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  • Writer's pictureEd Joice

Timing Yellow Morel Mushrooms

I see memes out there all the time about morel mushrooms, particularly about how elusive those little buggers are. How there’s all kinds of folksy and often contradictory indicators to note when morels will start to fruit. Like: look on south facing slopes some say, other say look on north facing slopes. Some say look at dead trees, other say live trees. Some say after the last frost but not after all the leaves have fully opened up. When the moon is in alignment with Venus and Taurus. All kinds of stuff out there. There may be some truth to some of these folksy sayings, but there's a lot of wacky falsehoods. The purpose of this article is to cut through the myths and help folks make data driven decisions on when to head out into the woods to start looking for morel mushrooms.


Soil Temperatures


There is very strong evidence that you will not find morel mushrooms until soil temperatures at depths of 4 to 6 inches deep (top soil horizon) have risen to an absolute minimum of about 47°F (Kuo, Morels). Unfortunately I cannot say why morels begin to fruit at these temperatures, merely that they do. From empirical evidence it’s also fair to say that 47°F would be on the colder end of the spectrum. I have seen 45°F cited as a minimum online, which might be true at deeper soil depths or for black morels, but could not find any evidence to support the 45°F value. Personally, the coldest temperatures at which I have found them was 49°F.




Earliest morel I'd ever found in Minnesota on Aprill 11th, 2021.

If you go out searching for morels in areas where soil temperatures are lower than this, you will not find morels or you will not find very many at all. Even if soil temperatures are >47°F, if they have only been at those temperatures for a couple days, the morels you find will start out very small. Like smaller than a thimble. Early season morels, with cooler temperatures, grow very slowly and can take as much as two weeks to grow to full size. Later in the season, particularly with lots of warmth and moisture, morels can grow to full size much more quickly but it will take several days of growth before they are to a good picking size. Point is, I wouldn’t go searching high and low the second the soil temperatures hit 47°F. 




April 30th, 2021 showing 49 deg F soil temps and picked morel stems left behind by previous hunter.

Another really important caveat which might seem obvious is that soil temperatures are highly variable. Not only from region to region, but even in the same city park, for example, there could be a 5-10°F range of soil temperatures. I once measured soil temperature in the same spot in the morning and then in the afternoon and it was a 13°F difference. The variability of soil temperatures is due to numerous factors such as soil moisture and composition, elevation, time of day, slope aspect, ground cover/vegetation, and more. The easiest factor to account for when considering where to hunt for morels is probably the elevation and slope aspect. Essentially, what both of these factors are getting at is sun exposure. With more sun exposure, soil temperatures can really heat up quickly. This is why folks will talk about hunting south facing slopes (the sun is slightly more in the southern part of the sky, thus south facing slopes experience more direct radiation from the sun). On the other end of the spectrum, north facing slopes get less sun exposure and therefore will reach the required soil temperature for morels to fruit a week or two later than south facing slopes. 




A great way to keep an eye on soil temperatures is to take advantage of agricultural or farming resources out there providing data for large regions like the image above (source: http://www.glasgowmfa.com/index.cfm?show=1&mapID=20). As you start to see soil temperatures in the areas you like to hunt in the high 40’s or low 50’s, then I would start heading out to your spots. Bring a basic kitchen temperature probe and check some soil temperatures as you hike around to verify that the online maps are accurate to your specific location. Assuming your soil temps are where they need to be, then keep going back to your spots and check over a couple week period. Just because you don’t see them right away does not mean that the morels aren’t out there. They might just be too small to see!


Degree Days


Another method for gauging timing of when to start looking for morels, rather than keeping an eye on soil temperatures, is to keep track of Degree Days (DD). There’s a few ways of calculating growing degree days, but the basic premise is that you get an average temperature for a day, subtract a “base” temperature (I use 32°F), and end up with a degree value for that day. If seeing the formula is helpful, here it is:





Let’s walk through a quick example for folks whose strong suit is not math: if a day’s high temperature was 64°F and the low was 20°F, then they add up to 84°F. 84/2 is 42°F. 42°F-32°F is 10°F for the DD. These degree days are cumulative, so if you have 3 days in a row, each with a degrees day of 10°F, then by the end of the third day the total degree days is 30°F. There are a number of great resources out there that are helpful for calculating degree days–here’s one of the ones that I used for the chart below: https://newa.cornell.edu/degree-day-calculator 


Unlike soil temperatures, there’s no exact number of degree days that are required for morels to fruit. However, soil temperatures are much more specific to each area and location and are highly variable, whereas degree days are more helpful from a broad brushstrokes standpoint in terms of determining when you should expect the hunt to be on in all places and how long you can expect it to go. 


