Greenhouse problems..


Last spring I purchased a small greenhouse that would fit on my deck. Last year was also the first year in over 25 years that I lived in place that I could have a garden. This year I set up the greenhouse and moved all of my tomato, green peppers, plus a couple of different flower seedling I had started in the house. Its a small walk-in type so I don't have any heat or fancy things going on in there. Anyway, the heat climbed to over a 100 degrees on a very cool day and I lost almost all of my tomato plants and several green pepper plants. Most of the flowers survived. My question is do you regulate the heat in this type of greenhouse? Should I leave something open just a bit on cooler nights knowing it will be sunny in the morning? Would like some advice on how to keep the heat just right. Thanks, Susie

Calgary, AB(Zone 3b)

Firstly, plants shouldn't be moved from indoors to outdoors without first being "hardened off" to sunlight. Indoor light sources, even metal halide, don't have the intensity of sunlight and tender, indoor-grown plants are "sunburnt" (foliage turns white and dies) when suddenly exposed to bright sunlight. I find setting the trays in the shade for a week does the trick (even shade is more intense light than most indoor light sources) - after that, the seedlings can take full sun here. Shade cloth also works very well, and is what I'm currently using over my outdoor wheeled plant stand. (The wheels are to bring it into the garage if there is to be frost overnight.)

The thing many people don't realize about greenhouses is that they require ventilation for many plants to do well (e.g. tomatoes don't do well over ~85 deg F), and the more useful ones will include an air-exchanger fan(s) to push hot air out and draw cool air in, or panels that at least tilt open to allow an air exchange. If your greenhouse doesn't allow that, or can't be modified to allow it, it may only be useful in the early season for seed starting, and to extend the seasons. It would probably help to use shade cloth with it.

Contra Costa County, CA(Zone 9b)

In a simple box sort of greenhouse you will have to be the temperature regulator, watching the weather and acting accordingly.

If the plants are just a little bit too cool, they grow more slowly, but are fine.
If the plants are just a little bit too hot, they die.
So err on the side of 'Keep it a bit cooler'.

Get a thermometer. Watch it through the day, as the sun rises and falls.
If it starts getting near 80*F, put shade cloth over the greenhouse, and open whatever flaps are there for more ventilation.
If there are not such flaps, then build them. Some near the top to allow the hottest air to escape. Some near the bottom to allow cooler air to enter.
In the latest part of the afternoon, as the sun is leaving, remove the shade cloth, and perhaps close the vents. Depends on the weather forecast.

You may be able to figure out how to place the shade cloth over part of the greenhouse that faces south and south-west, and allow the east and north-west sides to get sun. This would mean morning sun and late afternoon.

In the cooler seasons, even if there is a hint of frost, you will still be able to keep the greenhouse warm enough through the night, but you will not be using shade cloth, and you will probably be keeping the vents closed.


My greenhouse is simply plastic over a frame, 5 x 5 x 6. No fan etc. I do think you are right about slowly getting them use to the greenhouse, but mostly everything did fine moving directly from in-house to greenhouse (note to self for next year). My greenhouse has 2 doors with screens and outer plastic and 2 small (maybe 3 x 3) vent holes at the bottom on each side. I did put the shade cover over after I realized just how hot it had gotten in there. It was 105 degrees by 11:00 on a 50 degree day. Just a few days before that our nights were about 30 degrees and the greenhouse kept everything from freezing. Should I just leave the bottom of the one or both doors unzipped at night when its not too cold?

Russell, KY(Zone 6b)

Should I just leave the bottom of the one or both doors unzipped at night when its not too cold?

No, you should open it earlier in the morning :)


I will be opening the greenhouse very early in the morning from now on. Today, I actually placed all of the plants in the shade on my deck with mid 70's sunny weather. This is the 4th time I've done this but today was the longest I've left them out. They really perked up. Thanks for all the advice. It sure helps.

Contra Costa County, CA(Zone 9b)

If the weather is OK through the day it might be better to keep the plants on the deck, but have some shade available. Perhaps drape a light sheet or the shade cloth over some patio furniture.
Then it will not get anywhere near so hot.
Better ventilation, too.

Will the plants be going into the garden soon? Knowing that they will soon not need the greenhouse makes having to check on it several times per day easier. "Oh, it is just for another week or so..."

I have hoops over the vegetable beds, and can drape anything I want over those hoops:
Clear plastic in the cooler season, shade cloth in the heat, or use the hoops to support a trellis for peas and beans. I know some people could use them to keep rabbits or other critters out by adding some mesh to the hoops.


The plants are in the shade on the deck as I have an awning. It will be about 2 weeks before I can put the plants out at the earliest. My last average frost date is May 15th. So hopefully, not to much longer with the greenhouse drill. Thanks for all your great advice Diana.

Ayrshire Scotland, United Kingdom

Hi Gracesmum, as you have already by now gathered, having a greenhouse is a learning curve, you need to go by the weather and the type of plants you have inside the greenhouse, MOST plants don't need really hot temps other than tropical ones and even those would have there own way of protecting themselves from intense heat such as greenhouses have, but avoid the dry intense heat you have experienced
What happened in your case was there was no ventilation, so hot air built up and had no escape, no air movement and this caused the plants to just give up and die. By the way in an emergency, I can lay some old newspapers,(single sheets) over the plants to give the plants shade from strong sun light ad remove it as the sun moves away.
All the plants you are growing and lost, I have to grow inside my greenhouse all season due to our climate we have here in UK, so my regime is, open vents when you watch the forecast's and it is going to rise above the needs of the plants you have got inside the house, the seed packets tell you the temp required for germination and after care, buy a cheep thermometer to leave inside the house so in the morning when you open the vents /doors / windows etc, you can open one- two or more depending on the forecast or temp.
What the seedlings, then mature plants need more than very hot temp is constant air flow, plenty light, nor necessary hot sunshine at all, yes they need warmth in the night but not cooking temps, even in cooler days, I throw open the doors and windows as the sun can cause more damage the the coolness of the day temp, night temps are harder to gage so best to close the roof vents but leave the sides or lower vents open just a bit, IF it gets too cold or very windy, then nip outside and close a few more, also covering with sheets of newspaper gives protection from clod as well as from sun light burning in day time.

