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Mushrooms growing in raised beds are cause for celebration, not a time for worry. Mushrooms grow in moisture-retentive, nutrient-rich, places where water levels are healthy and partial shade allows them to thrive. Essentially, mushrooms are an indicator of perfect conditions for most vegetables.
However, there are some circumstances where mushrooms are unwanted guests, whether that’s the unmistakable odour of mushrooms growing in a greenhouse or the potential for poisonous mushroom varieties imported on woodchip or manure.
In this article, we’ll explore the benefits of mushrooms growing in raised beds, and explain how you can prevent them.
What are mushrooms?
It might sound like an obvious question, but most people still don’t realise the complexity and ingenuity of mushrooms.
Mushrooms are a fungus, but not the type that grows as parasites. They are spread by spores produced by the fruiting bodies on deep fungal networks called mycorrhizae. This network is made up of long web-like structures called mycelium, which is often mistaken for roots. They are in fact the main portion of the organism, with the mushroom you see overground being the fruiting body.
What is Mycelium, and how does it help plants?
Mycelium is one of the most common living structures on the planet. In every cubic inch of soil, there are more than eight miles of mycelial cells strung together, and naked to the human eye.
These structures, root-like when magnified, are actually built of thousands of tiny filaments called hyphae, which pass signals between each other and, in some cases, their hosts.
As mycelium nears the soil surface and finds an adequate host it produces mycorrhizae (myco: fungus; rhiza: root) which are substantially larger than their underground root system. These then produce the fruiting body we see as a mushroom.
For more information on the benefits of mycorrhizal fungus and its benefits in the garden (including additives for seedlings and young plants) read our article on mycorrhizal fungi and their use in the garden.
Why mushrooms like raised beds
Mushrooms grow in raised beds for several reasons:
- High moisture
- Shaded areas between beds
- Warmer soil
- Lots of organic matter
We use raised beds in vegetable gardens for a few different reasons, but the most important is that they provide a much more stable growing environment for our veg than simply planting in the soil.
Raised beds warm up faster than the rest of the garden in spring as they are slightly further from the water table and any standing ice. They also drain better, while typically having high amounts of organic matter like leaf mould or garden compost due to annual mulching.
The soil health, combined with timber edging and shaded gaps between raised beds creates the ideal space for mushrooms to mycelia to thrive, and mushrooms to fruit.
Are mushrooms a problem in raised beds?
Mushrooms do not cause a problem in raised beds. In fact, they have a few clear benefits for your plants:
- Mushrooms actively produce nutrients
- Mycelia has symbiotic relationships with deciduous fruit trees and bushes
- Mushrooms help break down fallen leaves in a similar manner to worms
If you don’t like the look of mushrooms or are worried about particular poisonous varieties, then mushrooms can be a problem.
While most mushrooms are harmless, some mushrooms are poisonous to humans and should be removed from any raised beds with edible crops to prevent contamination. These mushrooms are rare though, so if you find them growing in your garden, remember not to eat them, but consider letting them thrive quietly in a corner by themselves rather than destroying these rare and dangerous gems.
How do mushrooms develop in vegetable gardens?
Mycelia is present in all compost and all soil so it is very likely that there is the potential for mushroom growth in your garden, even if you have never noticed it.
Areas with higher humidity and slower drainage are more susceptible to mushrooms, and they can develop very quickly if areas are left untouched through late summer and fall.
The most common causes of mushrooms in raised beds are:
- Mushrooms imported on manure, or damp compost
- Untreated timber used for raised beds (or old treated timber)
- Slightly alkaline soil (can be caused by overfeeding with calcium, or over-liming)
How to prevent mushrooms in raised beds
Getting rid of mushrooms is relatively easy in most gardens, and is a case of leaving soils to dry out, and picking out fruiting mushrooms before they have a chance to spore.
Preventing mushrooms from growing in raised beds altogether can be a challenge, but with a little bit of hard work, you can keep your veg patch looking pristine year-on-year:
- Use only well-rotted manure
- Use stone raised beds instead of timber
- Improve drainage
- Encourage hedgehogs in your garden
1. The high temperatures and constant humidity of rotting manure create exactly the right environment for young mushrooms to thrive and establish young mycorrhizal networks of their own. Applying fresh manure to your own compost pile spreads the fungus, and it will inevitably return in the fall.
2. Timber raised beds are notorious for mushrooms, especially along the shaded side of the beds. Mycorrhizal fungi feed on decaying timber, so if this happens, they do cause damage by breaking down the timber. Either treat the timber sides with a 50:50 mix of vinegar and water or consider stone edging.
3. Because mushrooms love slightly damp, humid conditions, which are great for keeping your vegetables well fed, they are well suited to raised beds. Make sure any lining has holes pierced through so moisture can escape, and mix finer composts through the soil next year to improve drainage.
4. Squirrels, rabbits, deer and slugs also love mushrooms, and will often avoid other vegetables in favour of these high protein snacks, but are best kept away from the vegetable patch to avoid later devastation. Hedgehogs, on the other hand, love to munch on these tasty snacks, and will usually avoid the rest of your crops.
How to get rid of mushrooms in and on raised beds
There is only one way to get rid of mushrooms in raised beds when you spot them – pick them off before they spore.
It will take a few years, but eventually, the fruiting body of these fungal systems will learn that this just isn’t a good bed to put their energy into.
Plenty of forums will suggest raising the acidity with liver of sulphur or vinegar, but to raise the acidity to a point that gets rid of mushrooms will also destroy the rest of your soil health, so be patient, and pick them out by hand.
Can you add mushrooms to your compost pile?
Mushrooms are incredibly beneficial to a compost pile, and can actually help speed up the rate of decomposition. There is a chance that they will spore but it’s more likely that they will develop new hyphae, creating a new mycelium body that will spread through your compost and create a more even decomposition.
Final thoughts on mushrooms growing in your raised beds (and why you shouldn’t worry too much)
It’s easy to grow mushrooms on purpose in your home garden – as simple as inoculating a log with mushroom spawn plugs.
Our shaded greenhouse grows a crop of extra-large closed cap mushrooms every year as a previous tenant used to grow them on logs there. When we moved into the house, we didn’t realise and we threw out the logs, but they either spored or extended their mycorrhizae and now we have reliable mushrooms as a result.
They only seem to fruit in fall and spring when other things aren’t growing, and the soil has always been great there. Mushrooms growing in raised beds can be happy accidents in the vegetable garden, but be safe and check with an expert before eating any you find in shady corners!