Kim Wilde answers your gardening questions.
I have several healthy mother-in-law's plants but they leave a brown residue on nearby surfaces, such as windows. It's difficult to remove, requiring a lot of elbow grease and a scouring pad. Can I prevent it?
This tropical plant (Sansevieria trifasciata) is virtually indestructible - definitely the one for those who claim to be houseplant killers. However, it sounds as if your otherwise healthy plants are being attacked by scale insects, probably hemispherical scale (which are yellow/green when young, becoming more raised and brown in colour as they mature). These sap-sucking insects are common in warm environments such as greenhouses, and are clearly enjoying their window position. I would use an organic pest control, based on fatty acids, which will coat the insects and suffocate them. Several are available from garden centres, including Bio Pest Control and Nature's Answer. HDRA, the organic organisation, has a brilliantgardening catalogue with several good products. Call 0845 130 1304 for a copy.
We have recently moved to a London townhouse with a 25ft garden, which is overlooked. What manageable trees or shrubs could we grow at the end of the garden to gain some privacy? We would prefer native species, no more than 20ft high, which would help to absorb pollution.
All plants clean the air through photosynthesis - a process by which they remove carbon dioxide from the atmosphere. In a small garden, I'd suggest investing in a high hedge using either hornbeam (Carpinus betulus) or lime (Tilia platyphyllos) - both native, pleached trees. These are ideal in confined situations, providing complete privacy during the summer when you're mostly out in the garden, and a handsome framework to admire in the winter. Hornbeam is able to withstand severe clipping, making it a very manageable tree, and the leaves turn golden in autumn. Because these trees are trained horizontally and thrive on being cut back annually, the root systems become confined, thus preventing damage to the foundations of nearby buildings.