In The Garden

TIPS on how to combat Japanese knotweed — plus, find out what else needs doing in the garden this week.

Have weeds, will travel

I caught up with Alan Titchmarsh at the recent BBC Gardeners’ World Live show when he was giving advice on how to combat Japanese knotweed, the peril of many gardeners, a weed which is right up there on the list of seemingly indestructible thugs.

“It was brought into this country as an ornamental plant by the Victorians, who thought it was beautiful,” he lamented.

It has an underground network of tough, fast-growing rhizomes which makes it really difficult to control and in the summer the bamboo-like stems grow to more than 2.1m (7ft), suppressing all other growth.

The RHS advises that if you’re going to try digging it out, remove as much root as possible, then repeatedly destroy regrowth. In this way the energy reserves in the remaining underground parts will be gradually exhausted but this process could take a very long time.

Another option is to place a layer of carpet over it but you’ll have to leave it a long time before it dies and you can dig it up.

And when you dispose of the plant, do so carefully, allowing it to dry out before burning. Whatever you do, don’t put it on your compost heap or into your normal household waste because it colonises without seeding. It might be worth contacting your local council for advice on its disposal.

Titchmarsh, whose second ITV1 six-part series of Love Your Garden is currently being shown on Tuesday nights, says the best time to tackle this incessant weed is when it’s in flower.

“Chop it down to ground level and when it starts to regenerate and particularly when it’s flowering, that’s the best time to spray Japanese knotweed,” he says.

He says it’s more susceptible to systemic weedkillers such as Roundup later in the season.

“Give it one big blast in autumn or late summer when it starts to flower,” he advises.

If it’s coming through from a neighbour’s garden, you’ll need to put a physical vertical barrier in the soil, digging a trench not less than 2m deep and inserting a tough, impenetrable membrane into it.

My own nemesis is ground elder, introduced to Britain by the Romans as a pot herb and for its supposed medicinal purposes.

I well recall when I first moved to my present house, one border was covered in it and I ended up putting weed-suppressing membrane on the whole area, left it for about six months and then dug out every bit of root I could see.

Since then, it has threatened to appear from my neighbour’s garden, but I spray emerging leaves with systemic weedkiller on warm days in spring and that seems to keep it at bay.

Persistent hoeing will weaken the plant but won’t kill it as the rhizomes are brittle and the plant will regrow from any fragment.

If the weed creeps among your other garden plants, you may end up having to dig those up as well and untangle the runners from their roots, then allow time for any remaining bits to regrow and treat them with glyphosate.

Another rampant pest is couch grass, with its fast-growing rhizomes, which can produce hundreds of feet of roots underground, giving rise to a mass of shoots above ground.

It makes plants look untidy, with long blades of grass sticking out among your flowers, but can be dug up as the roots are shallow, rarely more than 20cm (8in) deep.

If you loosen the soil as you go, you may be able to pull out long, intact lengths of rhizome. Try not to leave bits of root in the soil because they will only come up again.

If your couch grass has spread from the lawn, create a clear edge between the lawn and flower beds to stop it spreading.

Aggressive perennial weeds are a pain - but planning your attack and patience will hopefully help to eradicate them.

Best of the bunch - Peony

They’re the blousiest blooms on the block, producing huge pom-pom flowers which are hard to beat, even though they are generally shortlived.

While many flower in May, some wait until June and they also make good cut flowers if you can bear to snip them off.

Herbaceous peonies look their best in borders among other perennials. Many reach around 1m (3ft) in height, their weighty flowers appearing over deep green leaves.

Good plant partners include bearded irises, or they can look stunning against a backdrop of climbing clematis.

Tree peonies, which are deciduous but keep their woody stems throughout winter, flower later than the herbaceous varieties and their flowers tend to be larger.

Newly planted peonies may not flower in the first year as they hate being moved, but it’ll be worth the wait for the magnificent blooms you’ll get in the years to come. They should be planted in a sunny spot in rich, fertile, well-drained soil.

Good varieties include the scented ‘Bowl of Beauty’, which has pink flowers with a creamy centre in June’, and ‘Duchesse de Nemours’, a white variety.

Good enough to eat... Protecting autumn fruit crops

Although the warm start to the year initially raised hopes of a good harvest, frosts in April and cold rainy weather in May and June reduced pollination and led to losses of remaining fruitlets. This will mean poorer plum crops this summer and a dearth of apples and pears later, says the RHS.

The charity suggests that to preserve whatever crop is left, it’s important to keep down weeds around trees, so that there is less competition for nutrients, especially in dry spells.

Careful control of pests and diseases will also help and there will be little need to thin out the remaining fruit.

Gardeners should summer-prune restricted forms of fruit trees such as cordons and espaliers. With few fruits to support, it’s likely that trees will grow too many branches and leaves.

“Because trees have dropped quite a lot of their developing fruits, gardeners should be wary about thinning fruits - and in many cases not thin out at all this year,” says Guy Barter, RHS chief horticultural adviser.

“This current warm, moist weather, in the absence of a heavy fruit crop, will also encourage lush growth, so summer pruning will help direct nutrients to the fruit and promote productivity for 2013.

“Adding potassium (high potash) fertiliser to the weed-free area at the base of the tree can help harden growth and promote fruitfulness.”

Three ways to... Lighten a shady spot

1. Make a garden seat or stone statue the focus of attention.

2. Make a backdrop of golden star flowers and cascading shoots of winter-flowering jasmine.

3. Add light and scent in summer with pots of all-white flowers such as Nicotiana alata, regale lilies and white busy lizzies.

What to do this week

Plant out bulbs of cannas and lilies potted up earlier in the season.

Remove the growing points from early peas which have finished flowering to concentrate energies on pod production.

Boost gladioli with a liquid feed every two weeks from now through to the first appearance of the flower.

Plant ‘De Caen’ anemone corms under cloches for flowering in the autumn and winter.

Deadhead border plants that have finished flowering, such as lupins, to prevent them from setting seed and encourage them to produce a second flush of blooms later in the year.

Pot up rooted basal cuttings of delphiniums taken last month.

Propagate strawberries from the plantlets that form on the runners. Plunge pots of compost into the ground and peg the plantlets down into the pots with bent wire. You can cut them from the main plant when they have rooted well.

Continue cutting back rock plants such as alyssum, aubrieta and helianthemum immediately after flowering, before they have had time to set seed.

Layer low-growing branches of chaenomeles, cotinus and magnolia now for good propagation results.

Continue to sow quick-growing salad crops such as lettuces, radishes and spring onions.

During dry weather raise the cutting blades on your lawnmower and mow without using the grass box so that the clippings help retain moisture.

Thin established seedlings of herbs such as chervil and dill to 15-30cm (6-12in) apart, according to the eventual spread of the plants.