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Facebook Q&A Session 2nd September


Thompson & Morgan Facebook Q&A Session 2nd September - Your horticultural questions answered.

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

Name: Jacqueline Hathaway

Question: My bay tree is slowly dying. Have you got any possible suggestions?

Answer: Hi Jaqueline. It’s impossible to say what the problem is without a description of the plants symptoms and its current growing conditions. However I can give you some general advice and hopefully something here will ring a bell with you. My immediate thought is that you may well be growing it in a container. If this is the case then it may have outgrown its pot as Bay trees have quite extensive root systems. Try re-potting it into a larger container, using a soil-based compost such as John Innes number 3. If the rootball is compacted then make sure you loosen some of the roots with a garden fork before you repot it. If the leaves are showing signs of distress such as yellowing then it may be suffering from a nutrient deficiency. Try feeding your Bay with a well balanced feed, or work some fish, blood and bone into the soil.

Although Bay trees are very resilient to drought they will be much happier and healthier if they receive a regular supply of water. This is particularly important for container-grown plants, and during hot dry periods when the soil can dry out surprisingly quickly. You could also apply a mulch next spring to help retain moisture at the roots.

Finally, it’s possible that your bay may have struggled to recover if it caught the frost last winter. If the stems have become bare and no further buds appear by late spring next year then the bare stems should to be cut back to live wood. Bays can tolerate hard pruning but are quite slow to recover so it is best to avoid this unless necessary. Try repotting, feeding and watering first to give it a chance to recover.Hopefully something here sounds familiar to you. If not then please provide some more information (and a picture if possible). Best of luck Jaqueline.

Name: Vicki Martin

Question: My onions are always tiny - never get beyond maybe 2" diameter - is this likely to be lack of watering in dry East Anglia or could it be something else? I plant the sets in autumn - your Seryu (or similarly named) variety. Last year was a little better than this but not much.

Answer: Hi Vicky. I think the variety that you are referring to is Senshyu. Senshyu will normally grow to about 8-10cm (4") in diameter so yours are definitely falling short of their potential.

Autumn planting onions are a long season crop so it is well worth improving the soil before planting. Prepare the planting area several weeks in advance by incorporating some well rotted garden compost or other organic matter to soil structure and fertility. Prior to planting it is also worth adding an onion fertiliser.

You can plant them from September, spacing them about 8cm (3") apart. The earlier that you get them in the ground, the better they will have settled in before winter. You may well be right that your onions lacked water this year. I had the same problem and my onions were also on the small side. We had a very dry spring this year which is a fairly crucial time for onions. Next year, try to stay on top of watering from April onwards and you may want to give them another feed in spring, as phosphate is quite important to developing onion bulbs.  Of course, if you are looking to grow really big onions then you would be better off choosing a Spring variety such as Hercules. I hope this helps Vicky. Better luck next year.

Name: Julie Pearce

Question: My Question - I often see tv gardeners using grit in their potting compost. Why? and more intriguingly, what do they do with the gritty compost after? sieve it and recycle the grit?

Answer: Hi Julie. Some plants need looser, more aerated compost than others, and adding grit helps to achieve this. The addition of fine grit will open pores in the compost that will allow water to drain more freely and draw air into these pores. This helps to prevent bulbs, roots and plant crowns from rotting. But you won’t need to do this for every plant. Any left over/ used gritty compost can be thrown onto borders to help improve the soil there. I certainly wouldn’t waste time sieving it!

Name: Alistair Conran

Question: Is there an organic home made spray that can be applied to conservatory plants to kill spider mite? Im unable to keep the plants damp due to work hours and travelling.

Answer: I’m afraid that spider mite is a particularly difficult pest to eradicate and generally requires something more substantial than a homemade remedy. Spraying with the Provado will help but spider mite readily develops tolerance to chemical controls and generally requires several applications in order to eradicate all stages of the lifecycle (mites, nymphs and eggs).

If you prefer to use organic methods then your best bet is Biological controls such as the predatory mite Phytoseiulus persimilis. This particular predator reproduces at twice the rate of spider mite. It is highly active under hot dry conditions, where just one mite of P. persimilis can consume 20 eggs or 5 adults per day! However, it requires a minimum temperature of over 16C (although daytime temperatures of 21 C are preferable) and there needs to be sufficient numbers of spider mite present for the predatory mites to feed on. Obviously, if you choose to use biological control then you can’t use chemical control as well because this will kill the predatory mites!

There are a number of cultural practices that you can also use to help prevent and reduce outbreaks. Spider mite thrives in warm dry conditions so moving your plant to a cooler, more humid environment may slow their reproduction slightly. Regular inspection of the undersides of leaves will also help to detect the problem before it spreads. Pick off and burn any infested leaves when you find them. Finally, at the end of the growing season, make sure that you take time to thoroughly clean away any plant debris as spider mites like to spend the winter hidden in dry crevices and plant debris until the weather warms up again in spring.

Name: Wes Halton

Question: I have a plum tree with really bad capsid infestation whats best way to deal with it? This year almost every leaf and plum is affected.

