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Big Bonus – The Horse Mushroom

Right now there are quite a few Agaricus (mushroom) species. I have already seen many species in varying urban habitats. I was especially lucky when I stumbled across these beauties literally round the corner from my house on a large grassy verge.

Horse Mushrooms in grassThe most welcome Horse Mushroom (Agaricus arvensis) is a great tasty mushroom that grows in most types of grassland, mainly permanent pasture land, but to reiterate, in this case it was a small grassy front lawn/verge.

They’re often found growing in large rings and this was no exception, even though it was only a partial ring. Still plenty to go around though.

The word ‘Horse’ used in the English name doesn’t reflect on where they can be found, such as fields with horses in (which is a common misconception) but is in reference to their large size. The largest in this group was 15cm across, the size of a small plate. 20cm is the maximum size on average and even at these dimensions, they are still relatively fresh and ready for the pan. If you find what appears to be a Horse Mushroom, but has a 30cm diametre cap, then you’ve probably found a Macro Mushroom (Agaricus urinascens), very similar indeed to our Horse Mushroom but slightly more scalier on the cap. That’s another story for another time.

People often avoid the Horse Mushroom because of the yellow (pale ochre) colouring that appears on the cap as it ages. Some are unsure that they could be dealing with the rather unwelcome ‘Yellow Stainer’, an extremely common look alike that could cause nasty gastro upsets (read all the about the Yellow Stainer in this post). In fact, some Agariucus xanthodermus were quite happily growing on a grassy verge nearby that very day!

But have no fear, the Horse Mushroom has some key characteristics that set it apart from the rest. Initially I always do the ‘Yellow Stainer’ test in which I rub the side of the cap and get the base of the stem out of the ground and snap it in half. If there’s some ‘strong’ chrome yellow colouring I simply avoid it. The Horse Mushroom has no extreme colouring like this and no colouring at all in the base of the stem flesh.

If you look around and find a very young example, the gills will be veiled by the what is to become the ‘ring’ on the stem (see picture below). A distinctive jaggedy ‘cogwheel’ pattern runs around the outer circumference of the membrane. This is always a good sign.

The young gills are white at first and turn pink, then eventually chocolate brown as time goes by. I found these at a good time and I didn’t hesitate at all in collecting some for my tea, leaving a few to do their thing.

They eventually ended up in a lovely mushroom soup (if I don’t mind saying so myself). I hope you too have some good luck in finding these beautiful and tasty mushrooms. Enjoy.

Horse mushroom pictures

The Horse Mushroom can grow up to 20cm in diametre. Notice the ‘cogwheel’ pattern on the veil, covering the gills of the younger mushroom (bottom left).

QUICK ID TABLE: HORSE MUSHROOM Agaricus arvensis

CAP / FLESH

5 – 20 cm across. Initially domed cap expanding out. Creamy white, yellowing with age. Flesh firm and thick. Slight smell of aniseed (more so when young). Veil on underside initially covers gills. Has a ‘cogwheel’ pattern.

STEM

8-10cm x 2-3cm. Same colour as cap. Often becomes hollow.

GILLS / SPORE PRINT

Free from stem apex. White at first, then pink, then chocolate brown with age.
Spore Print: Dark purple brown (see how to take a spore print here).

HABITAT / SEASON

Grassy pastures, lawns and sometimes grassy verges. Often in rings. Late summer – autumn. Common.

EDIBILITY

Edible and excellent. Good mushroomy flavour.

The Genus AGARICUS (Wood Mushrooms/Mushrooms): Characteristics to look out for:

• Many discolour yellowish, reddish or pinkish when cut or bruised.
• Those that discolour bright/chrome yellow should be avoided for consumption.
• Gills in young specimens are often pink (white in a few) – maturing darker brown.
• Make note of any smells, such as aniseed or a typical strong ‘supermarket’ mushroom smell.

Horse & Field Mushroom Imposter! – The Yellow Stainer

It’s a Sunday afternoon and I pass by a grass verge near my house. My heart jumps at the sight of a huge cluster of (what seem to be) Horse mushrooms or possibly Field mushrooms, but this is no field, just a grassy verge near trees at the side of the road! I was without a basket or bag so like a kid in a sweet shop I scooped up a good share, leaving some to drop their spores.

But my dreams of a nice fry up or even a creamy mushroom soup are soon quaffed because I suddenly realise these mushrooms are not what they appear to be. I wait until I get home around the corner to double check. Read on…

Image of yellow marking on Agaricus xanthodermus

Chrome yellow staining on cap edge. Bulbous base.

I’m not surprised at all that the Yellow Stainer (Agaricus xanthodermus) is responsible for the most cases of mushroom poisoning in this country. Although this evil twin of our favourite field, wood and Horse mushrooms is not deadly it can keep you in the loo for longer than you normally do! Gastric symptoms can persist for up to 24 hours. Fortunately I have not been caught out yet, but some people after consumption usually get away with only mild upset or sometimes have no reaction at all. My Uncle once told me “Ooh, I don’t get along with those Yellow Stainers – don’t like ’em” . Let’s hope that it was only his palette they offended – I didn’t ask!

So how on earth can I identify this poisonous peril when compared to a Horse mushroom? I hear you all cry!

Don’t rely on the ‘overall look’. They differ in colour from pure white to brown/grey, scaly and smooth, tall and short and so on. Horse mushrooms can display some paler yellow on the cap and stem – so if you do see some yellow it’s not always a bad thing.

Both the Horse Mushroom and Yellow Stainer ‘bruise’ yellow (There’s hardly any yellow about the Field mushroom). But the Yellow Stainer has a stronger chromium yellow once bruised. If you rub the cap with your thumb, there will be a very noticeable colour change. But the crunch test for me is at the very base. Take a knife to the very bottom of the stem (the base is more bulbous than the others) and cut in half (see picture below). If the colour changes to a vivid yellow, then you’ve got yourself a Yellow Stainer. Horse and Field mushrooms do not stain at the base like this.

Other good ID tips are:
1. The smell is an unpleasant (phenol/inky smell more apparent when being cooked)
2. The ring on the stem is large and floppy.
3. Before the veil drops it does not have the ‘cogwheel’ pattern like the Horse Mushroom.
4. Gills when very young are white unlike the Horse and Field mushrooms which are pink

In addition to point 3 – if you’re new to collecting mushrooms, avoid very young specimens as they also can be confused with much more poisonous (even deadly) young toadstools.

The Yellow Stainer - Poisonous UK mushroom

Horse and Field Mushroom lookalike – The Yellow Stainer. Notice the chrome yellow colouring at the base of a cut stem.

Large ring of Yellow Stainer mushroom

On open caps: The ring on the Yellow Stainer is noticably large and floppy. The Field Mushroom’s ring is a fine torn frill. The Horse Mushroom’s ring is formed of a double membrane. The lower part is ‘star shaped’

Quick ID checlist for Agaricus Xanthodermus.

The Genus AGARICUS (Wood Mushrooms/Mushrooms): Characteristics to look out for:

• Many discolour yellowish, reddish or pinkish when cut or bruised.
• Those that discolour bright/chrome yellow should be avoided for consumption.
• Gills in young specimens are often pink (white in a few) – maturing darker brown.
• Make note of any smells, such as aniseed or a typical strong ‘supermarket’ mushroom smell.