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Facebook Q&A Session 14th June 2013


Thompson & Morgan Facebook Q&A Session 14th June 2013 - Your horticultural questions answered.

Click here to view details of our previous Q&A sessions.

Name: Andy Lyttle

Question: I bought Sundaville Red last year and the two plants have grown well in a (heated) conservatory with plenty of foliage. Unfortunately, there is, so far no flowers. I have been feeding the plants over the last six weeks. Any suggestions to a cause or cure please. Andy

Answer: Hi Andy, in our climate, Mandevilla can bloom quite late in the summer so I wouldn’t be too concerned just yet. They flower on the current year’s growth and periods of cold or dull weather will slow them down. They are tropical plants, adapted to the high temperatures and humidity in South America. Provided temperatures are warm, preferably around 21°C (70°F) during the day with a minimum of 15°C (59°F) at night they should be happy enough to flower. Just make sure they are in full light all day and try increasing humidity by standing the pot in a tray of gravel with a little water in (make sure the pot is raised above the water level). I hope this helps and your Mandevillas bloom soon.

Name: Iona Wilson

Question: We have inherited 2 old apple trees and have begun the restoration of them to let them be loved again (following the suggested thinning and pruning etc) but they have been attacked by aphids on an epic scale over the last few days and all the leaves are curling. The aphids have now spread to our very young apple trees.... HELP ... what can we do?

Answer: Hi Iona, don’t worry - it will be possible to bring this back under control and there are measures you can take to prevent this happening next year. As the aphids have caused the leaves to curl and are probably hidden inside, organic contact insecticides are unlikely to be effective. The best course of action would be to use systemic insecticides (enters the plant’s vascular system) which are approved for use on fruit trees. You could try insecticides containing thiacloprid (Bayer Provado Ultimate Bug Killer) or acetamiprid (Scotts Bug Clear Ultra) - these active ingredients are normally written on the label.

This winter, use an oil winter wash such as Growing Success Winter Tree Wash to remove overwintering aphid eggs. This winter treatment is likely to be most effective at reducing the problem for next year but you will need to be vigilant in spring for a recurrence of the aphid which will need spraying. If the problem recurs next spring you could try using an organic spray such as Bayer Organic Pest Control. This has quite a short persistence and works on contact so you will need to be thorough and may have to spray several times throughout the season. However, this spray can be used right up to a day before harvesting which is a great benefit. I hope this helps, good luck.

Name: Elizabeth Crowe

Question: I have two oleander bushes, how dangerous are they to children and animals.

Answer: Hi Elizabeth, Nerium (Oleander) is highly toxic if ingested and contact with the foliage may irritate the skin. Fumes from burning this plant may also be toxic if inhaled.

But as with all poisonous plants, the potential risk really depends upon the circumstances. A poisonous plant growing in a children’s playground carries a far greater risk than the same plant growing in your own garden away from children and livestock. There are many other familiar plants that we grow without concern, which are also irritants and highly toxic if ingested. Rhus (Sumach) is one such plant, and yet it is commonly found in gardens, both public and private. Lilies are highly toxic to cats, but we happily send them in bouquets!

If your Oleanders are accessible to livestock, unsupervised children, or your cat is obsessed with chewing it then you should certainly consider removing them (carefully). However if the risk is minimal and you enjoy them in the privacy of your own garden then keep them - but be sure to wear gloves and long sleeves when pruning or handling them and make sure that your family (and anyone else with access to them) is fully aware of the risks. If you later decide to move home it would also be sensible to advise any new owners of its presence.

Name: Elaine Sipos

Question: Why are my previously purple sweet rocket now white, when they were only ever purple? Thanks.

Answer: Hi Elaine, there is a lot of natural variation within the Hesperis matronalis species and in nature you’ll often find various shades of purple and the odd white variety seeding itself nearby. We sell seeds of the white variety - click here to view. These plants are naturally short-lived, although their ability to self-seed often makes them appear to be long-lived perennials. This can give the appearance that the plant has changed colour. I hope this helps.