Given that degrees days are helpful for seeing overall trends, I want to show some of my data from years past in terms of degree days. The chart below shows the degree days in central Minnesota in the last 8 years, since I have lived in the region. It’s clear that 2016, 2017 and 2021 were warmer springs than 2018, 2019, 2020, 2022 and 2023. My morel finds reflect that because in 2016, 2017 and 2021 I found morels in late April that were pickable size (which I consider to be thumb size or larger - my “rule of thumb”). In 2018, 2019, 2020, 2022, and 2023 I did not find pickable morels until end of the first week of May or even the end of the second week of May. What that ultimately equates to is that according to these charts and my morel seasons from the corresponding years, quality pickable morels are found most frequently between the DD’s of 700 to 1200.


 




You can see in the chart above that this year is making a step change in the DD’s per date–we are about 100 DD’s ahead of those 3 warm years of 2016, 2017, and 2021. If those years trended about 1 to 2 weeks ahead of a typical MN morel season, then if trends continue I would expect this year to land 2-4 weeks ahead of a typical MN morel season. However, DD's alone do not tell the full story. In fact, a great research article that also analyzes a lot of what I've discussed here suggests that for warmer winters morels may actually require an increased number of degree days for morels to fruit. The author, Jeanne Mihail, suggests that the ~47°F soil temperature "trigger" for morels to fruit may be less sensitive due to the lack of a "chilling signal" and, paradoxically, results in a later fruiting than might be expected when considering DD's alone. My own experience does not necessarily back that up, but I would also say that my study was much less controlled than Mihail's work.


Precipitation


From what I have read and observed, morel mycelium (for more on mycelium read this article) only needs warmer temperatures to begin fruiting. With that said, increased amounts of precipitation and moisture yields more bountiful fruiting. It’s also important for complete development of the morels once they have fruited. As mentioned before, morels can take as much as two weeks (or even longer) to reach maturity. When morels fruit and the conditions are extremely dry and warm, the morels will dry out and have “burnt” shriveled caps.  My biggest concern with 2024 right now is not the warmer temperatures but the fact that we have such dry soil right now combined with warm temperatures.


Habitat


Tree in center showing classic dead elm shape with V in center of trunk and branches forming a "witches broom" shape.

Once all of the above conditions align and you have the right amount of degree days, the soil temperatures have risen into the upper 40’s or lower 50’s, and the snow melt and/or rain in your area have pumped a lot of moisture into the soil, morels won’t fruit just anywhere. The very final step is understanding that morels are mycorrhizal, which means that they have symbiotic relationships with other plants in the ecosystem. There are specific plants they form this relationship with–particularly elm and ash trees in Minnesota. Also, because a healthy elm tree and healthy morel mycelium can live a long and healthy life sharing resources, when the tree is healthy, morels do not typically fruit. It is when the elm or ash tree is stressed or dying that the mycelium say “hey we gotta move!” and so the mycelium creates fruit (morels) to spread spores (equivalent of a seed) to find a new tree to settle down with. All that to say that once the conditions are right in terms of warmth and moisture, go find some dead or dying elm trees (elm tree ID) and you’re likely to find morels nearby. If not, find some dead or dying elm or ash trees in a different area that has the conditions described above. 


If you keep an eye on the trends noted above and get out often once all of the boxes are checked, you’re sure to find some morels. Of course, at that point the very last step is understanding how to confidently identify a morel and then how to prepare morels in the kitchen.  If you continue to struggle with finding morels, hop on the blogs. I really like www.morels.com and the numerous Facebook groups devoted to morels and mushroom hunting. Folks post good, honest information on those sites that can be extremely helpful with timing, geographic areas, and also what types of morels are being found (black morels, early grey morels, big fat yellow morels, etc.) Jump on my Facebook page MN Forager if you're interested in learning more about guided mushroom hunts or just to keep up with my content on foraged foods and the mushroom hunting seasons. Feel free to contact me with questions or feedback on this article (ed.joice@mnforager.com). Happy hunting out there folks and good luck with the 2024 morel season!



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