Any change in their environment, especially any drastic change, requires some gradual adjusting time so basically the answer is yes. You need to "harden them off", let them gradually adjust to the GH.

On sunny days even an unheated greenhouse can get quite hot inside and nights out there are bound to be cooler than what it was in your basement. So a couple of cloudy, overcast days where nights aren't especially cold would work best but you'll still have to watch them closely.

Dave

Yes is the short answer. All plants need to be transitioned when you are changing the conditions they will be in. When moving from a basement that is in the upper 60s ( in my case ) I begin by putting them in a room in the 50s for several hours a day for at least a week before I move them to the colder outdoor or greenhouse areas permanantly.

BTW - greenhouses also can get very warm and the plants can benefit from being conditioned to warmer than home temps as well as colder imo.

An unheated g'house can get very cold too. This is the time of year I'd be moving my cole crops into an unheated greenhouse but mine are quite large and I usually use an insulating barrier if I expect the nights to be 'freezing' as opposed to frosty. Be careful with things like cauliflower, however because if it starts alternating from hot to cold to hot, it'll button up on you.

Thank you all for responding.....very useful feedback! I will indeed slowly acclimate them to their new home....

Jessica

I'm in La Crosse and i'm already planting onions and broccoli in the ground. I see that temps here are suppose to stay above 28F. I use a frost cloth should the need arise.

I think they'll be just fine inside a greenhouse. The nights in the teens/low 20F's should be done for...lets hope. Vent during sunny days or you'll fry those seedlings.. I had 104F in my low tunnel the other day.

They must adjust to UV light as well.

Dan

Dan-

I'm pretty sure a greenhouse blocks UV, if its plastic. They usually treat the plastic with UV blocks so the plastic doesn't fall apart in a year. I'm not sure about glass.