Name: Julie Atkinson

Question: Hi Sue, I bought some ground cover lilies (dazzler) from you and planted them in late autumn 2011. As it was unseasonably warm they started to grow and then got hit by frosts and died back. They didn't make an appearance the following spring, in 2012, but this year they have all come up. My question is their height - the website says 12 inches but mine are much much taller than this. Should I do anything to stop their growth as they are not in anyway going to be a ground cover flower? Thanks, Julie

Answer: Hi Julie, our apologies - we’ve just taken a look at the Lily ‘Dazzler’ product page and noticed it is displayed incorrectly at 60cm (12") - this should be 60cm (24"). Most lilies are tall-growing so we call these varieties ‘dwarf’ as they are naturally much shorter than normal. These lilies will naturally multiply every year and over time will form more bulbs and more flowering stems for a carpet effect. Lilies shouldn’t be pinched out as this will remove the current year’s flowers. Let us know if we can be of any further help.

Name: Laura Cooper

Question: I have been digging up and throwing away my favourite lilies because I am plagued with red lily beetle. Will they now attack my other plants? If so is there anything I can do stop this? Thank you!

Answer: Hi Laura, fortunately red lily beetles are specific to just lilies and fritillaries, so your other garden plants are safe. Lily beetles only attack the above-ground portion of the plant so there’s no need to throw the bulbs away. Even badly attacked lilies should return next year, although they may be slightly weakened until the following year.

The most effective control for lily beetle is to remove and crush any adults and larvae by hand from April or May onwards. Check the underside of leaves as they can often be found lurking there. The small eggs are laid in clusters and are bright orange in colour. You will need to inspect your plants throughout the growing season to keep on top of the problem. However if you have lots of lilies I appreciate this could be a mammoth task! Once the lily beetles become active from April onwards, you can spray your lilies with an insecticide containing ‘thiacloprid’ (Bayer Provado Ultimate Bug Killer) or ‘acetamiprid’ (Scotts Bug Clear Ultra) - you’ll find these active ingredients listed on the label. These pesticides are systemic (enters the plant’s vascular system) so will last for a few weeks before you need to spray again. You’ll need to be vigilant in spraying, until late July/August when the beetles stop laying eggs. I find that growing lilies in large containers (45cm tall) helps and the problem is much more manageable than for those grown in the ground. Best of luck Laura.

Name: Lindsay Mitchell

Question: Hello, any ideas why my sweet peas have gone almost yellow? I have watered and fed them but they seem to get more translucent every day!

Answer: Hi Lindsay, you may have been too kind to your sweet peas! Over-fertilisation and over-watering can cause the plant’s roots to rot away, meaning yellow leaves and possibly plant death. Depending on how bad the damage is they may recover from this if the soil is allowed to dry out slightly. It may also be worth looking closely at your plants and underneath the leaves for symptoms of a pest or disease - such as holes, discoloured areas or distortions/bumps. If you could take a photo of your sweet peas and any close-ups of the leaves and stems I may be able to help you further on this.

Name: Andrew Cottrell

Question: I have a question about trying to raise Lavender Plants from seed. Is there a more sure way to do it as I have tried twice this year so far and failed miserably?

Answer: Hi Andrew, to simulate their natural germination conditions, lavender seeds need to be chilled for about 6 weeks. The seed should be sown on top of good quality seed compost and then covered with a fine sprinkling of compost, as they need some light to germinate. Cover the tray with a clear plastic bag or clingfilm and place in the fridge for 6 weeks. It’s worth checking that none germinate in the fridge during this time - if so simply take out the tray and bring it into warmth. After 6 weeks remove the tray from the fridge and place in a well-lit propagator at a temperature of 15-18°C (59-64°F). Make sure the compost is damp but not wet. Take care not to place them on a sunny windowsill where temperatures get too hot, as this can cause the seed to die. Lavender seeds do take many weeks to germinate and it’s not unusual to wait up to 2 months! Lavender cuttings are a much quicker propagation method, although I appreciate you may not always get the varieties you want and in such quantity as you do from seed. I hope this helps Andrew, best of luck.

Name: Jo Macdonald

Question: I have a large mature (10+yrs) Photinia ‘Red Robin’ with a 10-12ft height and spread which this year seems very sparsely leafed. Is this normal as they get older? Would pruning encourage new growth? I don't want to cut anything off if it won’t grow back.

