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.
A recent Back to Basics column on pruning elicited numerous pleas for advice from readers with shrubs that have either “never really got going” or have become “old faithfuls” – badly maintained mature shrubs that are now desperately out of shape. What can be done about them?
Shrubs that never get going
The first step should be to familiarise yourself with the origins of the lacklustre shrub, what soil conditions suit it best, how much sun it needs etc. Armed with knowledge about your shrub’s basic requirements, you can take some useful steps. I recommend a good book (Hillier’s Manual of Trees & Shrubs) or the RHS website, but most advice will be along these lines:
1. Mineral deficiency
This is often indicated by pale leaves with green veins. The roots of acid-soil-lovers might be unable to take up nutrients (and therefore be slowly starving) because of too-limy soil. This may be resolved by an annual soluble iron tonic (Sequestrene or equivalent).
Another cause of yellowing and unhealthy-looking leaves is magnesium deficiency, corrected by a soil drench or foliar spray with magnesium sulphate (as in Epsom salts).
2. Lack of water and/or nutrients
Often caused by overcrowding and competition, culling one of a tight group of shrubs might be the only way out, or it may be a question of rescue from the clutches of ground cover or weeds. The greediest of these are evergreen spreaders such as ivy or periwinkle and the equally troublesome Hypericum calycinum (rose of Sharon).
There is no need for a scorched earth policy towards ground cover, however; just keep an eye on its progress and mount an annual “rescue” operation. For example, try to to keep the area under a shrub’s canopy (this area indicates the spread of its roots) free of competition so you can apply an annual general fertiliser (eg blood, fish and bone) and a bucket or two of soil-improving organic matter to the ground each spring.
An overnight dripping hose during droughts might hit the spot, too.
3. Lack of light
While some shrubs thrive in semi-shade, those that don’t will let you know by producing long, skinny shoots and fewer flowers. Small, young shrubs may need to be found more suitable, sunnier homes.
For shrubs that may have been happy once but have, over time, become overshadowed by other plants or trees, a partial solution might involve the pruning back or the raising of canopies of neighbouring offenders.
Old, out-of-shape shrubs
This is easier because at least you know they are basically happy. The way forward may have to be renovation (hard) pruning. Tackle it in early summer – although this may remove a year’s worth of flowers. It can also be done in stages, over two years.
Nearly all old shrubs will produce new green shoots from ground level or from brown wood. But there are some exceptions: for example, slightly tender flowering evergreens such as hebes must be treated with caution.
The solution is to cut back their outer growth first in late spring, to allow the visible lower inner growth to toughen up and eventually take over.
Tip for pruning roses
Prune your repeat-flowering bush roses (hybrid tea, floribunda, and modern shrub roses, including English roses) in late February and March.
Cut out skinny, damaged and (if you have not done so before) some of the very oldest wood, as well as anything that grows inwards to the centre of the bush. Finally, cut back what is left (stout, green shoots) down by about half, to an outward-facing “eye” (dormant leaf bud), imagining, in your mind’s eye, that you are creating a more or less candelabra-shaped bush.
The same basic technique (obviously modified) applies to roses trained as standards. Clear up, feed and mulch afterwards.
How do I stop my geraniums growing leggy?
The geranium/pelargonium cuttings I took last September are now quite leggy and flowering. This happened last year, so that when planted out in the garden I had to stake them because they were so tall. Can you please advise what I need to do? I am tempted to take cuttings from them.
Hilary Webster – via email
Follow your instincts and take cuttings from your cuttings. This time (and in the future) make them shorter, using young, green shoots about 10cm long, with two or three leaves on each, making a clean cut just below a leaf node. Carefully pinch out the growing/flowering tip of each before inserting them into pots of multi-purpose compost with a little added grit (or some ready-bagged seed and cuttings compost).
It will speed things up a bit to give each pot what used to be known as a “Wisley cloche”, which can be as simple as a clear plastic bag popped over the top, secured with a rubber band and prevented from collapsing on to the little cuttings by a trio of wooden kebab sticks or some such.
Make sure the cuttings have good light, but avoid putting them on sunny windowsills. When there are tiny signs of growth visible through the steamed-up “cloches”, snip the corner of each to let in fresh air. Within a few weeks, the cuttings will have developed roots and can be potted on, cloche-free, to a stronger loam-based compost.
From playhouse to potting shed
Gill Platt has ambitions to turn a part-glazed, octagonal, wooden, former garden playhouse into a greenhouse/potting shed to store overwintering dahlias, begonias, etc, as well as to nurture plug plants.
She asks what kind of heating would be needed – she doesn’t care for the idea of a paraffin heater and asks if it would be a good idea to install electricity.
Installing an electrical supply into a greenhouse or outhouse is fine, but there are certain regulations (easy to find online), and the work has to be signed off by a qualified electrician.
I would think a small electric tube heater with a thermostat (see twowests.co.uk) would be sufficient to keep the wooden shed frost-free, and would also make it an ideal place to overwinter tubers and other things that are not quite hardy (including dormant fuchsias, pelargoniums and so on).
I feel, however, that I may be putting a spanner in the works when I say Gill may run into trouble with her plug plants. They will be certain to suffer from the complete lack of overhead and all-round light and will grow stringy even if, as she says she intends to do, she takes them in and out of her new haven to remedy this.
She needs to consider, too, that the changes in temperature all the to-ing and fro-ing would involve would be bound to affect their growth adversely. Maybe a simple cold frame outside would provide a good halfway house.