|A Brussels sprout showing blown sprouts|
Brussels sprouts sometimes produce open, leafy sprouts instead of hard, round buttons. These are known as blown sprouts. The causes are similar to the factors responsible for heartless cabbage - not enough well-rotted organic matter in the soil, too little consolidation of the ground before planting and failure to plant firmly.
Remove blown sprouts promptly.
Make sure the plants are kept well watered during dry weather and avoid planting too closely. Choose an F1 hybrid variety.