Mildew in Mulch???

Springboro, OH(Zone 6a)

I have a white, fibrous, mildewy substance under the mulch in one of the areas I'm planting in this spring. I removed all of the mulch today and most of the top layer of soil. My question is.... could the mildew still be hanging out in the soil and potentially harm new plantings?? We plan on tilling and heavily amending this area before planting time, but I'm worried that the mildew will multiply and wreak havoc. The site is full sun.

I'm wondering if the mulch didn't have mildew in it when the builders laid it down??? They basically tried to create a shrub bed by laying 6" of topsoil over compacted clay and mulching like crazy. The mildew was between the mulch and topsoil layers.

Any thoughts??? I'd hate to plant new plantings in this area only to loose them from mildew damage. Thanks :)

P.S. Should also mention that we plan on using Round-up 2 weeks before planting time to eradicate any weeds...

Waxhaw (Charlotte), NC(Zone 7b)

The mildew is just a sign of the mulch decomposing; this is quite normal. This is not anything to worry about. You do not have to move the mulch. Just rake it to disturb it a bit.

Littleton, CO(Zone 5a)

I also have/had mildew in the mulch. I was very concerned and ask at the nursery where I purchased it if it would be a problem. They basically said the same thing as kdjoergense. It is just a sign of of decomposition. Actually a good thing. I use coco hull mulch and I just rake it around a bit. The mildew is ugly but I've been advised

Waxhaw (Charlotte), NC(Zone 7b)

Phew !!!... got off scott-free this time (would have been bad if the garden center had said "rake it and get rid of it", right ?)
If anything a bit of mold in the mulch shows that you have an active soil.

somewhere, PA

You probably already know this, but when the mulch breaks
down, the microbes are using nitrogen. This can deplete
the local nitrogen from the soil. You might want to add something
(perhaps bloodmeal if you are organic) to "feed" the
microbes and your plants.

Temecula, CA(Zone 8b)

Hiya hugahosta,

That mildew-like substance is indeed a sign that your mulch is decomposing properly via mesophyllic (cool temperature active) microorganisms. This is a marvelous sight to see, although a little daunting to those unlike Professor Egon (Ghostbusters) that don't normally collect molds, spores, and fungi as a I am in agreement with Tammy that small amounts of nitrogen to balance out the possibility of depletion during the decomposition process. Bloodmeal is great, but not so easy to find anymore. Other good N sources (non-synthetic) are Feather Meal-12%N, Hoof and Horn Meal-13.5%N, Fish Meal-10%N, Cottonseed Meal-6.5%N, Soybean Meal-8%N, or Alfalfa Meal 4.5%N. Hope these additional nitrogen sources are helpful to you...You may also try Corn Meal-9%N, but be aware that it is difficult to get seeds to germinate due to the allelopathic properties of this valuable soil amendment/ fertilizer.

Best of luck, and keep mulching


Springboro, OH(Zone 6a)

Thanks so much to everyone who replied :) How relieving to know all is well! I'll definitely look into the nitrogen sources. Thanks again, everyone!

Hugs :)

Little Rock, AR(Zone 8a)

I am glad I found this thread. Have been wondering what that wierd stuff in my mulch is. I ignored it, hoping it would go away. About blood meal, I rarely ever see it a nurseries, but always find it at farm stores.

Waxhaw (Charlotte), NC(Zone 7b)

Bloodmeal is just a nitrogen source. If you are concerned about your plants not getting enough nitrogen, then a sprinkle of lawn fertilizer will do the same (or ammonium nitrate, urea, etc). It does not necerssarily have to be blood meal.

However, mulch applied to the surface of the soil is not likely to "rob" nitrogen from the roots of the plants burred under ground. It would be different if you mixed the mulch deeply into the soil, but on top of the soil the decomposition process is not likely to be in competition with the plants.

This does not mean that you can not add nitrogen to the mulch if you like it to decompose fast (kind'a'defeats the purpose of putting the mulch down IMHO) but you can.

You do not have to for sake of your plants, however. The mentioned "conflict" (mulch borrowing nitrogen) is only a problem in say nurseries where mulch is used in 5 gallon pots and where the plants grow directly in the mulch (such as bare root roses being potted up, etc). But in soil, with a layer of mulch on top of the soil, the "robbing" effect is not a factor, really.

Temecula, CA(Zone 8b)

not sure if your soils are deficient in iron over in Ohio, but bloodmeal is also an excellent source of that essential plant nutrient. We feed our lawn with it and our grass is green, green. I also agree with kdjoergensen in that the N thievery of mulches is more often associated with larger particle material or the "nitrolized" shavings often sold at nurseries. Anhydrous ammonia is pumped into a mountain of the shavings to stabilize the N, however most of the N is lost into the atmosphere via volatization. Any slow-release N source will do. It doesn't have to be from an organic source, that is just a preference.

Best of luck!

Forestville, CA(Zone 9a)

The white fibrous mold in your mulch is actually actinomycetes, which is a decomposer microbe which is very common in compost piles. Actinomycetes is the microbe that gives the earth and compost that earthy smell.

Little Rock, AR(Zone 8a)


From the thread about your new garden I know you have horses. Have you ever used well composted muckings from the barn for mulch? I have access to some which has been composted for almost 2 years. I know it is not as rich as cow or chicken, and there is probably as much composted shavings as manure. I wonder if it would work for amending clay soil for roses and perennials.

Collierose (susie)

Temecula, CA(Zone 8b)

hiya collierose,

You bet!, That stuff if solid gold (well...not quite), for your garden, and "richness" is not really the answer to soil enrichment, it is a lovely, rather slow process. If you have continuous access to this resource you can mulch in the spring and in the fall (for enhancing winter protection of your plants). Each year we do our fertilizing just before we mulch in the spring (long release organics), and apply our minerals just before the fall mulching. We've been at our current ranch for less than 4 years and our garden soils are completely transformed. Wet the material thoroughly prior to application to your soil so as to leach any salts that may have accumulated over time. Congratulations on getting your hands on this stuff!

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