Thorny problems: What can I do about mildew? To answer the second part of your question: liberal mulching with organic matter in late spring to conserve moisture in the soil around your phlox roots may help, followed by preventive spraying with a systemic fungicide (such as Systhane) as they fill out in June. And seek out mildew-resistant varieties if possible. Definitely past it We have an enormous low-growing ceanothus in our front garden that we planted 25 or 30 years ago. It looks beautiful in spring when in flower, but has grown so large that it is now overwhelming other plants. Can we prune it harshly or are we likely to kill it? Judith Bailey, via email Judith, you are soon, I am sure, to experience a rare and wonderful gardening moment. But first let me congratulate you on two counts, first for your extremely long and clearly successful custodianship of a shrub that is known to be somewhat short-lived. Fifteen or so years is a ripe old age for small-leafed evergreen ceanothus (this one is presumably Ceanothus thyrsiflorus var. repens). Second, you should also be commended for tolerating this monstrous pile of a shrub, that earns its increasingly greedy place in a garden for only about two weeks of the year. At other times it just lies there in all its shiny, deep green boringness, ever-expanding and about as interesting as a comatose hippo. Well, that's the way I see it, anyway. But to answer your question. If you prune this shrub hard - into old wood - in order to stop it overwhelming even more of its neighbours, you will undoubtedly be saying goodbye to it. Not that it will die straight away, since it will very probably rally and try to sprout pathetically from its unprepossessing remains before it finally gasps its last. But before it does that, I can pretty much guarantee that one fine morning, seeing its ugliness and total unnecessariness through a red mist of utter rage, you will be up and at it with an axe. Then, and only then, will it dawn on you what a dreary monster you have been harbouring for so long. And lo, you will have created yards of space for something new and wonderful - no, possibly two or even three somethings new and wonderful that you have been itching to grow for years but had no room for. As I say, it will be a wonderful and liberating gardening moment. Enjoy. PS: For other readers who have yet to reach Judith's point of no return: if pruned every year after flowering once it is two or three years old (removing up to 12in of growth all over), a spring-flowering ceanothus can be kept relatively compact. Furthermore, you can grow late-flowering clematis through it to jolly it up from June to August. But it still will be relatively short-lived. Peony pruning Daphne Nightingirl has several peonies in her garden, some of which are herbaceous and some of which are shrubby "tree peonies." She has no idea what to do with them in the winter and wants advice. Herbaceous peonies are the easy ones. They just die back, some of them quite nicely since they have good autumn colour, and shoot up again each spring for their all-too-brief moment of glory early the following summer. Shrubby peonies are more complicated. Once they start flowering, which takes time and patience with some of the more exotic and showy things now on offer, they will produce their best flowers on the tips and side shoots of year-old growth. They become less productive, it would seem, if you leave a lot of old wood hanging around as they age and fill out. The answer, in a nutshell, is to take out the oldest shoots of mature plants at ground level once they have (very messily) dropped their leaves for the winter, and shorten one or two other middle-aged stems by half. No wonder poor Daphne had no flowers on her tree peonies this year, however, since, having been given conflicting advice, she cut all the stems back to the ground last winter. It is to be hoped she will be luckier next year - but only if this winter she leaves well alone all the new growth they have produced this year. Of rabbits, astrantias and rural hedges Penelope Laing's lovely witch-hazel, Hamamelis x intermedia 'Pallida," a gift from her husband three years ago, flowered well this year but, horror of horrors, the bark at the bottom of its stems has now been eaten by rabbits. Her husband says it will die. But can she not dig it up and replant it deeper to cover the damage or bind something around the stems, she asks. No, I am afraid not. Penelope's husband may be right, alas. Certainly those stems that have had their bark nibbled around their entire circumference will die, so even if it survives, the bush might end up a strange shape. And replanting more deeply is not the answer. Root tissue and stem tissue are quite different, and buried stems would simply rot. Her best hope? Clean removal with secateurs of obviously doomed stems. After that, a temporary fence of chicken wire should put off the nibblers - who, it seems, relish only the young stems of woody plants and leave them alone once mature. Rabbit guards, to protect lower stems of trees and shrubs, are also available at garden centres. Janine Blakemore from Herefordshire was disappointed that the leaves of her Astrantia major were irregularly blotched this year. The perpetrator of this disfigurement is a fly called Phytomyza astrantiae, which, as the name suggests, is host-specific. The fly punctures the leaves to lay its eggs, and it is the tiny grubs that hatch and feed within the leaf tissue that cause the patches. The flies arrive in April or May, so a systemic spray (Provado Ultimate Bug Killer) of the young astrantia foliage at that time will go some way to preventing the problem. Finally, Ben Flowers wants ideas for a tall, deciduous, rural, "productive hedge" to plant between land destined to be a vegetable garden and the public footpath that runs along it. How about a dizzy mixture of hazel and cherry plum (Prunus cerasifera, from fruit tree nurseries)? And rural hedges always get infested with brambles (ie free blackberries), in my experience. Pea sticks for Ben (eventually) and food for all - lucky passers-by and birds included. Write to Thorny Problems at helen.yemm@telegraph.co.uk or Gardening, The Daily Telegraph, 111 Buckingham Palace Road, London SW1W 0DT. Helen Yemm can answer questions only through this column .