Ask Monty

Q I’ve had black spot on my roses despite using fungicide. Can you help? Mary Hutchins, Lancashire A Black spot is caused by the fungus Diplocarpon rosae, which causes leaf markings and often leaf drop. It develops new strains quickly so it is very resistant to fungicides, which by and large are not very helpful. Prune to help good airflow. Collect and burn fallen leaves, rake round the plant in autumn, leave soil exposed all winter and mulch heavily in spring. Q I cut back my asparagus in midSeptember and gave it a mulch but it sent up fresh spears. Did I cut it early? Peter Hawkins, Middlesex A Normally asparagus is not harvested after June so the plants can grow new shoots that then develop full foliage which will feed back into the roots. This lets them grow stronger and ensures a good crop the next spring. By cutting back early you have shortcircuited this process. Next year, leave the shoots until they turn yellow, which is about now, and then cut back. Mulch heavily with good compost and repeat the mulch in early July. A Box blight is caused by two strains of fungae, which only affect Buxus. You cannot replant box in the same spot for at least five years but you can plant anything else immediately without any risk of cross- contamination. Write to Monty Don at Weekend, Daily Mail, 2 Derry Street, London W8 5TT or email monty.don@dailymail.co.uk. Please include your full name and address. We regret Monty can’t reply to letters personally.