Fighting fungus gnats at the flower farm

Recently, the joy of watching the green tips of seedlings emerge from the soil has been obliterated by these darn little black flies. Known as fungus gnats, they've stopped many of my young plants from flourishing.

But every cloud has a silver lining and dealing with these little critters has been a hard lesson in the value of quality compost. 

Know your energy - about fungus gnats

Fungus gnat larvae eat organic matter, including plant roots

Fungus gnats (Bradysia spp.) are tiny (3-4mm), dark-coloured flies that infest soil and plant roots. You’re most likely to spot the adults as they fly above the surface of plant pots in search of new homes, food and mates.

They're attracted to moist environments and thrive in rich, organic matter like potting compost.

Female fungus gnats can lay up to 200 eggs at a time in the top inch or two of soil. An infestation can quickly escalate!

The larvae hatch within a few days and spend the next week or two feeding on plant roots, fungi, algae and other organic matter in the soil.

Once mature, they pupate and emerge as adult flies. Female flies will live for another 7 to 10 days, feeding and laying eggs.

Getting rid of fungus gnats

After trying numerous methods ultimately it was eliminating the source of the infestation that was the key to getting on top of my fungus gnats.

I tried:

• Changing my watering routine - only watering when the soil was very nearly completely dry

• Covering the soil surface with diatomaceous earth – a type of sedimentary rock that is said to irritate the exoskeleton of the gnats and their larvae

• Using sticky traps poked into pots

I don’t like using inputs. Diatomaceous earth, while a natural product, is still a finite resource. It is mined so, as well as destroying ecosystems, fossil fuels are inevitably used to excavate and transport the product.  Sticky traps, on the other hand, are a single use plastic. Neither are in line with my ethos.

They did, however, help to reduce the population of gnats and enabled many of the seedlings to survive long enough to go outdoors where the predators in the soil’s ecosystem dealt with the gnats swiftly and effectively.

Sticky traps helped but are a single use plastic

It was only when I re-potted some house plants and brought the gnats into my home that I realised where the problem was coming from - the potting compost.

In a rush to get some newly arrived seeds planted, I'd recently bought some Miracle-Gro.

Although peat-free and a reputable brand, it was riddled with these bloomin’ gnats. So each time I sewed seeds and added their trays/pots to the greenhouse I was re-infecting the environment – doh!

I cleared out the greenhouse of seedlings, washed it down, thoroughly cleaned the pots and found a new source of starter compost for very last of the bulbs and seeds to be sown this year.  These went into the conservatory (rather than the greenhouse) for germination in the hope that, if there are any survivors of the deep clean, I can starve them out over the next couple of weeks!

Today, I’m happy to report that there are no signs of the gnats in the ranuculus, anemones and sweet peas.  They are all looking happy and healthy.

I’ve learnt some important lessons too…knee-jerk solutions to problems often don’t get to the root cause and good quality compost is essential. I've sworn to have more patience and wait for my delivery of Sylvagrow.