Greenhouses take a bit of time to get used to and to learn how fast the temps can alter up or down and me telling you what is best is like me telling you we had a heatwave here and and in your area, that could be just a coolish day so you will have to go by your instinks, as someone else mentioned, plants very rarely die from cold at this time of year, they just slow down and I would think in your area, once the plants are growing well, they need set outside for a few hours each day to harden them off and get used to the outside temp, bring back inside at night and do this for a few weeks, there after leave outside all time unless there is a sudden drop in temp to very cold nights, even then, maybe covering with newspaper would give enough protection for a one off drop in temp.

Your greenhouse needs to be cleaned out each year after winter storage of plants as these greenhouse environments can harbour lots of bugs, diseases or fungal problems that like to be given the nice warm damp area to spread spores or lay eggs or just be dirty from water splashes etc.
Don't get disheartened by one lot of failed seedlings, there is still time to sew more seeds and that's what's good about a greenhouse, it allows you to grow stuff either well before the outside temps allows you OR after that time as the temp hurries stuff on faster.
Just take your time to enjoy your new growing environment, this will allow you to grow more things as you have the perfect place to store or protect some very tender plants that otherwise could not live outdoors.

Hope this also helps a bit and you soon get used to how to control the differing and quick rise and fall in temps inside the greenhouse.
Best of luck. WeeNel.


Hello WeeNel, my little greenhouse can't even go up till early April at best. Its just plastic over a cheap frame that you take down when your done using it. We still get plenty of snow in my neck of the woods that could crush my little greenhouse, not to mention what the winds of March would do. I just got it last year and promptly burned up most of the plants I had bought and placed in there.

This year, I decided to start most of my vegetables and flowers from seed to save money. I've learned so much already this year! First of all, start a whole lot earlier. Not knowing, I started some things a bit late (especially my petunias, I think). Because I can't use my greenhouse earlier, I've had to start and keep all the plants here in the house. I did not use any grow lights, and my poor brandywine and sweet 100 tomato plants are very leggy. (I'll have grow lights on my Christmas list for Santa this year).

Once the weather was good enough for me set my greenhouse up, I spent a week "hardening" off the plants to the greenhouse. Most did well, till that dreaded day that I didn't check early enough and the temperature reach over 100 degrees in there. Since then, I've kept a shade cloth over the top and I've set the plants out under the shade awning on my deck everyday. On the cooler days, I've left them in there with both doors open (only the screen closed) so there is plenty of air flow.

Come this Saturday, Sunday and Monday our temperatures will be at or near freezing at night with 50 degree temps during the day. So I intend to just open one door and keep an eye on the temperature during the day and close it up at night. Beginning on Tuesday of next week the weather is suppose to be back in the 70's so I'm planning on setting the plants out into the sunshine a couple of hours each day as they will be going into the garden the weekend of the 17th.

Again, I've learned so much this year. I'm most excited about how well I did with germination of my seeds without grow lights and heat mats. I've had overall, about 85% germination rate. With the exception of my tomato plants, my pepper plants, geraniums etc are looking wonderful. And I've even learned to tell the difference in the leaves between the different plants.

Thank you for your input. I'll take all the advice I can get. Susie

Ayrshire Scotland, United Kingdom

Susie, 85% germination rate is very good indeed, honestly, I really don't think you will get better than that, especially when and how you did the germinating process.

I have never used Lights indoors to germinate seeds and after around 40 odd years, I don't intend to even though these have become very popular here in UK, I just start off a couple of weeks into February and do the seeds then, stuff like peas, beans ect.
I do some one day then 3 weeks later sew some more from the same packet so they don't all reach maturity at the one time, I prefer to pick and eat my veg rather than a glut and have to freeze ect, but were all different and work in different environments. I forgot to mention I dont use heat mats either but sometimes have to plug in my electric propagator with clear lid IF winter is getting too long and spring has got lost along the way.

I'm just delighted you have mastered your little greenhouse, no matter the size of g-house, the principles are the same regarding temp, ventilation and site where the house is, mine for instance runs North -South, Plants get better spread of light and for longer, this is because even in the hight if summer, I don't get enough sunlight and our seasons are too short for outdoor tomato's, cucumbers, peppers and tender stuff like that, so you will also learn how to take advantage of having your greenhouse, when to use it and how to control its temp, vents and lights. Do think in winter about a normal 40 watt light bulb as a means of light and helping to keep frost off the plants, most ordinary household bulbs will give off a little heat and can be enough to stop frost settling on the plastic or glass depending on your set up.

You will end up with a list for Santa that is like a roll of wall-paper ha, ha, ha, believe me, as you gain more experience and your confidence grows with it, your shopping and searching computer garden suppliers will be huge, just remember the old saying, a garden was not built in a day so take the time to enjoy your plot and you will have less worries, no gardener has a great year every year especially with veg.
Weather, soil, seeds and time all play a large part, just enjoy.
Best regards and good luck. WeeNel

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