Earthing up , he pointed out , is more about excluding light ( which turns spuds green and inedible ) , than increasing yield . • Peter even went so far as to say that those people who bought deep bags last year could try a different tack : simply roll them down a bit , use half the depth of compost this year - and halve the number of potatoes planted . • Indeed , the temptation to grow too many potatoes in each container should be resisted at all costs , the chaps agreed .