By Monte Leeper
Q. I have a problem with my driveway which nobody seems to be able to enlighten
me on. Three years ago we had a new blacktop driveway put in on top of an old blacktop driveway. Last year and especially this year, several growths have popped up from under the blacktop. Upon breaking away the lumps and bumps, they appear to be large toadstools and mushrooms. We broke them up and cleaned the empty space and filled them in and resealed them. The area we filled is about six to eight inches. I've never seen this before nor has anybody else I talk to. The first driveway was filled in (base) with blue
stone (no problem). The second was filled in with stone and gravel. I don't see how I can prevent this from happening, since the problem comes from underneath. (If it does happen again) I would like to know what caused it. Hoping you can enlighten me. Appreciate your help, if you can. Thank you very much.
A. The toadstools or mushrooms you described were hardy enough to push
through the asphalt because of the presence of moisture and a source of food from a place yet unknown. If the driveway below the one with problems was sound and unpenetrated, then the possible source of the mushrooms may, believe it or not, come from the gravel mixture. Since the gravel had to be unearthed from somewhere, it is quite possible that the mushroom spores were in the gravel mixture to begin with. It is unusual, but not inconceivable for this to happen.
To avoid problems such as plant growth, the gravel base should have a mixture of cement dust referred to as a "dry mix." The lime and calcium chloride in the mixture is usually enough to kill most weeds, since it forms an acid that is caustic to plants. The other reason for the dry mix is that you need a firm base below a driveway to prevent the top materials, which are flexible under the weight of a car or truck, from sinking or crushing. People with trees and shrubs next to the driveway often also experience your problem, as the roots force their way through.
To be even more thorough, a dose of weed killer (see the label for the effect on mushrooms) can be applied. If you want to be sensitive to the environment, and I hope you do, you may apply a mixture of 1 cup of baby shampoo to a 20-gallon hose sprayer or sprinkle dry laundry detergent on the area. This idea comes from Jerry Baker, "America's Master Gardener," and I trust
implicitly his experience in such matters.
You may also want to analyze whether you have a low lying or poorly draining area at the driveway, since moisture contributed to your problem to begin with. You may not be aware of a broken drainpipe, underground stream, or abandoned drywell under or near
the driveway. I suggest examining any of these issues to be certain of just why the growth developed. After the snow clears, wait until the temperature is averaging at least 45 to 50 degrees before doing any corrective work.
Until then, stay warm and thank you for your letter.
