Green mould on your grow kit: recognising Trichoderma and what you can still do

Our piece on what is normal on a grow kit had one exception: green. That exception deserves its own explanation, because a green patch is the only common problem where acting fast genuinely makes a difference. Here is what you are seeing, where it comes from and what you can still do about it.

How do you recognise Trichoderma?

Trichoderma starts as a small white spot indistinguishable from ordinary mycelium. Within a day or two the centre turns grass green to blue-green and the patch becomes powdery, as if green dust has settled on it. That dust is millions of spores. The sharp edge is characteristic: the green often sits like an island in white surroundings, and that island grows by the day.

Do not confuse it with the blue-green bruising some mushrooms develop when handled, or with the yellow and brown patches that come with older mycelium. Powdery plus green plus growing is the description to go by.

Where does it come from?

Trichoderma is one of the most common fungi on earth. It lives in every handful of garden soil, in compost, on old wood and in the air. At the nursery we keep it out with sterile work and clean air, but once a block is opened at your home, it lies in wait. A slit in the plastic is a door, and a block kept too warm and too wet is an invitation. So the contamination usually starts at home and rarely in transit, though we never rule the latter out.

Can a block with a green patch still be saved?

Often yes, if you catch it early. Rule of thumb: a patch smaller than a palm on the outside can be treated, a block that is green on the inside cannot.

1. Isolate the block, away from other grow kits and out of the kitchen where you prepare food.
2. Cut the green patch out generously with a clean knife, at least a centimetre into the healthy white part. Do this outside or over the sink, raising as little dust as possible.
3. Put the cut-away part straight into a sealed bag in the bin, not on the compost.
4. Wash your hands and the knife, and return the block to a cooler, airier spot.

If the green returns in the same place within a few days, it sits deeper than you can cut and the block is lost. Do still take the harvest that is on it: mushrooms growing on a healthy part of the block are perfectly edible, just rinse them.

Is Trichoderma dangerous?

Not for healthy people. It is a garden fungus you come into contact with daily. That said: do not breathe it in on purpose, so no green clouds while cutting, and anyone with a weakened immune system or asthma should leave the clean-up to someone else.

How do you prevent it next time?

Three things do ninety percent of the work. First, coolness: a block at 15 to 20 degrees has a clear advantage, at 25 degrees and above Trichoderma almost always wins. Second, air: no closed cupboard, but a spot where the air refreshes now and then. Third, moist but not soaking: misting is good, puddles in the bag are not. And opening a block within a week of receiving it and putting it to work straight away, rather than leaving it in the box for weeks, gives the mould hardly any time to get in.

When should you email us?

Always when the green is already there within the first days after delivery, or when it runs through the whole block. Send a photo and your order number via the contact page and we will look at it with you and sort it out. An infected block is nothing to be embarrassed about, it is a living product, and sometimes the wrong fungus wins. We will make sure you still get to harvest.