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Fungus Under Driveway

Jim Katen

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A friend called me the other day to ask my advice about lumps in his driveway. He had a gravel drive till last year when he had it paved with asphalt. This year, almost exactly a year after the asphalt went down, he noticed lumps forming here & there along a particular region of the driveway. They’re circular and vary from 1â€

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Wow, how thick was the gravel bed? The mushrooms live off of decaying organic matter. I had those dam pesky lawn mushrooms all thru my front yard. I went and picked them out of the lawn morning noon and night. I think they gave up and only a few dare try lately. But for a while there I was wondering who was going to win, them or me.

My understanding is that you got to get rid of the organic matter or wait until they get rid of it. Treating is temporary.

Chris, Oregon

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I would think that if the asphalt is thick enough and the compaction is good enough you should'nt have problems like this. By looking at the picture the material looks rather coarse also. The material that is more coarse will usually come apart easier. Coarse is good for a base layer. Then a top layer with finer aggregate for better durability.

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The paving crew didn't use roundup or a soil sterilizer.

I'd guess that the gravel bed was 6" - 8" thick and the asphalt was about 4". Certainly some areas were thinner. The asphalt was laid in a single lift.

The mushrooms had been growing the same spot before the asphalt came in and they were growing in a lateral line along the driveway so I'm inclined to strongly consider Bill's idea about decaying tree roots. This also fits with my friend’s statement that a few years ago he ran a ditch witch down the side of the driveway. He probably severed a bunch of Douglas Fir roots at that time.

As for the fungus being able to break through compacted gravel and asphalt, it's not really that hard to believe. The paving is hard when you press down on it, but it's much weaker when you press up on it from below. Also, emerging fruiting bodies, such as the ones in the picture, can exert a fantastic amount of pressure.

I suggested drilling holes over each lump and pouring boric acid down there. It's toxic to fungi but it's pretty benign to vertebrates. We experimented with a sledgehammer and found that the lumps were easy to beat down. But if there are roots down there, I suspect he's going to be fighting these things for years.

Fortunately I’m cheap and would never consider the expense of having my own driveway paved.

- Jim Katen, Oregon

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  • 9 years later...

Hi my name is Dominick Allocco Jr. My Brother Fred and I own ALLOCCO DRIVEWAY CO.,INC.a MUSHROOM CAN AND WILL GROW THROUGH ANYWHERE FROM 1 TO 5" OF FIRMLY COMPACTED ASPHALT. As asphalt is a flexible material, especially when in direct sun, this fungus will push up the blacktop in an almost perfect circle the size of the mushroom.Being in the driveway paving business and having responded to driveway repair calls as well as my own driveway, I have seen this again and again. The culprit is usually a decaying piece of wood or root. Mushrooms thrive on growing out of these and the asphalt becomes no match for Mother nature.They will either push the asphalt aside or more often bubble up the blacktop in a nice neat circle not even breaking the asphalt. The solution the said spot must be broken away with a sharp pick or similar tool trying to stay in the shape of the eruption so as to keep it a small area to repair.(Who wants a big eye sore in their driveway). Next dig out this wonderful specimen along with as much of the root or wood as possible. Using a strong weed killer or vegetation killer, spray into the hole liberally. Now being a Paving contractor we would then bring a small amount of hot asphalt leftover from a nearby job just finished. Applying it into the hole in layers and tamping it firmly. Hint: wet the bottom of the tamper with water so as not to scuff or scrape existing area. As a homeowner, since this small amount of hot asphalt cannot be purchased at any quarry, your local hardware store should carry bags of cold workable blacktop. Use the same method as above to fill the hole in keeping it as flush to the existing paved area as possible,i.e.not below or above for a better looking repair. I know you still may not believe this delicate fungus can push its way through hardened asphalt, until it happens in your driveway. 9 times out of 10 this is not the paving contractors fault who paved the area. Often a piece of wood or root missed in the excavation process because it was deep in the sub-base causes the problem. Oh well I hope this helps all you happy homeowners and may all your mushroom adventures be on your neighbors property.

You rs Truly,



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