Polythene Grow Tunnel Early Start. Polythene Grow Tunnel - 9.8' L x 18" H - Ideal for early starts - creates a warm and humid environment. Pre-assembled and ready to use. Sturdy wire frame. Tough polythene cover. Includes elasticated end closures and ground pegs. Plants can be accessed via the lift-up sides. Ideal for spring and fall use. Protects against frost and severe weather. Warms the soil for earlier spring crops. Gives some protection against plant pests.