Do Greenhouses Need Ventilation?

One common mistake made by new greenhouse owners is to limit the ventilation. At face value this makes sense – you want a greenhouse to stay warm so why open it up to outside air? In fact, ventilation is an important part of maintaining a healthy and successful greenhouse, as you’ll learn in this guide.

Why Should I Ventilate My Greenhouse?

There are several good reasons why greenhouses need ventilation. Here are the main reasons to allow in fresh air:

Reduces Mold & Mildew

Mold and fungus love damp, still air. While this is most likely to be a problem in the winter months, it can also cause issues in the summer too. 

In winter, greenhouse plants can go mouldy, leading to leaves being attacked and even whole plants dying off. 

In spring, damping off – when seedlings start strongly but then quickly buckle over and die off – is most commonly caused by overly damp, stagnant conditions. 

In summer, common plant diseases like blight can hit tomatoes if the air is damp and still. 

Ventilating your greenhouse helps to blow away mold and mildew spores, thereby keeping your plants fitter and healthier.

Controls Excessive Temperatures

Whilst greenhouses can be a god-send in the spring and fall, greatly extending your growing season, summer greenhouse temperatures can rapidly get too high. 

Even in spring my own greenhouse can quickly reach 30’C (85‘F) or higher on a reasonably sunny day. I’ve seen it push over 40’C (105‘F) by early summer. Such temperatures can lead to plants rapidly drying out and dying, or to leaves becoming scorches by the sun’s rays.

Ventilating your greenhouse in summer can help the excessive heat to escape, while still maintaining a warm and pleasant place for your beloved plants to thrive.

Allows Entry for Pollinating Insects

If you’re growing fruits and vegetables in your greenhouse then a further consideration is how they will be pollinated. While it’s not crucial for all food plants, many benefit from having bees and other insects feeding on their pollen, and transporting it to other plants.

Leaving greenhouse doors and windows open makes this a reality; I often find flying insects making the most of the flowers in my greenhouse. Of course, you could also argue that access to these early flowers also helps to support our struggling pollinators too. 

Assists Wind-Pollinated Plants

Not all plants are pollinated by insects – some need a gentle breeze to distribute their pollen. Properly ventilating your greenhouse allows this, and can greatly increase the crop yield from many plants. 

Allows Entry for Pest Controlling Insects

One final consideration comes in the form of biological control. Being warm and (for the most part) dry, greenhouses can suffer from a range of insect pests. 

While these can often be controlled using chemicals or predatory mites, one free and environmentally-friendly option is to allow in predatory insects. Ladybugs, for example, will feed ravenously on aphids that can otherwise damage your greenhouse plants.

When Should a Greenhouse Be Ventilated?

The more ventilation you can give your greenhouse the better, within reason. At the same time, consideration should be given to the growing conditions of your plants. 

In the colder months it is a good idea to open up your greenhouse on sunny days where the outside air temperature is at least 5’C (‘F). In this way your tender plants shouldn’t be too stressed by the gentle change in temperature.

In the summer months it makes sense to open your greenhouse in the morning – often before work and before the temperature in your greenhouse rises too much. 

In all but the hottest of weather it generally makes sense to shut up your greenhouse before the temperatures drop too low at night. 

How Do I Ventilate My Greenhouse? 

There are two common ways to ventilate a greenhouse. First are the doors, and second are the roof vents. For best results it is wise to open up both of these. 

Hot air trapped near the roof of your greenhouse can then rise away through the roof vents, while cool fresh air can be drawn in through the open doors. 

Considerations When Ventilating Your Greenhouse

We’ve established the importance of ventilating your greenhouse but before you go throwing open those doors there are a few other important considerations.

Are There Cats in Your Neighborhood?

Cats have a nasty habit of digging in flower beds, or climbing up onto potting tables and greenhouse staging to lie in the sun. When you first start ventilating your greenhouse keep an eye out for any unwanted cats which may cause damage.

If you experience feline-related damage there are options open to you. Two simple options are to consider adding a cat scarer to your greenhouse, or to make a net door cover.

Good quality cat scarers are generally safe for other wildlife so are an environmentally-friendly option. They also give full access to the greenhouse for beneficial insects.

A net can be fashioned to cover the opening in your greenhouse as an alternative. These are lightweight, cheap and quite effective. That said, some DIY knowhow may be required, and there is a risk that fine netting may keep some insects out of your greenhouse.

Are Birds Likely to Do Damage?

From the big fat pigeons that live in my garden, to the constant chattering of sparrows, my greenhouse seems to pose a strange attraction for many birds in my area. Fortunately they haven’t yet done any real damage, save for the sparrows taking sand baths in the dry earth.

If you’re unlucky enough to suffer damage from birds – such as them pecking at soft green shoots – then you may want to consider a net as was recommended for cats. Consider a larger gauge of netting if the option is available to bees and ladybugs still have full access.

Are Any Strong Winds Expected?

Strong winds can cause damage to greenhouses. A greenhouse that is all shut up tends to weather storms much better than one where the doors and roof vents are open. Before you go opening up your greenhouse in the morning, therefore, check your local weather forecast. This is especially important if you’re going to be out all day.

Check the Weather If You’re at Work All Day

It’s not just wind that can cause problems with greenhouse ventilation. Heavy rain, snow and hale can also all cause issues if your greenhouse doors are left wide-open. If you’re going to be out for the day – such as working a full-time job – then try to get into the habit of checking the weather forecast before you leave home. 

Are You Forgetful?

We all lead busy lives, and it can be all too easy to forget about your greenhouse at the end of a long day. 

It is possible to buy “smart” greenhouse vent openers which can be very useful. As the greenhouse warms up, so the vents automatically open. Then, in the evening as temperatures drop again the vents close. 

If you want to ventilate your greenhouse with the minimum of effort then they really can be worth their weight in gold. 

Do Heated Greenhouses Need Ventilation?

One final point worth discussing is whether artificially heated/cooled greenhouses require ventilation. Generally the answer is still yes, though you’ll likely want to employ rather more technology than just opening the door. 

Electric fans can be used to circulate air in winter, for example, rather than risking the loss of too much heat by opening the doors. A number of high quality air exchange systems are available for more advanced greenhouse gardeners who opt to heat their greenhouse over winter. 

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