Collars for cabbage root fly

Good morning Nobbie,  I think the sharp edges of the roofing felt (if I'm thinking of the same stuff) may graze the stem and allow in infection.  Carpet underlay sounds better.

Cheers,  Tony.

You can buy specific collars for this purpose, I’ve used them before or I’ve made them from underlay but neither are fit to the set again in my experience, the ones I now use were made by my dad out of damp proof membrane cut into a square approximately 3” with a cut to the middle & then a few snips around the centre so it fits snugly around the plant stem. He then bent a piece of green coated wire to slip on top of the plastic material to anchor it into the ground. It’ll be 17 years in October since he died & I've been using them ever since 

I sometimes use torn rhubarb leaves. They can be fitted snug round the brassica stem and the outer edges buried just below the soil to prevent lifting. Apart from forming a physical barrier, cabbage root fly doesn't like the smell of rhubarb leaves and oxalic acid within the leaf helps to ward off club root.
By the time the brassica is established the rhubarb leaf will have decomposed into the ground so there's no tidying up to do later.

Interesting discussion as I tried collars but found the faff of putting them round spring cabbages, sprouts, cauliflowers, winter cabbages, broccoli and over wintering cauliflowers was more than I could be bothered with and I went to a sturdy fleece which has worked perfectly for me.

I like the rhubarb leaf idea and might try it in a limited manner.