Overheating greenhouse

[Cutecumber]

The greenhouse in question is about 8x6, glass on an aluminium frame, in a North facing narrow suburban garden (but at the sunny end). The longest side is parallel to the house, so you walk down the garden and turn left into the door, if you see what I mean. The back (ie opposite the door) is only a foot or so from the fence. There are two roof vents and two side vents. An ivy-clad Hazel tree and a dense Hawthorn shade it for a while during the early and middle part of the day in summer.

So, my question is why does it get so prohibitively hot and what can be done about it? The only things that can survive in there between May and October are chillis and aubergine and I've had some success with mini-cucumbers. The temperatures easily achieve 100 degrees by mid-Spring even with shading (which is counter-productive anyway). Tomatoes - which I would quite like to put in there - obviously can't stand it.

I have tried the paint on white-wash shading and mesh draped on the outside, but the difference is minimal. I've tried electric fans to keep air moving and to extract the hot air as much as possible. I dowse the path with water as often as I can.

Many, many people grow tomatoes successfully in greenhouses here in the UK, so is my experience unusual? I've read Terry marshall's book about growing tomatoes and he manages to keep temperatures in the seventies - how?

Should I just be content with growing chillis and aubergines in there?

Your thoughts and ideas would be most welcome.

Thanks.

[OverWyreGrower]

Can you prop the doors open at all?

Last summer, we found that we had to open the 4 roof vents fully and the door (fairly wide open) to get any sort of reduction in temperature or air movement (and damping down the paths)

We did have tomatoes successfully, but they required watering in the morning, damping down at least once during the day (twice on the hottest days) and watering again at night to keep the soil moist enough for them.

[Cutecumber]

Thanks for your comments.

Even with everything open - doors, vents, the lot - it will get to 38 degrees very quickly. Last summer, that was the temperature that it spent most of its time entertaining!

I just wonder if it is the location of the house itself that is the problem - can't get enough air flow or perhaps the angle to the sun.

Interestingly, it's not really what you would call a sheltered garden and the wind is a problem.

[Alison]

I've not had this problem and have only reached those temperatures on the very odd occaision. I have auto opening roof and side vents on opposite sides of the greenhouse and if it's very hot open the door myself for the day. Just a thought, what is the floor of your greenhouse, are you getting a lot of heat reflected back up from there? Mine has gravel paths and don't seem to cause a problem. In addition, I provide net shading on the sunny side during summer months.

[Cutecumber]

Thanks.

There is a central path of stone slabs and it's gravel on each side - we don't use the soil itself. There is quite a lot of "stuff" in there - pots, tools, old but this comes out during the summer months so air can circulate better.

It's peaked at 33 degrees this week, but I'm really hoping that was with a closed door!

[Matt.]

How confident are you that your thermometer is correct? It sounds so high especially at this time of year.

All I would do is try and keep the door open as much as possible. If you have a problem with cats getting you get get stiff wire mesh panels from DIY stores. I have one that just nicely fits the gap to stop them getting in but doesn't prevent cooler air entering like a board would.

[Cutecumber]

Good point, Matt, but I am confident the thermometer is ok - it's a remote sensor system that I am using, but there is a traditional max/min thermometer in there too and it gives pretty similar read outs.

That 33 degrees was probably quite a brief episode - it has been very sunny down here on and off recently and I expect that the vents weren't open for some reason. Might have been during the morning before anyone opened up. It's only been 12 or 14 in there during most of the days this week, which is clearly more normal.

I suppose it's fair to say that we have had some stinking hot summers here over the last four or five years - I'm not far from where the highest temperatures are recorded in north east Kent and it's a built up area, so it hangs on to that heat more than some places.

If it didn't potentially cause so much disruption, I would be tempted to move the greenhouse into the centre of the garden to try and improve the airflow.

The irony is that any vegetables I manage to get in the ground suffer from shading rather than over-heating! Typical!

[smallblueplanet]

Have you tried growing toms in there? (We've got Terry's book too and although he's a guru, he's not always right. ) We grow toms/aubs/peppers/chilis and a (Turkish) cuke in ours (and a melon last year), they didn't seem to mind the heat (kept very hot all summer - I love my aubs!). However we do grow in the greenhouse borders so maybe that helps?

[Digger-07]

I resorted to taking a couple of the glass panel out of the sides to get some cross ventilation. It worked last year and only needed some wicker panels over the holes when the wind got up.

[beefy]

Your lucky Cutecumber .At work Ive got a double glazed greenhouse / conservatory type thing. Before you all get jelous and start ignoring me there is a downside.There is no automatic ventillation and temps can get to over 50 dec C.Yes thats the sun hitting it full on for a few minutes and raising the temp which is allright through the week when Im at work and have the windows open .But I finish at 12.00 on a Friday and when I leave I have to close the windows and lock the door as you can acess the school through the greenhouse and thats it to Monday morning.Usually on a Monday morning the max/min thermometer has pushed the max to the top of the tube.Will take a pic tomorrow to let you all see.
All I can do is water everything and keep the humidity as high as possible to help as best I can.

[Norm]

I had a similar problem a couple of years ago. The thermometer read 44 degrees c. It felt hot in the greenhouse, but not that hot, so I dangled the thermo from one of the wooden staging support legs and out of direct sunlight. In 15 minutes it was down to 28. So I was just wondering ........?

[beefy]

No it is that hot the therm is hung in shade where the sun cant get to it.

[Snadger]

No it is that hot the therm is hung in shade where the sun cant get to it.

Spread some Eucalyptus oil on the staging, chuck a bucket of water inside, strip off, and use it as a sauna!

[nick the grief]

Hi CC,

Part of the problem is the size of your greenhouse. Small houses (8x6 or less) fluctuate greatly they heat up quickly & cool just as fast as there isn't a large volume of air in there.

I have an 8x6 & a 12 x 8 and the small one gets hotter than the big one despite being behind it !

Tomatoes will grow in yours OK providing you keed the humidity up & they don't dry out. They frow the damn things in spain & Italy & I'm sure it gets just as hot there.

As to Mr Marshall, If I remember right he has a greenhouse bigger than mine so thats how he could keep it so stable.

[Cutecumber]

Thanks for all your ideas - even the sauna one! lol!

I am interested in the concept that the size matters (sniggers!) - I never really thought about that. Oh well.

I have tried growing tomatoes in there, but they don't get on very well - and it obviously produces serious watering difficulties.

Perhaps when Mr Marshall gets fed up with growing tomatoes he could pass on one of his super-duper greenhouses to me