Don't be afraid of ivy - it's a good plant in the right place, says garden expert Helen Yemm

How to over-winter dahlias, a rescue plan for a choisya and how to prevent whitefly on outdoor cabbages

Ivy covered by morning frost
Ivy covered by morning frost Credit: gapphotos.com
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.

Ivy in gardens is a subject that divides the nation. There are those who loathe it, viewing its presence as an infestation that destroys walls and topples trees. Some even confuse it with the serious blister rash-inducing North American poison ivy (Toxicodendron radicans) to which our own ivy (Hedera helix), with its mildly irritant sap, is not even related.

Others are more sanguine and appreciate ivy as an important conservation plant, providing cover, nesting, breeding and hibernation sites for a mass of wildlife. What’s more, if allowed to grow to maturity, ivy carries autumn flowers and berries that are an important food source for numerous insects and birds.

It has recently been acknowledged, too, as a useful insulating layer for buildings, keeping them cooler in summer and warmer in winter.

I belong, as readers might expect, in the latter camp, but without doubt there are places where planting ivy is a liability. For example, the previous owner of my house planted a huge-leafed ivy to hide and embellish the deeply shady walls of the garage.

By the time I moved in, it virtually covered the roof: painstaking removal was undertaken as a matter of urgency. It now forms an admirable shiny green backdrop to a variegated dogwood, massed white astrantias and Japanese anemones, but I have to get a ladder out at least three times a year to keep it out of the gutters and ensure that new growth swoops elegantly downwards. A small-leafed or variegated (therefore less vigorous) variety would perhaps have been a better choice.

This week, a few ivy basics to encourage a balanced view:

  • The hairy little aerial roots of ivy that cling to structures simply enable it to climb, they don’t seek or absorb moisture, and will do no harm to sound brickwork or mortar.
  • They will, however, sneak into existing cracks in old and crumbling walls and enable ivy to climb over and between roof tiles and even cling to smooth slates and painted woodwork. After removal, you need a wire brush to clean off the dead remains.
  • Native ivy that is allowed to spread over the ground, where it may usefully smother weeds, can only be allowed a free rein in the wildest gardens. In tidy gardens it can, if annually culled, be kept under control.
  • To kill rampant ivy, you need to seek out its huge main stem (not easy: it may not even be on your own property), sever it close to the ground and poison it with a stump killer (e.g. Roundup Tree Stump & Root Killer).
  • As ivy matures, it changes in character, becoming woody and starting to flower, attracting insects and ultimately flocks of blackbirds. It can then be pruned annually in spring to keep it in check.
  • Mature ivy that gets into the canopy of a lofty deciduous tree creates a weighty blob in winter. If this happens, sever the main stem low down, and check regrowth annually.
  • But before you wage war on a mature ivy that has become a major, valuably evergreen and attractively flowering component of a hedge, you should consider what you would plant to fill the gap created by its removal.
     

Tip for overwintering dahlias 

Overwintering dahlias - placing them upside down in a tray to dry off Credit: gapphotos.com

When frost blackens the tops of your dahlias, it is time to act. In mild areas, you can cut down the foliage, leave the tubers in situ and expect them to come back next year. If in doubt, cover each plant with a dry mulch (old fern fronds, for example) pegged down under fleece. Be aware that dahlias will need slug/snail defences from early spring.

In cold areas, dig dahlias up. After drying them off upside down, store tubers in labelled paper bags in a frost-free garage or shed. Give similar treatment to those grown in pots.

A rescue plan for Choisya 'Sundance'

Last winter, I cleared an area on the north side of my house of conifers. I would like to keep a Choisya ternata ‘Sundance’ that also grew there, but it is now leggy, sparse and badly shaped. Can I prune it fairly hard to promote a tighter growth in future? And, if so, when should I do it?

Heather Mather – via email

Choisya ternata 'Sundance'  Credit: gapphotos.com

You can certainly prune choisyas hard, but now is not a good time to do so. Go ahead and shorten the leggiest stems a little to stop them from flailing around in the wind, but wait until the weather warms up in March before cutting it back (you can be quite radical, so that it looks like antlers sticking out of the ground). Give it a fistful of a general fertiliser and a mulch to get it going again and it should soon start to recover its golden foliage, but do not expect it to flower properly until the following year.

As I am sure you know, golden-leaved shrubs are particularly valuable for “lighting up” dark areas of a garden (which is presumably why yours was planted near conifers in the first place). When subjected to full sunlight, however (and particularly harsh mid-day sun) their foliage can become unattractively bleached out. Even though this border is on the north side of your house, the removal of the conifers will have made a difference. If your renovated ‘Sundance’ suffers, you may have to move it.

Whitefly wars

I managed this year to keep my Brussels sprouts and broccoli free of caterpillars by protecting them with mesh cages, but they became infested with whiteflies instead. What can I do next year to prevent this from happening? I will use sprays if I have to, but is there a biological control or plant (marigolds perhaps?) that I can grow nearby to deter them?

Carol Startup – via email

This is complicated. The biological control that you have probably heard of, Encarsia formosa (sold for use against glasshouse whitefly), is not effective on cabbage whitefly (a different species). This is known to be kept in check to a degree by a different, naturally occurring encarsia species that would, of course, be knocked out if you were to use persistent or systemic chemical sprays to try to control the white fly.

I found a shortlist of suggested companion plants, that might, I understand, help to deter/ control cabbage whitefly if grown close by, but marigolds are not on it: hyssop, rosemary, sage and thyme.

Probably your best bet next year would be to experiment with one or some of the companion plants listed above, and also use organic sprays containing pyrethrum, such as Bug Clear Gun for Fruit and Veg, plant oils (e.g. Vitax Plant Guard Pest and Disease Control) or fatty acids (Doff Greenfly and Blackfly Killer). The effects of these products do not last very long, so you will have to spray repeatedly. Good luck.