It will depend where you are but I start all my seeds off in our unheated greenhouse. I don’t start until mid March, this year a little later due to Covid. All my tomato, pepper, cosmos seedlings are racing away. Will be doing the rest today as still catching up.
We over winter our lemon tree and dahlias in the greenhouse. Most years they are fine. Admittedly, this winter has wrecked havoc and we’ve lost our dahlias and the lemon tree has lost its leaves but it will recover ( looks like Monty’s on GW and his in an an insulated shed).
Edited - oh just seen that you are in Surrey. Our night time temperatures are similar but it’s warmer than that in the greenhouse, especially if we’ve had a sunny day. I did wrap some bubble wrap around my tomato and pepper trays but only until they started germinating
If you have a garden and a library, you have everything you need.”—Marcus Tullius Cicero East facing, top of a hill clay-loam, cultivated for centuries (7 years by me). Birmingham
I planted seeds of tomatoes, beetroot, cosmos and and basil and put them straight in my unheated greenhouse at the end of March and they have all germinated without using heat or using window sills in the house. I did this last year as well and they were all fine. If temperatures are going to be low overnight, as they were last week, I cover them up with bubble wrap.
During the day the greenhouse gets surprisingly warm even when temperatures are nothing special outside so there is hope!
The only failure is my Shirley variety of tomatoes, all my other varieties have germinated but not one single Shirley seed has, but they were planted in exactly the same way as the others so no idea why 🤨
My OH starts to sow his seeds( many flowers though) in late March early April in the greenhouse. Unheated. He sowed cosmos, marigold, Californian poppies and some astrantia and has now two tomato plants. All doing well and have have germinated except the astrantia and the marigolds which he only put in last week. We have many more seedlings growing on the windowsill too which we sowed weeks ago. As others have said though, it does depend on where you are and what your conditions are. We have a green house which is in the sunniest part of the garden and it gets lovely and warm during the day. I have put a potted dahlia in there too but nothing happening above soil level yet.
I have a tiny unheated greenhouse, and have only moved my nasturtium seedling in so far. Cosmos going in as soon as the rain stops long enough for me to pot them up. I also have some pots in there with a "plants for pollinators" mix, but no germination as yet. I am in the north west and night temps still not much over 5 degrees. I'll sow again and skip the windowsill in another week.
We have always planted all of our seeds in our unheated greenhouse, in Mar/Apr. Only if there was a hard frost would we consider bringing them temporarily indoors for the night. We are also in the South East, and have never had any problems with this method.
Even when there has been a mild frost, I don't recall ever losing any seedlings that were left in the greenhouse overnight. We also have Dahlia/Canna/Calla Lily's in pots in the greenhouse, and they remain there all Winter, regardless of temperatures. They always survive, we have never lost any. The only thing we lost this year was all of our succulents (in one night!). I would get on planting and make the most of your greenhouse now. One of the advantages of having one, is that you can start sowing much earlier.
The problem with any greenhouse/cold frame or similar at this time of year is - they get very, very hot when the sun's out, even just for a few hours, then the temps drop rapidly at night. That's what's difficult for seedlings and small plants. The swing between the two - far less of a difference in a house. A consistent temp is what's best, but that's why there can be a bit of juggling involved....
It's a place where beautiful isn't enough of a word....
I live in west central Scotland - not where that photo is...
Over the winter and early spring my greenhouse is used for the slightly tender plants in pots and to house pots which wil crack if frosted. In Spring I germinate seeds in the house but transfer all to the greenhouse after germination. If you have a heated mat or propagator in the greenhouse you can keep even tomatoes at a reasonable temperature without spending a fortune and they grow better in the extra light than on a windowsill, although a bit nore slowly. If it gets really cold (and I use a max-min thermometer to check what is happening in there) you can add a layer of fleece overnight.
@LilyW I jumped here from my Cherry tomato discussion. A very wise member of this forum wrote when I started membership, "I grow everything in the greenhouse. If it doesn't survive in the greenhouse, it will not survive outside". Like tomatoes that get pressed in soil after they were harvested, and choose the right moment to germinate in Spring, or last years episode in Beechgrove "when the weeds start to germinate, it's time to plant". I have my pots with seeds in the greenhouse since March. Some decided to come now in mid April, others germinated earlier. Everything is late this year, and I have the suspicion it will not get better weather-wise.
Plants tell you when it's the right time and they want to show up.
I keep my greenhouse door open and the window ajar all year so the hedgehogs can get in and out. It's not heated but against a brick wall in a sheltered spot.
I tend to think we over complicate things, each year I leave dropped tomatoes on the ground of the greenhouse and they turn into plants by the next year. I have a soil floor with a central path made with old storage heater bricks which retain heat from the sun.
I’ve got a propagating bench with a soil warming cable set to 8 degrees at one end of my 8x6’. On the bench I have 4 varieties of fairly mature chillies, amaranthus, cosmos and Orlaya grandiflora seedlings. Off the heat there’s dahlias, begonias, sweet peas and 3 varieties of tomatoes. At the side I’ve got 2 toms in their final pots with flower trusses and a 5l water container for a bit of night heat. It all gets put to bed with fleece at night and uncovered in the morning. I’m in central belt Scotland and have done this for many years with slight leaf scorch as the only frost damage suffered.
I always keep an eye on my weather app just in case there’s another beast from the east or something and then plan b (whatever that is) would swing into action.🤔
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We over winter our lemon tree and dahlias in the greenhouse. Most years they are fine. Admittedly, this winter has wrecked havoc and we’ve lost our dahlias and the lemon tree has lost its leaves but it will recover ( looks like Monty’s on GW and his in an an insulated shed).
Edited - oh just seen that you are in Surrey. Our night time temperatures are similar but it’s warmer than that in the greenhouse, especially if we’ve had a sunny day. I did wrap some bubble wrap around my tomato and pepper trays but only until they started germinating
East facing, top of a hill clay-loam, cultivated for centuries (7 years by me). Birmingham
As others have said though, it does depend on where you are and what your conditions are. We have a green house which is in the sunniest part of the garden and it gets lovely and warm during the day. I have put a potted dahlia in there too but nothing happening above soil level yet.
I also have some pots in there with a "plants for pollinators" mix, but no germination as yet.
I am in the north west and night temps still not much over 5 degrees.
I'll sow again and skip the windowsill in another week.
Even when there has been a mild frost, I don't recall ever losing any seedlings that were left in the greenhouse overnight. We also have Dahlia/Canna/Calla Lily's in pots in the greenhouse, and they remain there all Winter, regardless of temperatures. They always survive, we have never lost any. The only thing we lost this year was all of our succulents (in one night!). I would get on planting and make the most of your greenhouse now. One of the advantages of having one, is that you can start sowing much earlier.
A consistent temp is what's best, but that's why there can be a bit of juggling involved....
I live in west central Scotland - not where that photo is...
A very wise member of this forum wrote when I started membership, "I grow everything in the greenhouse. If it doesn't survive in the greenhouse, it will not survive outside".
Like tomatoes that get pressed in soil after they were harvested, and choose the right moment to germinate in Spring, or last years episode in Beechgrove "when the weeds start to germinate, it's time to plant".
I have my pots with seeds in the greenhouse since March. Some decided to come now in mid April, others germinated earlier.
Everything is late this year, and I have the suspicion it will not get better weather-wise.
Plants tell you when it's the right time and they want to show up.
I ♥ my garden.