Most mum varieties hold up better in the garden if pinched a time or two. Otherwise they may flop over when in bloom. Pinch back mums when they have grown to about 6 inches tall. Pinch back just the growing tips to promote more compact growth. Repeat pinching in late June if the plant is still leggy. Avoid pinching again after the first of July, to avoid removing developing flower buds. Alternatively, you could grow cushion or carpet mums, low-growing types that get no taller than 8-12 inches. These shorter types form a solid mound without the need for pinching.