Barf! An ode to the fascinating life of slime mold

It may seem odd to sing the praises of a weird, sometimes gross, looking phenomenon that sometimes freaks gardeners out, but every summer I love educating gardeners about the fascinating lives of slime molds. Here in Nebraska we often get calls about “this weird thing that’s all over my mulch” after a rainy period in the summer and whether or not it will 1) hurt their plants, 2) poison them, or 3) some other horrible outcome they’ve dreamed up. But the fact is, not only are slime molds harmless they’re also quite fascinating. In fact, I’ve often thought if I ever had the right terrarium setup, I’d love to have a pet slime mold. No joke.

What is a slime mold?

Basically speaking, slime molds are a group of eukaryotic organisms similar to amoebae that feed on microorganisms and do some pretty amazing things for simple-celled organisms. They can often be found in soils, forests, especially wetter ones, where they live on the microorganisms that are breaking down fallen logs and detritus on the forest floor. In gardens, you’ll find them in areas with heavy, woody mulches or high organic matter. (They also live in rain gutters if they’re full of leaves and also air conditioner units with poor drainage, FYI.) Most of the calls that we receive at the extension office are from suddenly finding slime mold in woodchip mulch. Usually after a large or extended rain event during the summer, and usually in thickly layered (several inches) mulch.

When food (microorganisms) is abundant, the majority of a slime molds, at least the ones in the “cellular slime mold” category live most of their lifecycle is as a single cell, imperceptible to humans. But when food becomes scarce is when the fun begins. In mulch, this can happen when the weather conditions (wet and hot) favor a slime mold population boom thanks to sexual reproduction and they eat themselves out of house and home. When that happens, the individual cells join together as a single body (one big cell blob with multiple nuclei), forming a visible and for many disconcerting presence in the landscape. Many of them look like actual slime and several look like webbed networks or neural structures.

Slime mold on mulch | Jody Green, UNL Extension @JodyBugsMeUNL

The most fun thing about slime mold, though, and the thing that I love about them is that THEY MOVE! Yep, you heard that right. These single-celled organisms, that stream together to form a single body, can chemically sense the presence of food in the nearby environment and will change shape (together) to move toward the food. And they’ll follow the food wherever it is – on the ground, up on a plant, into the canopy of a tree, etc. Scientists have demonstrated this by having slime molds solve mazes or even create designs, like the Tokyo subway system, in vitro. The trail of slime that it leaves in its path even relays a chemical signal that it uses to “remember” where it has been so it doesn’t backtrack or go the wrong direction.

Several of our concerned callers call because a slime mold has moved up onto one of their plants and they want to know if 1) it will kill their plant and 2) if it is toxic. Neither of which is the case. But most of our calls happen when our most common slime mold in the area moves onto its next phase of life – spore production.

You see, when environmental conditions change or the slime mold finds a good supply of food it is time to settle down for some good old-fashioned asexual reproduction. The shape and texture of the slime mold changes, usually becoming more pronounced and dryer looking as it produces sporangia that produces spores.

Slime mold life cycle | Wikimedia Commons

Callers often describe what looks like dog vomit in their landscape, and that’s where our common slime mold gets its name – the Dog Vomit Slime Mold (Fuligo septica).

Dog vomit slime mold | Peter Stevens, Flickr

Shortly after this phase, the collective body of the slime mold will dissipate. Spores will turn into new single-celled organisms that will repeat the process over again.

Can it be stopped or controlled?

First of all, why would you want to stop this fascinating phenomenon in your garden? (I’m kidding). Really, there’s nothing you can do as a control and there really isn’t a need to do so. As I stated earlier, slime molds do no harm to plants and aren’t dangerous for people or pets. If you just don’t like the look of it, you can reduce the likelihood of it popping up in your landscape by reducing the thickness of woodchip mulch (don’t let my GP colleague Linda hear me say that) or reduce irrigation if it is occurring outside of larger rain events.

