Posted: 26/04/2016 at 12:59
Sophie the best netting for brassicas is either enviromesh (expensive, but very good) or debris netting which when bought on a roll is cost effective. The shade netting will be similar to debris netting so should work fine. I think Steve 309 advice a bit strange as butterflies will literally fly through 1/2" netting and fine netting doesn't cut out enough light to worry about.
If you want to make your own hoops pop down to the builders merchant and buy a roll of 20mm water supply pipe, you can then cut it into whatever length of hoop you want. I've used this method for years and it keeps out the butterflies and pigeons. Please make sure it's taut and secured to the ground so that birds won't get trapped. Another thing to consider is giving enough room above your crop to stop the butterflies landing on the netting and laying eggs through the mesh. Final thing to mention is to really firm your soil, brassicas need firm soil and raised beds are normally a bit loose for that purpose.