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How to weed gravel, deal with sap-suckers and revive a straggly shrub, by garden expert Helen Yemm

A gravel courtyard
A well-kept gravel courtyard Credit: Andrea Jones / Garden Exposures Photo Library

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

WEEDS ON GRAVEL

The gravel laid on a weed-smothering membrane in an area in front of Sandra Loder’s garden seems, over time, to have become a perfect weed-seed bed, mostly for sycamore seedlings, she tells me. Can she use Roundup with a clear conscience to kill these weeds? She is slightly worried, also, about the roots of her climbers that must be somewhere beneath the membrane.

What follows is just the tip of a huge weedkiller iceberg – there is a detailed explanation of the various functions of numerous weed, moss and algae killers on the RHS website.

First, to clarify: original Roundup (still with us) is one of several products that contain a single active ingredient, glyphosate, which acts by direct contact with greenery, and travels down through that greenery to plant roots and kills them.

Roots of other plants in proximity whose leaves are not treated are not affected. However, spray can drift invisibly, so careful application is crucial (a watering can with a weed-bar is best). Once dry it is totally safe for pets and children, and it does not, the manufacturers insist (and this is the controversial bit) harm the environment by lingering in the soil.

Weedol Pathclear also contains glyphosate combined, vitally, with another chemical called diflufenican – this inhibits the germination of further generations of seeds by hanging around in the soil for several weeks.

Just to complicate and confuse matters there is a “new” Roundup (similar green bottle, same lettering) of which the active ingredient is acetic acid (similar to vinegar). This quickly “burns off” young foliage of weed growth and can kill their roots if used early enough in the season. The small print does state, however, that it is “for use on unwanted vegetation and areas not intended to bear vegetation”. On balance, I think this would seem to be the one for Sandra to use on her weedling-infested gravel.

OFF THE SCALE

A major branch of my four-year-old flowering currant is covered in what looks like lichen and is barely alive. Should I cut this off at the base and temporarily live with a lopsided shrub? Or is there any treatment that will stop things from getting worse?

Maggie Turner – via email

A greater spotted woodpecker perched on branch covered in lichen  Credit: Getty Images

When I enlarged your picture it became pretty obvious that although the branch of your flowering currant does indeed have some greyish lichen on it, it is smothered with an infestation of white cushion scale insects. These are notoriously hard-to-eradicate sap suckers that do just that: sap the very life out of plants, once they get to branch-smothering stage.

In this case I would cut out the branch, and also prune the whole shrub quite hard (after it flowers is a good time). The spring-hatched juveniles will be on the move now, so removal and disposal of as many affected branches as possible is a good (if somewhat drastic) way of controlling them, followed by a spray with Bug Clear Ultra to nobble any leftovers and help protect new growth. With feeding and mulching, this should be rapid.

You could, of course, replace your pink currant with an elegant white-flowered variety Ribes sanguineum White Icicle. This makes a less eye-watering partner for yellow forsythia with whose flowering, in so many gardens, it coincides.

A STRAGGLY SARCOCOCCA

My sarcococca, planted in partial shade on the north side of our house, is trimmed annually in spring but, although the growth looks healthy, the plant is now a mess, growing in all directions but with a sparse and twiggy centre. How can I improve it?

Eunice Harradine – via email

Sarcococca confusa Credit: Martin Pope

You should have been a bit more savage. Sarcococcas are evergreens grown mostly for their ability to tolerate shade, pollution and poor soil while still coming up smiling every winter. What you are experiencing is their downside, which can largely be remedied by pretty ruthless cutting back after flowering. All the new growth made during the year will flower the next.

However, before you grab your secateurs, please be aware that your straggly shrub may either be: (a) a naturally suckering variety, or (b) have layered itself (put down roots from shoots that made contact with the soil, as many evergreens do).

Either scenario would explain why the outer growth is lush while the centre is moribund. If so, you may decide to ditch the centre and create a new plant out of the best of the layered or suckering shoots. So grab a spade too.