Fat hen, sometimes called white goosefoot, is a very common annual weed, appearing in both cultivated and uncultivated bare soil.
Plants grow quickly, and each one produces thousands of seeds each year, so quickly covers beds and borders and grows through cultivated plants.
These seeds can remain dormant in the ground for several years. And as “One year’s seeding means seven years weeding”, allowing plants to flower and produce seeds, means several years of trying to control and remove it.