Answer: Hi Jo, Photinia ‘Red Robin’ does have a tendency for sparse growth, particularly as it reaches maturity. Regular trimming each year will help to keep it bushy and vigorous. Photinias can also suffer die-back in extreme or prolonged winters such as the one we’ve just had. Depending on how straggly the growth has become there are two things you can do to help. You can remove any badly positioned or straggly stems and then tip prune every other stem, removing about 15cm (6") to give a nicely balanced shape. Alternatively you can carry out some renovation pruning to which Photinia x fraseri normally responds well (it will re-grow from old wood). For renovation pruning you cut all stems back hard to a low framework and thin out congested shoots as they re-grow. Light trimming can be carried out now but for drastic renovation I would be tempted to wait until next year, cutting back in April or May when temperatures have stabilised above freezing, and the plant is full of spring vigour. I hope this helps Jo - Photinias are very resilient plants and I feel sure by next year you will have a healthy-looking shrub once again!

Name: Julie Pearce

Question: I have a mature cherry tree, age unknown. Two years ago, we had a bumper crop of cherries. Last year, none at all, but that may have been due to the weather. This year the tree has produced only about 50% of the foliage of other cherry trees nearby, and a small crop of fruit, yet to ripen. Many of the branches have no leaves at all, and are breaking very easily in the wind. It also has a big split in the bark on the trunk which is leaking a sticky brown substance. Is it diseased? And if so, what can we do to recover it? Or should we get rid of it before any disease spreads?

Answer: Hi Julie, it sounds like your cherry tree may have bacterial canker, which commonly affects members of the Prunus family. Symptoms include dark sunken areas (which can look like cracks) in the bark in spring and early summer, which ooze a gummy sap. You may also notice the leaves, and sometimes fruit, have small brown spots on them which will fall out later in the summer to leave ‘shotholes’. If canker spreads all the way around the branches, they will rapidly die, which could be why so many of your cherry tree’s branches are brittle with no new growth. To try and save your tree you’ll need to prune out every branch displaying symptoms, cutting back to healthy wood. This is best done in July when tissues are most resistant. Use a wound paint to protect the exposed wood from re-infection. Don’t compost the prunings - burn them or dispose of them in household waste to prevent spread.

Unfortunately there isn’t much that can be done about the lesion on the trunk of the tree. To give your tree a fighting chance of recovering you’ll need to make 3 separate applications of a copper-based fungicide such as Bordeaux mixture, from late summer through to mid-autumn, making sure you completely cover the leaves and stems. This will give some control over the disease for next spring. If your tree is too badly affected and displays the same symptoms next year you may want to consider removing it completely. It would be preferable to plant your replacement tree in a different location. But if this is impossible then I would recommend getting into the habit of spraying your replacement tree with copper fungicide in autumn as a preventative measure. The varieties ‘Merton Glory’, ‘Merton Premier’, ‘Merla’ and ‘Merpet show some resistance to canker. Let us know how you get on Julie.

Name: Emma Stevens

Question: I have a Japanese maple in a pot that is way too small for it so it has grown out the bottom. Can I change the pot now and what compost do you recommend?

Answer: Hi Emma, it’s best to re-pot Japanese maples in April or September but they can be re-potted now provided they’re given some shade if we get hot weather. As Japanese maples are long-term container plants I would recommend using a loam-based compost such as John Innes No. 2 or No. 3, mixed with some ordinary multipurpose compost. This will make the pot very heavy but provides a much better growing environment for your Acer! Multipurpose composts break down over a number of years, becoming prone to water-logging and having little nutritional value. Acers will need feeding each spring with a slow-release fertiliser which can be applied to the compost surface. I hope this helps.

Name: Sophie Harrington

Question: Something (maybe root weevils) has eaten the root on every plant in my raised bed vegetable patch. Strawberries, curly kale and celery all suddenly shrink and root is gone but virtually no leaf damage. The other plant that is OK is parsley. What could it be and how do I stop it. Thank you. I have been out at night and not seen any weevils.

Answer: Hi Sophie, it is well documented that vine weevils love strawberry roots and the symptoms do sound like vine weevil. However, I’ve never heard of kale or celery being attacked, although if your raised bed is filled with compost it could be the cause as these pests prefer the loose texture of container composts. Another possibility with Brassicas such as Kale is the cabbage root fly. The best way to identify what is the problem is to have a dig around and check for larvae. Vine weevil larvae are white and often ‘C’ shaped, with a light brown head. Cabbage root fly larvae are white, straight, headless maggots. It could also be wireworms, which are the larvae of click beetles - the larvae are white when young but develop a tough golden skin as they grow. Alternatively the plant roots may have rotted from over-watering.

