Vitax Biodegradable Cabbage Collars Are A Simple And Efficient Non-chemical Way To Protect Your Spring, Summer And Winter Greens From Cabbage Root Fly Attack.vitax Cabbage Collars Protect Your Plants From The Cabbage Cabbage Root Fly By Preventing The Adult Fly From Placing Eggs In A Sheltered Position By The Base Of The Plant.after Planting And Firming The Plant In The Ground, Slide The Collar Round The Stem So That A Snug Fit Is Achieved. As The Plant Grows And The Stem Thickens, The Cabbage Collar Opens To Avoid Any Restriction To Growth.30 Biodegradable Cabbage Collars That Fit Snugly Around The Base Of Brassicas To Protect Against The Cabbage Root Fly Whilst Keeping The Soil Warm, Moist And Weed Free.biodegradable Cabbage Collarsprotects Against Cabbage Root Flyencourages Rapid Plant Establishmentsupplied In A Pack Of 30measures 8cm Dia.