Premium

How to get rid of fungus gnats, according to expert Helen Yemm

This week: the ongoing battle with fungus
gnats, a citrus tree that has outgrown its
home and the benefits of the daily grind
This week: the ongoing battle with fungus gnats, a citrus tree that has outgrown its home and the benefits of the daily grind Credit: Alamy Stock Photo

Every week, Telegraph gardening expert Helen Yemm gives tips and advice on all your gardening problems whether at home or on your allotment. If you've got a question, see below for how to contact her. 

Fungus gnat battles 

In the wake of recent press coverage of the planet’s beleaguered insect population, I will not make light of attempts to vanquish one of the smallest of winged irritations – the fungus gnat, as seen in greenhouses and indoors.

Also known as sciarid flies, these tiny insects, which hover close to the surface of compost, are themselves harmless. However, their minute, wormlike white larvae feast mainly on algae and organic matter and also damage tiny fragile surface roots in constantly damp compost, which can cause havoc among seedlings.

Biological control is available for the larvae, but if the flies are introduced to a house or conservatory plant collection (in my own case via an impulse-bought amaryllis from Ikea), steps should be taken to get the upper hand – and it is possible to do this, without using chemicals.

I was tickled, on a recent Twitter trawl, to read a despairing tweet from fellow garden writer Alex Mitchell (@alexmitchelleg). Yellow Post-it notes smeared in Vaseline, she observed, absolutely do not do the same job as the traps traditionally hung indoors and in greenhouses to achieve “control” of flying insects (i.e. hasten them to a sticky end). The traps (from Vitax, for example) are chemical-free but are covered with a powerful sticky substance – and the bright yellow colour is irresistible to the gnats.

Alex’s tweet struck a chord. Steve Woodward (@plugwoodward) observed that carnivorous sticky-leafed sundew plants (Droserae) are helpful. Others asked for yet more non-chemical answers: Alex keeps her compost drier than heretofore, which helps, she says, but has become “resigned”.

I am still battling: by shoving short strips of cut-up yellow traps vertically into the compost around the edge of my amaryllis pot, I trapped a gnatty squadron, although the stockade (stickade?) looked hideous. I am also topping my pots with a recommended ½in (1cm) layer of grit to discourage egg-laying.

Coffee grounds in the garden? 

For years, I put my coffee grounds and tea leaves straight on to the garden, something I was told recently was not good for my plants. Though I could put them in my wormery or compost bins, it’s not quite the same as forcing myself out into my London garden each day to choose a new spot. I should love to know what you think. Christine Hastings – via email

By putting coffee grounds and tea leaves on your garden you are adding, albeit in small quantities and modified form, something “organic” to the soil. This cannot be a bad thing. Furthermore, the daily trip around your patch to find a new place to tip your grounds and/or leaves gives you the chance to observe, troubleshoot, fiddle around and tweak at things, which is an essential part of proper gardening, in my view. Some gardeners advocate begging grounds from coffee bars and the like to use as a short-life anti-slug/snail barrier around precious plants. Like you, I make worm compost in a Worm Café (from wigglywigglers.com), which is where I tip all my drink dregs to join the veg trimmings, egg boxes, etc, to create, with the worms’ help, a sludgy compost as well as a rich liquid plant food. I use this as a “booster” for the contents of my regular bins.

When life gives you lemon (trees)

Lemon tree Credit:  Getty Images Contributor

I have three citrus trees, grown from pips and now almost 4ft (1.2m) tall, that are too large to be accommodated in our fairly full conservatory. I appreciate that they may never flower or fruit, but I should hate to lose them after all this time. How frost-tolerant are they, and what insulation do you recommend? Gerry Gibson, Peterborough
 – via email

Alas, citrus trees are not hardy in the UK. They need a cool and bright situation under glass in winter but from June to October, they need to be soaking up the sun outdoors.

That said, I have friends who live in the highest village on Dartmoor who proudly showed me, a few years ago, a squat, sullen-looking “outdoor” lemon tree bearing a solitary lumpen fruit with peel resembling a Sherpa’s jacket. And I am sure there are people with hot little courtyard gardens somewhere to the west of Torquay who will delight in sending me pictures. But outdoors in Peterborough? I fear not.

You have done well to get your citrus charges this far, and I sympathise: but they will go on growing and so will the problem. If you want to keep them, you may have to say goodbye to something else in your conservatory to make room for them and then keep their increasingly weighty pots on trolleys so that they can be wheeled outside.

Is there any chance you can give them to an appreciative friend?