I would guess that this may be the most commonly recognized edible wild mushroom in the Southern Rocky Mountains. It goes by many names: porcini, king bolete or king, Rocky Mountain red-capped bolete, Rocky Mountain red, penny bun, or often simply bolete. People may also refer to this mushroom as Boletus edulis because it was classified as that species until 2014 when David Arora and Jonathan Frank published a paper that distinguished our Rocky Mountain porcini as its own species!
The porcini species holds a special place in my heart! When I started mushroom hunting around age 10, this was the first species we looked for. It was my introduction to the world of fungi.
This is a great mushroom for beginners because they're often plentiful, easy to find, simple to identify if you follow a couple of rules, and delicious. In the rest of this post, we will go over the main characteristics of the Rocky Mountain king bolete, where and when to look for them, common look-a-likes and the traits that differentiate them, and anything else that I think you should know about this species!
Identifying King Boletes
Boletus rubriceps is a stout mushroom with a convex cap that is reddish brown and often slightly sticky or greasy when fresh. It has a robust stem, pale cream to white with white or sometimes light pinkish reticulations (net-shaped patterns). The spore-bearing structures are sponge-like pores. They are white in young specimens but progress to yellow and eventually greenish brown as the mushroom matures. The flesh is white and does not stain when cut, though it can be accidentally stained during cutting by pulling some of the cap or pore color through with the knife.
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How to Find King Boletes
These mushrooms usually begin fruiting with the summer rains in July, if we get rain, and can be found into late September or even October. Early in the fruiting season, they can be found as low as 8,000 ft. and will continue to grow up in elevation to the tree line. In Colorado, they mainly associate with Engelmann spruce (Picea engelmannii), and sometimes firs (Abies spp.) and Douglas fir (Pseudotsuga menziesii). They can also be associated with pines (Pinus spp.), which is less common in my experience.
Now that we've gone over when they fruit, what trees to look out for, and which elevations to target, let's get more specific about where to look. I tend to focus my searches on northern aspects, these north facing slopes tend to stay a little cooler and hold moisture longer. This can be especially useful during our dry years in Colorado! Porcini can often be found fruiting along trails or near creeks or other water-holding features, though I do tend to find them a little further away from the moisture than some species. While looking for king boletes, you should also look out for fly agaric mushrooms, Amanita muscaria, and hawk's wing, Sarcodon imbricatus, as these species share environmental preferences with them.
King boletes have very few look-a-likes, and the few that it might be confused with are very easy to tell apart once you learn some key traits. The most commonly misidentified species are from the genus Leccinum, also known as scaber stalks. At first, these mushrooms can appear to be Boletus rubriceps. However, a few obvious characteristics will give them away upon closer examination. The first is how the genus Leccinum gets its common name, the presence of black or brown scabers on the stalk. The word scaber comes from scabrous, meaning rough or scab-like, but the scabers on these mushrooms are fairly soft, feeling almost like the mushroom has fur on its stalk. If you miss the scabers, the next very obvious trait is staining flesh. The species we have in Colorado can stain blue, purple, grey, or even reddish, though the first two are most common.
It's worth noting that many people consider the Leccinum species we get in Colorado to be edible. However, most field guides, as well as myself, advise caution. There have been many instances of poisonings concurring from orange-capped Leccinum, often referred to as aspen boletes. There have not been any definitive answers as to why the poisonings are occurring, but the symptoms sound bad enough that I choose to leave these mushrooms in the forests.
The other mushrooms I sometimes see confused with king boletes are slippery jacks from the Suillus genus. The mushrooms in this genus are also easily differentiated, with a few traits. The first is simply shape and size. Slippery jacks in Colorado are usually pretty small, rarely having caps over 5 inches. Their stems are skinny and small compared to the stout stem of a king bolete, and they also have a unique trait called glandular dots. These little dots spread down the stem, starting at the cap. They're often brown or tan but can also be light or even white. The other obvious feature of mushrooms in the Suillus genus is a very sticky cap. These mushrooms will often have pine needles and dirt stuck to their caps. This stickiness can sometimes be faint or completely missing in some specimens in dry conditions. From there traits can vary between species, some will stain when cut or damaged, some can have residual rings from a partial veil, some can even have a velvety appearance to their caps.
Harvesting King Boletes
Finally, here are a few notes about harvesting king boletes once you find some! Many people will suggest cutting wild mushrooms to preserve mycelium health. We will save that discussion for another day, but here is a link if you'd like to do more research on the topic: http://www.fungimag.com/spring-2012-articles/LR_Agaricidal.pdf.
I would recommend a gentle pulling and twisting motion to harvest them. These mushrooms are often surrounded by dirt and organic material, especially when young, making it very hard to get a knife in for cutting them cleanly. Pulling also results in more food to take home because their stems are large and usually mostly below ground level. Once removed from the earth, brush and cut away any debris without wasting too much meat.
These mushrooms, and many others, will often house small fly larvae once they've matured. If you start to clean one and find a lot of small holes or larvae, just put the mushroom back where you picked it or break it into pieces and disperse it around the area. This is a personal decision. Some people don't mind the extra protein. I usually take specimens with a few holes, but any with lots of larvae get left behind.
Lastly, as mentioned above, once these mushrooms mature, their pores become yellow or greenish-brown. I will leave the pores on young porcini if they are still white or off-white and consume them with the mushrooms, but once they become yellow, I remove them and spread them around the area before leaving. I find that the pores can become bitter at this point, though many people keep them and enjoy the flavor, or they separate and dry them to use as a mushroom powder or thickening agent!
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