Monthly Q&A May 2024

Q&A with Ashley Edwards, Humaira Ikram, Christine Walkden and Matt Biggs

Replant dahlia tubers in May, when all risk of frost has passed

Ashley Edwards says: You’ll find an estimated ‘last frost’ date online which should help you decide when to take the covers off. There’s no exact time as every year will vary and will also depend on how exposed your garden is, but the end of May to mid-June should be ok. In really wet winter, dahlia tubers can rot off, so make sure to plant them in a well drained soil, or in pots where they can be moved out of the rain. Watch out for slugs as they will feast on new shoots, consider spraying with a garlic solutions to make the dahlias unpalatable.

Our acer is losing a top layer of bark. When I removed it there were lots of woodlice and little holes. Should I remove the affected branch? Alexandra, East Sussex

If growing in pots, wrap with fleece or bubblewrap to protect the rootball from frost in winter

Matt Biggs says: Woodlice, loss of bark and little holes mean that the wood is dead, so you have find a long term solution to ensure it doesn’t happen again. Acer’s need a sheltered spot, ideally in dappled shade or away from scorching sunlight, protection from frost and cold winds – wrap with horticultural fleece in winter if necessary. They should be planted at the same depth they were in the pot – don’t plant too deeply. They do best in well drained soil with plenty of organic matter. Remove the branch in mid-summer, when the Acer is in full leaf – any earlier and the cut is likely to bleed.

When should I prune my Victoria plum trees? Gill, Devon

'Victoria' is a great choice for a small garden as it's self fertile, so doesn't need a pollination partner nearby — 'Victoria' is great for a small garden as it's self fertile, so doesn't need a pollination partner nearby

Ashley Edwards says: The best time to prune your Victoria plum is in summer. This is the same for many stone fruits including cherry and nectarine as it reduces the chances of them getting silver leaf. This fungal disease spreads via spores from brackets, produced during winter. It causes die back of branches and can kill the whole tree. When pruning your plum you should aim to remove central branches to allow maximum air flow through the tree. If they are young trees, you could consider pruning them into attractive espaliers. When pruning, make sure that tools are sharp and clean to reduce the chances of infections entering the tree. It’s good practice to prune in dry weather.

My box hedge has developed white web-like areas and I've found bright yellow and green caterpillars along the leaves. What do I do? Paul, East Midlands

Look out for webbing, dieback and droppings on your box as a sign of box moth

Matt Biggs says: Box moth can defoliate a plant in days so they need treating quickly. Remove caterpillars by hand daily, searching right into the plant and shoot tips that are covered with webbing. This is only practical if you have a few plants. Biological control containing a micro-organism, Bacillus thuringiensis that doesn’t harm other wildlife is effective and can be bought online but is not licenced for amateur use. Several treatments will be needed over the growing season, thoroughly coating both sides of the leaves. Pheromone traps help to have some success in disrupting the life cycle and birds like blue tits and jackdaws will eat the larvae.

How can I get my greenhouse aubergine plants to fruit? Sue, North West England

Once plants have started to flower, feed weekly with a high-potash fertiliser or tomato feed

Ashley Edwards says: Like many fruiting plants, aubergines rely on pollinators to produce fruit. If your plants were outdoors (which is not advisable in this climate!) there would hopefully be an abundance of pollinators around. In the greenhouse, you will have to play the role of pollinator! By using a paintbrush you can go between flowers, giving them a tickle and that should do the trick. You could also open the doors and windows of your greenhouse in the summer and invite the pollinators in. Planting pollinator friendly flowers, such as pot marigold, in the greenhouse will also encourage insects in.

