We know that the inside of a greenhouse will be higher in temperature than the outdoor temperature – we would expect an unheated greenhouse to be approximately 5 degrees Celsius higher than the outside temperature. But how do I know if my greenhouse is too hot?
In this guide we will show you the temperature at which plants can be damaged within your greenhouse, how you should monitor the temperature and what things you can do if you see the temperature getting too hot.
How Do I Know If My Greenhouse Is Too Hot? What Temperature Is Too High?
According to the RHS if your greenhouse reaches a temperature over 27 degrees Celsius / 81 degrees Fahrenheit then plants can be damaged – in effect the temperature in your greenhouse is too high.
Greenhouses are designed to soak up the sun – which works perfectly in extending the growing season and protecting plants over winter – but in the summer when outside temperatures can be sweltering, the temperature within your greenhouse can be intolerable for you to work in and damaging to your plants.
How Do I Know If My Greenhouse Is Too Hot? What Will High Temperatures Do To My Plants?
Different types of plants will tolerate different temperatures before they start to be damaged. So lets take a look at a few examples of what you might be growing in your greenhouse, what temperatures they can cope with and what will happen when the temperature in your greenhouse becomes too hot for them.
Tomato Plants - A typical greenhouse crop in the UK, tomatoes can tolerate high temperatures and it’s only when temperatures climb above 32 degrees Celsius / 90 degrees Fahrenheit that they will suffer dramatically. In the worst case, your tomato plants won’t set fruit if the temperature is too high, but you could also see poor pollination and damage to immature fruits.
Cucumber Plants – Another popular greenhouse crop, cucumber plants produce the tastiest crops when growing in temperatures from 21 to 28 degrees Celsius / 69 to 82 degrees Fahrenheit. When there are prolonged periods of too high temperatures you will see blossom and fruit drop, you could also see discoloured, blistered or sunken areas on the sides of the fruits most exposed to the sun.
Sweet Peppers – Pepper plants do not like to be grown in temperatures over 30 degrees Celsius / 86 degrees Fahrenheit and will produce less fruits when these temperatures are reached. You might also see the leaves of pepper plants develop pale, sunburned patches when exposed to too much sunlight.
In general, if your greenhouse is experiencing temperatures over 27 degrees Celsius, then you could expect to see your plants showing the following symptoms:
Sweating
You might sweat stood in your hot greenhouse but your plants will ‘sweat’ too! When plants ‘sweat’ it is referred to as ‘transpiration’ and this is a normal process. Transpiration is basically the plant pulling water up from the soil through its roots, through the stem and into the leaves. Pores on the leaves then release this water into the air. Transpiration is needed for healthy growth.
HOWEVER when the temperature in your greenhouse is too high the plants will ‘transpire’ or ‘sweat’ too quickly – they will draw up so much water from the soil so the soil dries out and subsequently the plants can’t get more water. Their leaves will start to go brown and frazzled at the tips but this will spread across the whole of the leaves, down the stem and into the roots.
Leaf Scorch
Plants with leaf scorch will have leaves that will begin to turn brown or yellow – indicating that the leaf tissue is dying. If you have just read about ‘sweating’ and ‘transpiration’ you will know that this can be because of your plant loosing too much water and the plant isn’t able to draw the water up from the soil sufficiently to remain healthy. So, this can be caused by the temperature in your greenhouse being too high and humidity too low, but it can also be as a result of drought – this could be you not providing sufficient water, by root damage or compaction - so the plant can’t draw water from the soil, or from excessive fertilisation or the build up of salts in the soil.
Drooping and Wilting
There are several reasons why your plants might start to wilt or droop. It’s usually caused by incorrect watering – so either under or over watering. However, it can also be caused by exposure to cold drafts, too much light or intense heat. So, if your greenhouse is too hot then you could see your plants responding in this way. The plants are drooping as they are drying out too quickly – if you can alter the temperature then your plants may improve, but it might take up to 7 days before you see that improvement.