The best course of action if you have vine weevils or wireworms is to be vigilant and treat the affected areas regularly with biological or chemical controls. Chemical controls such as Provado Vine Weevil Killer can be used to drench the compost of container-grown plants but cannot be used in open soil as they will kill all soil dwelling creatures, including worms. Biological controls such as nematodes can be used in both containers and open ground as they are harmless to wildlife (and pets and children). They are effective in killing the larvae in the soil and can be applied once the soil has warmed up to at least 5°C (they are simply watered into the soil). Unfortunately there is no chemical control available for cabbage root fly - the best defence is to place a cabbage collar around the base of the stems of your kale, cabbages and cauliflowers. I hope these suggestions help Sophie, let me know if you need any further help.

Name: Nick Hamilton

Question: Hi, I have a plum that got silver leaf. I chopped it back hard into the main trunks. The growth this year is vigorous but one new branch shows silver leaf again. What would advise? Is silver leaf that much of a problem to risk losing the tree? How much more chopping back is worthwhile? Thanks, Nick.

Answer: Hi Nick, if it’s just the one branch I would promptly prune it right back to the trunk or to where the brown staining in the centre of the branch stops. All of the wood above the stained area of branch will be infected. The appearance of the silvery leaf colour indicates the branch will shortly die back, and it is from this dead tissue that more fungal spores will be released in the autumn. You also need to try and catch the fungus before it spreads through the rest of the tree. Dispose of any prunings in household waste or through burning. Unfortunately silver leaf can be fatal if the fungus manages to work its way through the tree, as it kills woody tissue. Sometimes the tree will recover from the infection without pruning but more often than not, if the whole tree is infected it is best removed entirely. Maintaining the health of the tree can help, so make sure it is getting enough water and try feeding it each spring with a slow-release fertiliser. Mulching with organic matter such as well-rotted manure will help improve the soil and it has nutritional value too. I hope you manage to catch the infection in time Nick, best of luck.

Name: Alison Cross

Question: What's a good way to get rid of greenfly from herbs?

Answer: Hi Alison, as these are edible plants you may prefer to use organic methods of control. I find the easiest (and most wildlife friendly) method to get rid of aphids is to use a strong jet of water to blast them off - you’ll need to repeat this every so often. You can also wipe them off with kitchen paper if you have the patience! Alternatively you can buy sprays. Chemical sprays are widely available but you do have to check the label to see whether they are suitable for use on edible crops. Organic sprays are based on natural plant oils or fatty acids and may need applying more often than chemical sprays to be effective. The benefit to organic substances is that crops can be sprayed up to the day before harvesting. You should find these readily available at all good garden centres. I hope this helps Alison, good luck.

Name: Tony Jarrett

Question: Should peppers be trimmed in any way? Or just fed, sprayed against flies and left?

Answer: Hi Tony, peppers shouldn’t need any trimming - this will slow their growth and flowering. Some varieties are naturally very tall and others only reach 30cm (12") - it’s best just to let them grow away. Regular watering and feeding (peppers are greedy plants) is all they need and it’s worth keeping an eye out for aphids and red spider mite. Garden centres will stock insecticide sprays if needed, although do check whether they are suitable for use on edible crops before buying. I hope this helps.

Name: Christina Goozee

Question: Who's been drilling into my Peach Avalon you supplied last year? It is the only one of the 5 peaches that has a hole and of 3/4 Peregrine peaches as well no nibbles.

Answer: Hi Christina, immature peach fruits are commonly gnawed by squirrels - this certainly looks like some kind of animal or insect damage. It could have happened earlier in the fruit’s development and has simply grown out as the peach has started to swell. You should still get an edible fruit as the plant will naturally repair the damage, however it will be a little misshapen. If you suspect squirrels could be problem you could net your whole tree or cover each cluster of fruit. If your tree isn’t touching any other trees or telephone wires (so access is only from the ground) then you can try wrapping something smooth such as sheet metal around the trunk, about five or six feet off the ground so the squirrels cannot grip. You’d need to prune out any particularly low branches for this to be effective. I hope this helps Christina, let us know how you get on.