I have used a similar method with good results, but I use a slightly different approach. Since peas are basically a cold weather crop anyway and very frost tolerant once they are up, I start them under a cold frame for better germination but place the trellis just outside the back of the frame. In this way the trellis does not interfere with the placement of the cover on the frame, and the tendrils very quickly reach the trellis anyway. Any time after the first of May peas in the BC interior (Zone 3) are safe. And most years they can be started in cold frames early in April. You can then do a second sowing late in May.

I was under the impression that the cold frame had to completely enclose the plants. Does frost get in from the sides if you leave a gap?
Thanks in advance.

I need some cold frames can any one tell me which is the best way to get these frames.

I need some cold frames can any one tell me which is the best way to get these frames.

They are relatively easy to build yourself. Lots of plans on the Web and not too costly. Cheaper sill if you can pick up old windows, glass or screen doors from your local landfill.

Back to the original question. Once you have your raised bed build your cold frame. You will need this in the spring. Build it so that the top is easily removable. The trellis is not needed until much later until the pants are well in early summer. Put any kind of low trellis work within your frame it can be netting supported by sticks. It only needs to be a few cm high to get them started. When you are SURE the last frost has passed remove the top and put in your larger 1.5 metre permanent trellis work.

This year, I experimented with plastic covered raised beds, and a cold frame I have made from scrap lumber and 3 old windows I have laying around.

First, the plastic covering. I had a partial roll of 6mm plastic sheet (like vapour barrier) that I had left over from a reno project. I bought some pvc pipe and some brackets. I attached the brackets to the raised bed I have, bowed the pvc pipe over the bed and into the brackets (looks like a covered wagon lol) and then spread the plastic sheet over the pipe. This sort of thing is called a tunnel or some thing, easy to find exact instructions on Youtube/Internet.

As a comparison project, I slapped my cold frame I had built on another raised bed I have, beside the plastic tunnel one. I wanted to see if there was any difference in how they worked.

I started lettuce and some bell peppers on April 8th this year. I live in Southern Ontario, where everyone is convinced nothing should be planted till May 24. I had the lettuce come up and do well in both beds (there was a bit of a two day snow in my area in mid-April) and they were already harvested about a week ago.
The peppers, much to my surprise, came up -- although reluctantly. They seemed to stop growing for a little while, and now they are going again. The ones in the cold frame did slightly better (they are taller), but only by about an inch in height.

I've taken the tunnel and the cold frame down around May 24. I've since lost my peppers (too much rain...and my raised beds are made of wood and they couldn't really cope with the amount of water to be drained it seems). I've replaced them with some starts I've had in the house.

Next year, I'm definitely going to be starting out with the plastic sheeting and pvc again. I didn't think this setup would keep it warm enough in the beds to germinate, but they do. I'm excited to see what happens next year.

As a side note, I planted sweet corn this year. Half my plot is on raised beds (just mounded up and leveled off - about 2 feet across, so 2 rows of corn per 'mound') and the other is just straight planted. I report that despite it taking longer (couple of hours) to do the mounding, my corn is coming along nicely. But, the straight up planted corn is learning to swim, as the flat area got completely saturated and pooled up with rain water.