Blossom and Fruit Drop
If your greenhouse temperature remains too high for long periods then some plants will respond by dropping their buds or flowers. This is their way of trying to conserve resources, so all their energy is used on surviving, not producing flowers or fruit. You could see this happening with plants such as peppers or cucumbers for example. If you can get the greenhouse temperature to be lower and remain that way than these types of plants will then start to flower and fruit as normal.
How Do I Know If My Greenhouse Is Too Hot? How Do I Monitor The Temperature?
When the days are hot and sunny, we know that the temperature within our greenhouse will be increasing and if it becomes too hot for you to be stood in there then it’s likely to be too hot for your plants too. However, to correctly monitor the greenhouse temperature so that you can take action to try to control and reduce the temperature within your greenhouse you will need two pieces of equipment:
- A thermometer - to measure the temperature
- A hygrometer - to monitor the level of humidity.
These are the two key components which combine together to potentially cause your greenhouse to be too hot. By being able to balance the temperature and humidity within your greenhouse you should then be able to create the best growing conditions in which your plants will flourish.
How Do I Know If My Greenhouse Is Too Hot? Use A Thermometer
Simply walking into your greenhouse and feeling it’s too hot for you is not really sufficient so you should use a thermometer to measure and monitor the temperature.
We would always recommend using a max/ min thermometer as these are designed to record the highest and lowest temperatures reached throughout the day and night – so you don’t need to be there to see what temperature your greenhouse got to – simply look on the thermometer the following day or when you get home from work etc.
|
A good example of a max/min thermometer is the digital one shown opposite. Its large digital screen clearly displays the maximum temperature reached, the current temperature and the lowest temperature recorded. And at the push of a button you can swop between Celsius and Fahrenheit. |
 |
Keep a written record of the temperatures – maximum and minimum – which are being reached in your greenhouse and this will not only help you to decide when you need to reduce the temperature, but it will also give you a way of seeing if the methods you use have a positive effect and the temperature decreases.
How Do I Know If My Greenhouse Is Too Hot? Use A Hygrometer
Using a hygrometer enables you to measure the humidity in the air – so the humidity level within your greenhouse. Depending on which types of plants you are growing in your greenhouse will affect what level of humidity you should be aiming for – in the UK most amateur gardeners are aiming for the humidity within their greenhouse to be between 40-80%.
|
|
There are lots of hygrometers available to purchase, including ones which are included with thermometers such as the one shown opposite. The inset dial on the face shows the humidity level from 0 to 100% - on a very wet day you would expect outside humidity to be around 90-100%. If your greenhouse is showing humidity levels of over 80% consistently, then you want to be on the lookout for such plant diseases as botrytis or powdery mildew. |
Consistently high humidity levels – unless this is what your plants prefer – indicate that you need to be increasing ventilation and air circulation within your greenhouse.
How Do I Know If My Greenhouse Is Too Hot? What Can I Do To Reduce The Temperature?
When it comes to reducing the temperature in your greenhouse, then you can look at shading, ventilation and humidity.
How Do I Know If My Greenhouse Is Too Hot? Improve Ventilation
Improving greenhouse ventilation starts with opening the greenhouse door and roof vents during the day when it’s sunny. And if your max/min thermometer is showing temperatures are remaining high overnight then you could leave them open overnight to.
Top Tip
When we are experiencing a heat wave, then the RHS recommend temporarily removing some of the glazing panes from your greenhouse to help with ventilation. Just be cautious though, as this takes time and is involved. It also isn’t something that’s quick to replace if there is a downpour . . . and we all know how unpredictable the Great British Weather can be!
