Dividing the Flower Bed

Lord Emsworth has a large circular flower bed in his park, whose centre is marked by a single tall rose stem. It is divided into two parts by a straight line through the centre. He now wishes to divide it into four equal quadrants with a second diameter at right angles to the first. The gardeners can mark straight lines by sighting along lines of pegs and hammering in a fresh peg where two lines cross, but cannot measure distances or draw any kind of circular arc. How can they find the second diameter?