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Gardening jobs to do now before things get extreme; pruning roses, moving shrubs and mildew

Best foot forward: with any luck, September will bring good weather for gardening before the more extreme weather arrives for winter - Alamy
Best foot forward: with any luck, September will bring good weather for gardening before the more extreme weather arrives for winter - Alamy

Every week, Telegraph gardening expert Helen Yemm gives tips and advice on all your gardening problems whether at home or on the allotment. If you have a question, see below for how to contact her.

For most of us lucky ones, memories of the extremes of lockdown are, I suspect, starting to blur. Meanwhile, its effects on our homes (much culling, clearing, bagging and boxing-up of the evidence of decades of shopaholism went on in mine) and on our life/garden styles may be starting to filter into our gardens.

With the arrival of autumn and the lurking anxiety of more lockdowns, I want to act while fair weather permits, rethinking and simplifying, even culling no-hopers or giving a shrub or two new and improved quarters. Hence a few paragraphs this week on how to move a sizeable shrub successfully – for those who didn’t realise they could.

September is a good time. It is lovely to work while light levels are high and the soil still warm. With hopefully a month or two before any really harsh weather kicks in, transplantees should bed-in well.

Take your time and do things in a sensible order, described below. Very large subjects may also need to be pruned by half or more first, making them less vulnerable and more manageable (even though flowering may be poor or nonexistent next year).

How to make planting hole for shrubs

Plantation of an heptacodium in a garden -  Flora Press
Plantation of an heptacodium in a garden - Flora Press

Preliminary excavations of the proposed new site will reveal how dry the soil gets in summer (which may have a bearing on suitability). If you hit rock or impenetrable clay, wrist-width tree roots close to the surface or a supply pipe, go back to the drawing board.

To estimate the size of the new planting hole, discover the extent of the shrub’s roots by sticking a fork in the ground all around it, roughly in line with the furthest extent of its canopy, and work inwards to find where major roots start. The rough breadth of its root ball area established, stick a spade downwards all around and try to gauge its approximate depth. Drip an overnight hose into the area.

Prepare a planting hole based on the above, but larger and deeper, throwing the dug soil into a wheelbarrow to be mixed with 30-50 per cent garden compost or other organic stuff and one or two fistfuls of bonemeal. Return some of this mixture to the planting hole, fork it around to incorporate it into the soil, then water it.

Spread a plastic sheet on the ground next to the shrub, ease it out of the ground and on to the sheet. Cleanly trim any inevitably damaged roots before dragging the loaded sheet to the new site and plopping the shrub into the ground (having first ensured that the hole is the right depth and having added to or taken away some of the base compost/soil layer). Also, make sure the shrub is “facing the right way” in its new site. This is often forgotten in the heat of the moment.

Finally, backfill with your barrow mixture, making sure that there are no air pockets, using your hands and/or by treading everything gently, watering the area slowly at the halfway point. Finish the job with the remaining soil and apply a mulch.

Pruning ‘Compassion’

'Compassion' Rosa  - Chris Bosworth
'Compassion' Rosa - Chris Bosworth

John Emms sent me a picture of a rose growing against a fence, which he rightly says has grown far too tall (well over 2m), and asks how and how far he can prune it in order to regain control.

It looks like a ‘Compassion’ rose, a super-tall scented, pale-coppery-pink hybrid tea rose that produces its fragrant flowers, infuriatingly out of reach, on the end of ramrod-straight shoots, while resisting all attempts to be trained to grow horizontally like a “proper” climber (for which purpose it is often mis-sold). Popular in the 1980s (the pastel years, I call them) there are a now a lot of elderly specimens around in need of shock pruning.

I suspect the top growth of this rose has been regularly cut back, probably to prevent wind damage. This year, John needs to go further. He should identify the thickest and oldest shoots, trace them downwards with his hand and saw them out very low down.

This will shockingly reduce the rose height and bulk at a stroke, and force it to produce most of its new growth from below his cuts. What is left of the rose undercarriage should be shortened to a height of about 60cm. Yes, yes, I know – by this time there may be almost nothing left. But replacement growth will be rapid next summer and should flower (perhaps slightly later than usual) at nose height. You really can’t kill a rose by pruning it hard.

Dear Helen

My garden is plagued by powdery mildew and this year seems particularly bad. The list of plants affected is endless, but worryingly includes trees and a hedge. As my garden has matured, the mildew seems to have got worse, spreading from plant to plant. I never blitz with fungicides, but I do spray individual plants when I notice the dusty leaf-coating. Have you some words of comfort or advice?

– Penny Clamp, via email

Dear Penny

There are loads of mildew strains and they tend to be host-specific, so plants not related to each other may not be affected but may be susceptible to others. The real enemy, particularly this year (although it all seems like a distant memory now), was the prolonged heat causing dry soil conditions. The other important factor encouraging mildew is the overcrowding of plants, leading to a lack of air circulation around them.

This autumn thin out any crammed colonies of mildew-prone forget-me-nots etc. Dig up and perhaps split your older perennials and replant them in soil generously enriched with moisture-retaining organic matter.

Next spring while the soil is still moist, mulch around trees, shrubs and clematis; under hedges and over borders, and in summer give stems and foliage of early-performing perennials a regenerative cutback. You will need a ton or more organic “stuff” if your garden can’t generate enough: mushroom compost may be easy to come by.

Lastly, systemic fungicides work best if they are used preventively, so spray plants that suffered this year before the problem shows up next.

Helen’s tip of the week

Pampering your hellebores

Hellebore 'Harvington Picotee' - Gap Photos 
Hellebore 'Harvington Picotee' - Gap Photos

Largely ignored and treated as ground cover for the past few months, it is time to give hellebores some TLC. Both the Oriental hybrids that produce flower stems and new leaves annually from below ground and others (e.g. H. foetidus – with biennial shoots that flower in their tips) will be forming next spring’s buds soon.

A general “groom” – that may mean removal of any threadbare and damaged leaves for the Orientals – plus a little general fertiliser lightly fiddled into the soil and watered-in, followed by a leafy mulch, will benefit them all.

Send your questions

Write: Gardening, The Daily Telegraph, 111 Buckingham Palace Road, London SW1W 0DT

Tweet: @TeleGardening

Email: helen.yemm@telegraph.co.uk

For more tips and advice from Helen Yemm, visit telegraph.co.uk/authors/helen-yemm

Helen Yemm can answer questions only through this column.

Read more: our guide to the best secateurs

What gardening jobs are next on your to do list? Tell us in the comments section below