A cold frame means you are going to be checking on things and opening and closing it at least twice a day. You can't just plant in it and leave it. It has to be opened on a daily basis so your plants don't fry and closed up at night to retain some of the heat when the sun goes down. Even on a cloudy day it Must be opened a bit because it only takes a bit of sun to heat up that small place

I presume you are going to plant in the ground not use pots. If that's the case don't use a soilless mixture. They are meant for pots not soil. What you want is good soil like you'd find in a well tended flower bed. First make sure there is nothing growing in your raised bed. Pull out any weeds and grass making sure you get all the roots. Loosen the soil by digging and top dress it with compost and rake it in. Now you are ready for planting.

Make sure when you make the cold frame it either has a slant or an arch to run excess rain water at night or if you want to keep it off the bed. Also include some way to open just part of it when you don't want it open all the way. Such as a cold but sunny day. Put the top on and leave it closed for a week or 2 before you plant to warm the soil. This will also kill any weed seeds that are there.

Note--A cold frame heats up fast in the sun. It only takes about 20 minutes with the sun shining on it to reach a too hot for plants stage. When I used cold frames before my greenhouse was built I'd open then early in the morning and close them around 4 Pm to retain the heat for the night

Agree that in your zone controlling the heat build-up will be the major factor. If you can swing an automated temp control vent it will help a great deal. You'll be amazed at how quickly it will heat up inside.

As to soil or mix - it really is your choice, especially for perennials as they aren't as very picky. I might be inclined to incorporate some soil-less mix into the soil just to lighten it up a bit (depends on the quality of your soil) but otherwise just plant them in what they will be growing in when transplanted.

Thank you both for the information! I was planning on using a soilless medium to lessen the risk of dampening off. Is there less risk of dampening off when growing in a cold frame vs. growing indoors? I am thrilled that I won't have to get rid of all of that good soil in my raised bed, which is a tricky thing to do in an urban area.

And thanks for the tip about the temp controlled vent. They're a bit pricey, but with my historic tendency toward forgetfulness, I think necessary!

Thanks again.

yes there's less risk of damping off. In a pot the moisture is contained by the pot. In the ground the moisture can spread.

Agree. Assuming you don't over-water there is less risk than in containers. Of course if you do over-water and you allow stagnant heated air to build up in the frame then all that advantage is loss. When the fungus is present and the conditions are right damp-off can develop in even an open air garden bed.