Answer: Hi Wes. The majority of Capsid damage occurs in spring when the nymphs hatch and begin to feed on sap in the leaves, as well as the blossom and developing fruitlets. By July these nymphs will have matured into adults and begin laying eggs in bark crevices which will overwinter and hatch the following spring.

As with all pests and diseases, control will be most effective if you can break or impede their life cycle. Capsid bugs are most vulnerable as nymphs from late spring so schedule in a preventative spraying programme for May, June and July after the tree has finished flowering. Use Bayer Sprayday Greenfly Killer or Westland Plant Rescue Fruit & Vegetable Bug Killer. Hopefully next year’s crop will see less damage.

Name: Kay Rogers

Question: I have some really stubborn bramble in a border. What is the best way to get rid of it permanently? Roots seem to go on forever if I try digging it out so is there a product I can use please.

Answer: Oh dear Kay. Brambles can be a real nuisance, but I’m afraid that digging is the best method. That’s the way I cleared my plot over 10 years ago and although occasional stems still return in crevices here and there, they have never been a significant problem since they were cleared. Cut them back to ground level and dig up what you can see. Thereafter, hoe off any new shoots as they emerge. The secret is to stay on top of them, and eventually they will become starved of energy and give up invading your border.

Alternatively you could try painting a systemic glyphosate based herbicide such as Round Up Tree Stump and Root Killer on to the leaves of the new shoots when they emerge. However your will still need to clear the plot in the first place and its often quicker to dash round with a hoe once a week than to fiddle about painting on herbicides. Good luck Kay.

Name: Zoe Stewartpepper

Question: I have chillies and sweet peppers this year, and the fruits on both of them have gone from green to black (instead of my promised red!). Any idea why this is?

Answer: Hi Zoe. If they look normal except for the colour and the fruits remain glossy and firm then this is probably a natural pigmentation. You can see from this picture that our peppers here at T&M are starting to do the same thing. Many peppers go through a black stage before ripening to red and this is perfectly normal. Your chillies should still have time to ripen to red, but it may be worth moving the sweet peppers into a greenhouse if you have one, to try to ripen them before the end of summer.

Name: John Goss

Question: How do I treat / prevent leaf spot on my Gerbera plants?

Answer: Fungal leaf spot infections tend to develop in humid, moist conditions and the spores are easily spread by wind and splashes of water. Spray the plants with a fungicide. There are lots available from the garden centre but make sure that you choose one that is appropriate for use on ornamentals.

There are also a couple of things that you can do in order to reduce the spread of the spores. Ensure that your gerberas receive adequate ventilation particularly after watering. If they are in the greenhouse then I would recommend that you water before midday and open windows to improve air circulation during the daytime and dry out any splashed water. This will ensure that the foliage of the plants remains dry and that the humidity is reduced. The windows can then be closed late in the afternoon. If they are in containers outdoors then space them out a little more to improve airflow around each plant.

I would also strongly suggest that plants are watered from beneath their foliage as overhead watering is the main cause of fungal diseases spreading. You should also clear up and destroy any plant debris that might harbour fungal spores. I hope that helps John.

Name: Victoria Oliver

Question: Just moved into a garden with heavy clay soil. Whilst I am adding in grit and sand to break up the clay and multi-purpose compost to help my plants establish themselves, is there any plant that particuarly thrives in clay soil?

Answer: Hi Victoria, it sounds like you’re doing the right thing - lots of organic matter is the key to improving clay soil. In addition to compost you can use manure or recycled green waste to help improve the soil structure. Keep mulching around your plants annually and the organic matter will naturally work its way into the soil.

There are quite a few plants which will thrive on clay soils. If you’re looking for trees then you could try crab apples, birch, Japanese maple (Acer palmatum), Sorbus (Rowan) or a weeping pear (Pyrus salicifolia ‘Pendula’). Good shrubs include Berberis, Hydrangea, Roses, Weigela, Mahonia and Fuchsias.

If you’re looking for herbaceous perennials you’ll find Aster novi-belgii, Astilbe, Astrantia, Hosta, Rodgersia, Filipendula and Solidago thrive in the damp conditions provided by clay soil. You can also grow a range of grasses in clay, including Deschampsia, Calamagrostis, Elymus and Ophiopogon.

It’s best to start planting in the spring as even the toughest plants may not survive a waterlogged clay soil over winter. When you do plant, make sure you break up the soil at the bottom of the planting hole first to prevent water collecting - you could even add some grit for extra drainage. I hope this gives you some ideas to start Victoria, best of luck.

Name: Amanda Dawson

Question: Was doing well with my container crop veggies till one day the caterpillars ate the lot just days before i harvested. any advice please?

Answer: Hi Amanda, I’m sorry to hear your crops got eaten - I know how soul-destroying it can be! If you are growing cabbages, cauliflowers, kale, pak choi or broccoli the most likely candidate is the cabbage caterpillar (caterpillars of the cabbage white butterfly) which will devour Brassicas very quickly once they reach a certain size. Caterpillars of the small white butterfly and the cabbage moth are green, and the caterpillars of the large white butterfly are black and yellow.