As we aren’t always around to open doors and roof vents when temperatures increase – and if you don’t want to leave them open all the time when you are at work for example – then the best solution for the roof vents is to fit automatic openers.
|
Automatic openers work by wax within a cylinder expanding when the temperature increases – which in turn pushes the window in your greenhouse open. In the same way, when temperatures fall the wax shrinks and the window is pulled closed. You just need to be aware that the expansion of the wax isn’t instant – it takes time – so windows are not opened and closed quickly, but rather gradually. |
 |
Another way to reduce the temperature within an overheating greenhouse is to start to move the cooler air from the outside of the greenhouse inside. So fans positioned near roof vents, louvre vents or the door should help to bring the cooler air in faster.
|
|
The image opposite shows an Air Blower which is supplied with chains to enable you to fit it in the roof area of your greenhouse – so close to the roof vents. This model moves 420 cubic metres of air per hour, using just 69 watts of power. You will also find some greenhouse fan heaters which have a fan only setting – which then lets you utilise your heater in the summer but this time just to circulate the air.
|
Top Tip
If you are using a fan in your greenhouse, always make sure that it is positioned so that it does not blow the air directly at your plants. Ideally the air should be directed over the top of your plants – so this works well if you can suspend the fan I the roof space of your greenhouse. If the fan directs the air straight at the plants it could result in over-drying the leaves and could damage or in the worst case, kill the plants.
Ideally, for maximum ventilation in your greenhouse, you should combine opening roof vents, side vents and the greenhouse door, as these are all ways to allow ait into your hotter greenhouse environment.
Adding more roof vents into a greenhouse can be virtually impossible – these usually must be bought and fitted at the same time as your greenhouse. However, you can increase / add side ventilation to an existing greenhouse by fitting louvre windows.
|
To make the best use of louvre windows and ensure maximum circulation of air around the area of your greenhouse, louvre windows should be fitted near to the ground to allow the cooler air to enter. Using a louvre kit enables the DIY gardener to add these relatively easily to a greenhouse, reusing the existing glass of your greenhouse to form the louvre blinds. |
 |
How Do I Know If My Greenhouse Is Too Hot? Provide Shade
Another way to reduce the temperature within your greenhouse is to adding shading – this could be in the form of a paint-on shading, shade netting or shading blinds.
A traditional way to provide shade to your greenhouse is to use a length of shade netting, which can be attached to either the exterior or interior of your greenhouse.
When attached to the outside of your greenhouse, fixing it in place might be tricky and it will be exposed to the elements. However, by fixing the material to the outside it stops the sunshine getting into the greenhouse and in this way is more effective than if used on the interior of the greenhouse.
|
|
Fitting the netting to the inside of your greenhouse is much easier – usually you can reuse the fittings that held your insulation to the greenhouse walls, such as alliplugs, to hold the green shade netting securely in place. When selecting a shade net look at the shade density so you can see how effective it is going to be and if it will suit your plants – some plants will thrive in different levels of shade. Our shade netting has a 50% shade value.
|
With shade blinds, you will find some which can be fitted to the outside of greenhouses and some which can be fitted to the interior. Gardeners tend to prefer the interior fitting blinds as they are easier to fit and won’t be affected by the weather – so heavy rain or high winds won’t damage them.
|
When fitting blinds inside your greenhouse – as shown opposite – just be careful not to obstruct the opening roof vents. Ideally on the panels in the roof where the window is, fit the blind below the window – this will then still allow you to manually open the roof vent or add an automatic opener. |
 |
How Do I Know If My Greenhouse Is Too Hot? Add Water
One way to rapidly cool down your greenhouse if it is overheating is to spray water on the floor of the greenhouse and on the plants.
Applying water like this will lower the temperature through ‘evaporative cooling’ - the air is cooled down to a comfortable temperature due to water evaporation. It will also help to increase the humidity within the greenhouse, which should slow down plant water loss due to excessive transpiration.
Our blog post How Do I Know If My Greenhouse Is Too Hot? has been created from our personal knowledge, information gathered by speaking to other gardeners or manufacturers in the gardening industry, by reading gardening magazines and devouring information from books and the internet. We aim to be as accurate as we can, so if you find a mistake, please remember, we’re only human. if you have any queries you can contact us today!