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What to grow on a windy balcony and honeysuckle tips, by garden expert Helen Yemm

Bosco Verticale in Milan features plants and trees on its balconies
Bosco Verticale in Milan features plants and trees on its balconies Credit: Pawel Toczynski; Photographer's Choice

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.

Roses on a balcony

Trevor and Pat Shaw could not have been kinder to their rambling roses since they planted them in large (50-litre) pots four years ago to form part of an extensive and largely successful plant menagerie on their balcony. Since they spend time abroad each year, the couple have installed an irrigation system, mulch pot surfaces with slate chips to maintain moisture and carefully top up compost nutrient levels with slow-acting fertiliser. Yet still the roses fail to thrive, flowering hardly at all, dropping their leaves early each year and making little significant strong, trainable growth (as expected of ramblers). What are they doing wrong, they ask?

I feel I have to be slightly cruel to be kind. In any situation it is difficult to persuade roses to grow and flower well in containers, and furthermore this balcony is exposed to the elements, being on the sixth floor, with spectacular westerly views of the Mersey and Liverpool docks. I fear that no amount of generous root room and ongoing TLC will help these roses cope with the conditions and after four years it might be time to give up and try something else.

The wind would appear to be the main issue, not the cold, since two tough-leaved non-native evergreens, an olive tree and a hebe (both visible in the background of one of Trevor’s pictures) are, surprisingly perhaps, growing extremely well.Therefore it would seem logical to suggest that Trevor and Pat should experiment a little in their choice of replacements, adding some more wind-tolerant evergreens, many from New Zealand and Australia (such as brachyglottis, ozothamnus, olearia, coprosma, griselinia, muehlenbeckia) to provide additional protective bulk to their planting scheme, relying on spring bulbs and hummocky summer annuals for a bit of floral fandango.

A regroup for spreaders

Most of my perennials form thick mats of growth that I (very occasionally) split down the middle with a spade, giving bits to friends. However, a relatively “young” monarda and an anthemis have spread sideways and look totally dead in the middle. Should I replace them completely, or can I dig up the rather scattered bits of each and replant them?

Deborah Wilson – via email

The outermost parts of herbaceous perennial clumps are always the most vigorous, which is why it is best to dig up entire plants every few years, cut away the middle section, then divide and replant the rest. This central, weaker part is less obvious in a dense mat-forming perennial such as a phlox, but more worrying in the case of looser spreaders like your monarda and anthemis, which may need to be “regrouped” more frequently.

Carefully dig up the “scattered bits” and sort out the most vigorous to keep and replant. Also dig up and bin the dead-looking central core of the original plant. Once you have cleared the area, add lots of compost and a sprinkling of bonemeal and replant a few of the saved bits 10cm or so apart. These should run into each other and form a loose clump of sorts next year.

Honeysuckle heave-ho

I have three honeysuckle plants struggling along the east-facing fence in a north-facing garden, all very straggly. I’m wondering if I should remove them and plant star jasmine there, or should I just cut them down to their base?

Allie Barnicoat – via email

Your pictures revealed an all-too-common problem. So-called “evergreen” honeysuckle (Lonicera japonica) drops its lower leaves constantly and most of the rest in winter. It is best grown where its rather unlovely twining lower growth clambers through other plants so that its flowers just “appear” aloft in tree canopies.

It is, as you have found, a climber that provides unsatisfactory fence coverage in small gardens and your three plants are shoehorned into a rather narrow strip of soil at the base of a bare wooden fence with not even a trellis for it to cling to. I don’t think cutting them down will solve the problem. You could replace them with a (single) evergreen star jasmine (Trachelospermum jasminoides), if a climber you absolutely must have.

Enrich the soil beforehand and provide an extensive grid of wire on the fence for it to climb. But I feel you would achieve greater and quicker coverage if you were to plant (instead or as well), an evergreen shrub or two that could be encouraged, via subtle pruning and tying, to hide the fence and add a bit of year-round interest and shape to your garden. Rhamnus alaternus ‘Argenteovariegata’ and Escallonia ‘Iveyi’ spring to mind.