If you prefer to garden organically you can rub off the clusters of tiny yellow eggs from the undersides of the leaves and pick off the caterpillars by hand. Make sure you check your plants regularly as the butterflies may still be laying eggs at this time of year. You can also use nematodes, small parasitic worms which infect the caterpillars and kill them. It is perfectly safe to use on edible crops and is harmless to children, pets and wildlife.

If you don’t mind using chemicals then there are quite a few sprays available in garden centres which will state on the bottle which pests they are effective against. I have found the best method of control is to net all my Brassicas with a fine mesh, preventing the butterflies laying their eggs in the first place. Make sure you use a mesh with holes no bigger than 7mm - it’s surprising how the butterflies can squeeze into even the smallest gaps. I’ve found netting very effective and a lot easier than picking the caterpillars off by hand each week. I hope this helps Amanda, better luck for your next crop.

Name: Sarah Griffiths

Question: When is the best time to wrap up fragile fern plants ready for winter? Nearly lost 3 last year as left too late. Do you prune off any remaining foliage before covering with straw and hessian sacks? Thanks

Answer: Hi Sarah, you can wait until the first frosts before wrapping up more tender ferns such as the tree fern (tree ferns can cope with temperatures to about -5?C). If the leaves are still green then leave them - you can still pack straw in and around the crown of the fern with the leaves on. It’s also a good idea to draw up the leaves into a bundle after doing this, and tie with string before wrapping hessian around the trunk. This makes the wrapping slightly easier and also further protects the crown. If the leaves are completely brown and dead they can be removed. I hope this helps Sarah.

Name: Deirdre Snook

Question: I have just bought some new herbs - lemon curd thyme / chocolate mint & am wondering if I should plant them out now with the other herbs or keep them indoors? Should the other herbs be trimmed back before winter?

Answer: Hi Deirdre, these herbs sound delicious! Both mint and thyme are fully hardy plants so should be planted outside for the winter. If you’re unable to plant them and want to keep them in their containers put them in a sheltered place against a wall or in a cold frame to over-winter. It’s best not to cut back herbs now as this will encourage new growth which is easily frosted. Deciduous herbs such as fennel, chives and mint will soon be starting to die down and will need their leaves for energy, which is drawn into the crown and roots and preserved for the winter. It’s best to leave any dead foliage on the plant to help protect it through the winter months. Trim your herbs in the spring when growth is vigorous, to encourage fresh new shoots and leaves. If you’d like to find out more about growing herbs take a look at our ‘How to grow herbs’ article. I hope this helps Deirde.

Name: Daniel Stewart Marshall

Question: Hi Sue. I have a young Goji Berry plant that I have left to its own devices in a pot. It has grown 4/5 really long straggly (6ft+) branches. What should I do with it. The RHS website recommend training against a wall or fence but I don't have any spare. What should I do to make it more compact (and still fruitful!) ? Thanks

Answer: Hi Daniel, pruning is normally carried out in early spring and is only needed to remove damaged or crowded growth. However it sounds as though you have a very vigorous plant! To keep plants compact it’s best to prune during the summer as you would with a cordon apple tree to restrict its size. There are no strict rules - you can prune it to fit the available space, making sure you cut just above a bud each time. Bear in mind that these plants fruit on wood that is a year or more old so don’t prune too hard - you can reduce the branch length by up to a half.

If you’re planning to keep your Goji in a pot make sure you feed it during the growing season. Try using a fruit fertiliser which contains a high proportion of potassium and encourages flowering and fruiting. Good luck Daniel, let us know how you get on.

Name: Mary O' Donovan

Question: Is it better to sow hardy annuals in the autumn or the spring?

Answer: Hi Mary, this is a good question! Some hardy annuals perform better when sown in the autumn and others have variable survival rates so are better sown in the spring. The hardiest annuals which will thrive on an autumn sowing include cornflowers, corn cockle, poached egg plant, love-in-a-mist, larkspur and annual poppies.

Hardy annuals which may need some shelter in the winter, or are better sown in the spring, include sweet peas, Californian poppies, Lavatera, Cerinthe, night-scented stock, Nemophila and Salvia viridis. Success can also vary depending on where you live and the type of soil you have. You will find many hardy annuals self-sow very well so you may only need to plant them once! The best thing to do would be to experiment and find what works best for you. Good luck Mary, let us know you get on.

Name: Lucy Garden

Question: Hi, Sue, my romanesco plants are nice and healthy, but show no sign of forming anything edible unless I pick the leaves and eat them! Is there still time, or do I give up? I'm not even sure where to look - would the curds form at ground level or at the top of the long stem? I've only seen pictures of the end product, not of it when it's still growing.

Answer: Hi Lucy, the curds form on the top of the stems and are nestled in the leaves similarly to a cauliflower. You can normally expect to see them anytime between now and December, so don’t worry just yet! Although your plants may just be taking their time, do make sure you’re not feeding them too much nitrogen as this concentrates all their energy into growing new leaves rather than flowering. Hopefully you’ll be seeing some crops soon, let us know how you get on.