I'm just wondering ... how many plants are you talking about, where do they normally live/thrive and how moveable are they at short notice?

Hi Alisonoz

around 40 plants - they are outside in a well shaded area from september to mid april, they are movable, but as there are a lot of them i would prefer not to have to move them every night!!

Well, at those temps, you might even have to consider incorporating a heater in the greenhouse. Didn't know there was anywhere in Brisbane where temps got that low!

B

Hi Funnelweb,

most of brisbane stays much warmer than that during winter, however quite a few of the western suburbs of brissy get really cold, especially those close to the mountains/on the mountain. We had the fireplace going most of the winter last year (but that may be because its very pretty and we are very soft! hehehe)

in that case, perhaps keeping the plants indoors for a few months, supplemented with a growing light might be better/cheaper?

It's often the wind chill factor more than the ground temp. I'm luckier in that I have plenty of patio area which, with brick house walls and paving under, seems to hold some warmth as well as being more protected.
I really don't know.... The "hothouse" aspect is more to do with day temps I think, and holding humidity, I think as Bill says, the plastic doesn't really "hold" heat all night.
But, unless I misunderstood, you meant less than 5c a night, not minus 5? If you actually had frosts below zero that's another matter.
I am just wondering firstly if you can put the greenhouse somewhere where it will get filtered light instead of stewing them, and again, can you vent it in case you actually get warm days in winter.
The cost, too.... but that's up to you, there might be cheaper options but you have to be a bit DIY with frames and tunnels.
Unless of course you can pick up something cheap on gumtree or e-bay, you have a bit of time to plan.
I tried some options last year as I'd bought some tropical palms that I was afraid of losing, but I didn't need as much space as you, so I found a couple of plastic blanket boxes that I put round the plants at night, I found a hot water bottle hardly lasted any time, you might need a small brazier in there? Sorry a lot of waffle, I think you need input from southerners

I live in the Southern Tablelands (near Canberra) - winter temp down to -5/6/7 and heavy frosts.

I have a small walk-in plastic greenhouse which spends winter on a concrete slab, up against a north facing brick wall. I grow chillies in pots - the greenhouse extends their life for several weeks, but eventually the cold kills them.

Other plants, like geraniums, seedling kaffir limes, etc., have survived over winter.

Our night time temp drops quite low in early winter and spring, while the days can be quite warm. Important to roll up the door in the morning, otherwise the plants get cooked.

I had a neighbour with a small glass house. She used to use a kerosene lamp at night. It provided enough warmth to keep her collection of tropical orchids from carking it.

The plastic greenhouses are quite reasonably priced. It would be worth having a go. But make sure you anchor it down so it doesn't blow over. Personal experience tells me it's a pain to have to repot everything :-)