Nine unusual aubergines to grow

My 20-year-0ld olive tree is looking very unhappy with yellowing leaves and not much fresh growth. I did wonder if this winter's cold and wet weather contributed as it has come through many very cold winters unscathed. What do you think? Liz, Cambridgeshire

Protect olive trees in winter with horticultural fleece and mulch around the base

Matt Biggs says: Olives certainly don’t thrive in a combination of cold and wet, though as you have discovered cold and dry is fine. Ideally they should be planted in a sheltered, sunny spot on free draining soil, near a wall to replicate their favoured Mediterranean climate. Add plenty of grit and plant on a mound 30cm high in heavier soils. They gain greater resilience with age, so it should have the strength to survive and will certainly enjoy a hot summer to recover. Providing shelter by wrapping with horticultural fleece will help but may not be practical with larger trees. Although they are evergreen, olives do shed leaves, mostly in late spring as older leaves create space for new shoots.

How can I stop pigeons eating my honeysuckle? Jan, South Staffordshire

All honeysuckles will grow in most soil types but prefer a well-drained, humus-rich soil.

Ashley Edwards says: If you’re not keen on a scarecrow, you could try physical barriers. Using string and bamboo canes to create a sort of frame could prevent the pigeons landing near your honeysuckle. Reflective tapes or CDs are a common sight on allotments, but they will not only deter pigeons but all types of smaller birds too. You could put out bird feeders which would be far more attractive to the pigeons and also encourage other birds to visit your garden. Honeysuckle is a vigorous plant and even if the pigeons are a nuisance, they are unlikely to do any permanent damage.

My amelanchier grows new leaves in spring, blossoms, leaves begin to change to green then die back from the bottom of branch. What is the problem? Claire, Cambridgeshire

Fireblight causes leaves to shrivel and drop, shoots to dieback and white liquid to ooze from infected wood — Fireblight causes leaves to shrivel, shoots to dieback and white liquid to ooze from infected wood

Matt Biggs says: Dieback such as this can be caused by late air frosts; once the danger of frost has passed, cut back amelanchier stems to new growth. If adjacent stems are also dying back, it is likely to be fireblight, which gradually kills the plant over successive seasons. It is often scattered through the plants, with healthy and diseased stems growing side by side. Pare back the bark and there is a foxy brown stain, and later a bacterial ooze from the bark. Prune back damaged stems to at least 60cm beyond infected wood, sterilise tools afterwards and burn infected wood. If the infection is very bad, it would be worth removing the plant.

I've fed and watered my rhubarb and split the crown but the stalks remain thin and weak. How can I remedy this? Anna, England

Rhubarb needs an open, sunny or partially shaded site, and rich, moist soil.

Ashley Edwards says: It could be the timing of your harvesting. Try not to harvest after June to allow the plant to build up enough energy for next years crop. If you have just recently split your plant, it can take a few years for the plant to produce thick stems again. Resist harvesting for at least two years. Rhubarb is a greedy plant so give your plant a good mulch of well-rotted garden compost in spring to give it a boost. It sounds as though you are providing a liquid feed, which is also good; I’d recommend organic liquid seaweed as it is high in nitrogen, important for healthy leaves and stems.

I'm looking to fill a very dry area with dappled shade under some neighbouring overhanging leylandii trees. Can you recommend some ideal attractive flowering perennial plants/shrubs that will add colour throughout the year against a prominent and boring fence backdrop. Lee, Sandhurst

Cyclamen hederifolium will grow well in partial shade and self seeds freely — *Cyclamen hederifolium* will grow well in partial shade and self seeds freely

Matt Biggs says: The dry soils should be improved by adding well-rotted organic matter, installing drip irrigation from your water butt and removing some of the branches from the ‘Leylandii’ with care to avoid looking at bare stems. Autumn flowering Cyclamen hederifolium and Cyclamen coum in their pink or white forms have attractive leaves, snowdrops and small daffodils, which need less water than large varieties will flourish in these conditions. Forms of Geranium macrorrhizum, like ‘White Ness, the biennial honesty which takes two years to flower with blooms in shades from dark to pale purple and white followed by silvery penny shaped seed heads, foxgloves and cultivars of Liriope muscari like ‘Royal Purple’ have been spotted growing under yew trees. These will all be happy but need watering until they establish.