The Problem SOLVER
Q What’s attacking my desert rose? Ian Mitchell ,by email A
Desert rose or Adenium obesum, with impressive pink flowers some 5cm (2in) across, is a succulent native to areas of sub-Saharan Africa and the Arabian Peninsula.
The browning of leaf edges could be due to poor root function. I would change your potting mix from the current 3 parts general purpose, 2 parts coarse sand and 1 part cactus compost to a cactus compost, with 30 per cent added Perlite or coarse sand.
I would also dispense with water-retaining gel and controlled-release fertiliser. The former probably doesn’t do anything at all, while the latter will release excessive amounts of nutrients in warmer weather, which may damage the roots of your desert rose. Instead, stick with half-strength Phostrogen about once a week during the growing season.
Q What has attacked our brassicas? They’re protected. Ann Lockey, by email A
Some damage could be due to slugs, but eating out the centre leaves is due to caterpillars. Just two or three butterflies finding their way in via gaps or holes in netting is all that’s needed. The netting also has to be finer than anti-bird netting and must be suspended above the crop and not laid on top as butterflies will lay their eggs through it.
If you grew brassicas in the same plot last year, then the damage has been done by cabbage moth caterpillars. These pupate in the soil so adults can emerge the following year beneath the protective netting.
Plants can probably be saved. Either apply a biological control, such as Nemasys Natural Fruit and Veg Protection or an insecticide for caterpillars. Check product details for use on edible crops.
Q Why has my dahlia produced different coloured flowers? Nigel Hughes, Wrexham A
Unfortunately, your dahlia has virus infection. While blotching on leaves could be easily overlooked, the white streaking or ‘breaking’ on flower petals is a classic symptom. Viruses cause a wide range of symptoms on foliage, including yellow spots or blotches, mosaic patterns or various yellowing along the veins and margins.
Dahlias are affected by a number of viruses. Dahlia mosaic virus and cucumber mosaic, spread by aphids, are the most common but also, occasionally, tomato spotted wilt virus, which is spread by thrips. Gardeners can spread infection wider when taking cuttings.
There are no cures and infected plants should be lifted and destroyed. Regular spraying with insecticides can control insects that spread viruses, while sterilising tools when taking cuttings or dividing plants will reduce transmission.
Q Why is our containergrown ‘bargain bin’ gerbera wilting? Ray and Linda Damsell, by email A
The Garvinea Series is a strain of hardy gerberas for outdoors. In containers, grow in a rich, gritty, well-drained potting compost of John Innes No 2, with plenty of extra grit. I’d lift and replant it, but make sure you break up the rootball and knock out some of the existing compost. I think the wilting may simply be due to the roots failing to grow out into the new potting mix.
It’s also important to set the plant slightly high so the compost doesn’t wash into the crown. Water well and feed fortnightly during the growing season with liquid tomato fertiliser, but keep it drier over winter. Remove spent flowers to encourage more.
The millipede you found in the container wouldn’t damage the plant. Millipedes are detritivores, mainly living on decaying organic matter. They occasionally damage seedlings, but their mouthparts aren’t strong enough to damage growing plants.
Q Can I rescue my dahlia tubers for next year? Paul Lee, Portsmouth, Hampshire A
The dahlias you inherited are unlikely to store for next year having been out of the ground for many months already. I’d be tempted to plant them on your allotment. Just before the frosts, for extra protection, spread a 10cm (4in) mulch of organic matter or a deep layer of autumn leaves, secured with anti-bird netting, over the top.
If they do survive, next year’s growth is likely to be poor, but the tubers may build up for the following year. Alternatively, if they do shoot next spring, you could take basal cuttings of the shoots, dip into Clonex rooting gel and insert a number in small pots of free-draining compost and place in a propagator at 21C (70F) for two weeks to root. These could then be potted on and eventually planted out to create new plants.