Lay the 120-inch and 96-inch willow branches out on the ground. Cut off any shoots coming out from the main branch on each willow branch with branch cutters so that you are left with a straight branch.
Push one 120-inch branch into each corner of the garden bed so that the ends are 24 inches into the soil. Use a tape measure to evenly space three more branches in between the corner branches on the 96-inch sides of the garden bed. The branches should be spaced 24 inches apart. Push the ends of each branch into the soil 24 inches.
Bend the ends of each branch over to the center of the garden bed so that they overlap the opposite branches across from them by 3 inches. Wrap garden twine around the overlapping bent over branches and tie the twine in a double knot to secure the branches together so that they form a hooped frame. Cut the garden twine off close to the tied branches with shears.
Lay one 96-inch branch across the top of the hooped bent branch frame. Wrap and tie the branch to the tied-off bent-over branches at each point where they intersect the 96-inch branch. Cut off the extra garden twine close to the knot.
Position one 96-inch branch halfway between the top 96-inch branch and the ground level on each side of the hooped frame in the same way that you secured the 96-inch branch in Step 4.
Pull the poly tunnel cover over the frame and clamp the ends closed with two spring clamps.