the most out of your plants before the season comes to an end.

Remember to Ventilate​​​​​​​

Although temperatures are falling, it is still important to ensure good ventilation in the greenhouse. Damp autumn days can quickly create a warm and humid environment where mould and fungal diseases thrive. Open windows or ventilation vents on milder days to allow the air to circulate. Just remember to close them again when the temperature drops, so the plants are not exposed to unnecessary cold.

Tidy Up After Summer​​​​​​​

Once your crops have finished, it is important to remove dead plants, spent crops and other plant material from the greenhouse. Plant debris can harbour pests and diseases that may otherwise survive and cause problems next season. At the same time, clean shelves, surfaces and pots, and wash your tools thoroughly. A good tidy-up now will leave you with a healthier and more organised greenhouse.​​​​​​​

Gardening Tips for September

Outside the greenhouse, there are also plenty of gardening tasks to get on with in September.


Remove moss from the lawn using a rake or scarifier. Tip: Apply lime in spring – it helps prevent moss.


Thin out and prune evergreen trees and shrubs to keep them in shape.


Cut the raspberry canes right down to ground level as soon as the last raspberries have been picked. This helps prevent grey mould and other fungal diseases from affecting the new shoots. Be aware of whether the variety produces fruit on first-year or second-year canes.

Give Your Greenhouse a Fresh Start

Autumn is just around the corner, and it’s time to consider how you want to use your greenhouse once most of the plants have finished for the season. Perhaps some of the plants can stay, while others are replaced with furniture, pots and cosy décor, allowing you to use the greenhouse as an extra living space.


With shelves and tables, you can create more space and keep your greenhouse organised, while also giving yourself the flexibility to arrange it according to your needs. This is also a good time to start thinking about next year’s greenhouse season and consider what changes or improvements may be needed, so the greenhouse is ready when spring returns.

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JULIANA GROUP

Juliana Group is a leading global supplier of quality residential greenhouses. Founded in 1963 by Mogens A. Stærmose in Odense, Denmark, the family-owned company is today run by third generation, Nikolaj Stærmose. Headquarter and production remains Odense-based with subsidiaries in the UK and Germany and export to more than 20 countries.

’We help people grow’. That is why our brands Halls and Juliana set the frame for experiences that are as natural as they are magical. Despite our long history, we’re future-focused and curious on how we can improve the sustainability of our greenhouses. Learn more about Juliana and find inspiration for life in the greenhouse in our inspirational universe.