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I have a small all-glass conservatory. It's essentially a greenhouse joined onto the side of the house, and it's getting remarkly hot in the current sunny weather (North London, UK).

When the doors/windows are fully closed, a metal digital thermometer has been just about hitting 60 degrees in the direct sun.

Even with 2 windows and the doors full open, the temperature is comfortably getting into the 40s. I have some tomato saplings, and I pushed a digital thermometer a few cm into the soil, and yesterday it peaked at 39 degrees (Celsius).

Is this too hot for plants in general?

What about tomatoes specifically?

Is there anything I can / should do (beyond keeping a careful eye on watering, obviously!)

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I don't think this will be so much of a problem by Tuesday/Wednesday this week, because the weather is set to be cooler (nearer normal temperatures for early June), with some cloud and rain. Even so, you probably need to keep windows open, and the door whenever possible when it is sunny to try to keep the excessive temperatures down, as well as provide some shading for the tomato plants during the middle part of the day (11-3) if they're in full sun, as one would in a greenhouse. As the weeks progress, the UV levels will get lower, even if it is very sunny - April and May are when UV rays are at their strongest in the UK.

I've had my tomato plants on a windowsill indoors, and without shading, I notice some of the leaves have crisped up at the edges with sun/heat damage, even without high room temperature, so sun exposure might need to be restricted when it is very sunny, up till you're trying to ripen the fruits. My own plants are now almost 4 feet high and one of them even has a fruit forming so they do like the warmth and sun - just not too much of it until a little later in the season.

If you're growing more tropical houseplants, these should cope pretty well, so long as they don't prefer shady conditions.

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If you are really concerned about the temperature in your conservatory, you can get a greenhouse shade to put up. They are usually a mesh cloth that adds some shade inside the greenhouse or conservatory which helps drop the temperature.

Most regular greenhouse shades are inexpensive and widely available online. I have seen some expensive greenhouse shades that are motorized too.

The tomatoes and most tropical plants can deal with the temperatures you mentioned (39c / 102f) for short periods of time with adequate water and humidity. Although I do highly recommend that you get a greenhouse shade to keep the temperature down a little bit.

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I live in a hotter country, yet greenhouses (at-least in nurseries) are tuned to a lower temperature. The cooling system involves shading and also cooling radiators which operate by evaporative cooling and provide extra humidity. They require air circulation so the plants get fresh air and moisture constantly.

In your country that same setup may require less cooling. However, you live at a higher lattitude and have more cloudy days, so you may need to use a less dense shading cloth . Consult local nurserymen on shading cloth density and (if necessary) complementing light with artificial lighting. There are good LEDs which produce abundant light and little heat. 39 C is marginally good for your plants, and seedlings may suffer. The main problem is that cold air (outside) holds less humidity than hot air. Now that it heats-up inside the greenhouse and with the same level of moisture, it has a lower relative humidity. This may add more stress on tropicals and especially on seedlings as they dry-out more quickly. I have failed to find cooling radiators for hobbyists, and coolers for commercial greenhouses can be very large. You may experiment with humidifiers, but these provide moisture without lowering temperature.

Regarding ventillation - I have not seen a picture of the layout, but remember that hot air rises, cold air falls. To vent excess heat, it is very helpful to use a roof vent in addition to the ventillation windows. If you want an energy-efficient and less labor-intensive system, you will like this video by GrowingSpaces.

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    Evaporative cooling is an extremely simple concept that can be achieved with simple homemade methods. There's no need to invest in an expensive industrial swamp cooler. If you google "DIY greenhouse swamp cooler" you can find instructions. – csk Nov 28 '20 at 3:44
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It sounds like you have an excellent greenhouse that was costly . You are missing a low cost final step ; a window fan. Here in a warmer climate ( USA zone 8 ) commercial greenhouses will use fans up to 6 ft diameter . Here a cheap 20 in. widow fan can be $ 25, get 2 or 3 if necessary. Some will have a thermostat . Or with some expense you could rig the fans with a regular home thermostat ( possibly such a unit is available off-the-shelf) . Some plants even seem to benefit from air circulation. A local serious orchid grower seems to have his big fan running most of the time.

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