Otherwise, just enjoy your pet! It won’t hang around for long. The visible, fascinating life of a slime mold is fleeting but exciting. Next time you see a slime mold, check it out! And keep an eye on it to see if it moves or if it progresses through its life cycle – maybe you’ll just develop an appreciation for them, too. And I’m not joking about pet slime molds, someone even wrote a guide to dog vomit slime mold pet care!

To mulch or not to mulch? It shouldn’t even be a question.

There’s wood chip mulch peeking out of all of our landscape beds

One of the popular arguments against mulching landscape and garden soils is that mulch delays soil warming and thus retards plant growth. Given that a well-chosen mulch will moderate temperature extremes – both hot and cold – is this an argument supported with evidence? In today’s post, I’m reporting the data I collected in visiting various parts of my home landscape and gardens and measuring soil temperatures.

My trusty soil thermometer

For measurements, I used a soil thermometer placed at the same depth in every soil tested. This required movement of mulch if mulch was present, so that thermometers were inserted completely into the soil. These thermometers read the entire length of the probe, so readings represent the average temperature in the top 5” of soil. I took close-up photos of each of the areas tested. I took 5 measurements for each location.

Our evening temperatures have been near or below freezing, but the past several days have been sunny and the air temperatures are well into the 50F range. On March 17, it was 68F at 2 pm in the sun, though it was 27F that morning. The morning after (March 18), it was 35F.

There are several interesting trends to see on the box-and-whisker graph:

The variation of soil temperatures is most extreme in unprotected soils
  • Mulched raised beds have the most consistent temperatures, with no differences seen at any time or in any location measured.
  • Unmulched soil mounds have extreme changes, mirroring air temperatures.
  • Bare soil in beds under sunny conditions have extreme changes mirroring air temperatures, but not as great as that in raised beds. They are warmest during the day and coldest during the night.
  • Bare soil in beds under shaded conditions are the coldest soils during the day and even colder at night.
  • Soil under living mulch (turf) and beds with varying depths of wood chip are cooler during the day than bare soil in sunny conditions, but warmer at night.
  • Bare soil in beds that were newly mulched are much warmer than bare soils not near mulched areas.
  • The soil temperature under turf or in beds at least partially mulched did not change at night (data not shown on graph).

Extreme temperature swings can result in the death of germinating seeds, seedlings, expanding buds, and other tissues that aren’t cold hardy. This is especially true of tissues near the soil surface, where temperature are colder than they are at increased depths. Unprotected soil mounds show huge daily vacillations; comparative raised structures under mulch are cooler during the day but warmer at night. And bare soil in the shade is colder than any mulched soils. Consistency is important for young tissues, as they have few protections against environmental extremes.

What my little experiment demonstrates is what mulch research has consistently shown: appropriate mulch materials will moderate soil temperature extremes due to air temperature fluctuations. Just because a bare soil is 55F in the daytime doesn’t mean it won’t be 35F at night.

Landscape fabric – a cautionary tale

This isn’t the first time I’ve ranted about bad mulch choices and it certainly won’t be the last. But this pictorial cautionary tale is too important to pass up.

We already know that sheet mulches can be death to microbes, plant roots and animals living in the soil underneath. Our newly published research shows that landscape fabric reduces carbon dioxide movement between the soil and atmosphere about 1,000 times more than wood chip mulches do: plastic mulches are even worse. Oxygen movement will be likewise affected.  And while gaps and holes in these barriers can lessen the impact, the question remains: why would you use ANY mulch that reduces gas movement? Yet people persist in using fabrics and plastics, usually to “smother” weeds (and that verb should set off alarm bells for anyone thinking about collateral damage to soil life). But weeds are weeds for a reason, and they will eventually colonize the surface of sheet mulches as soil, organic matter, and water collect over time.

So without further ado, here is a case study of what happens when sheet mulch is used for landscape weed control.

These irrigated landscape beds are in Wenatchee, Washington, which has hot, dry summers. As you can see, bark mulch has been used to hide the shame of sheet mulching. And from a distance it looks…okay.

Upon closer inspection, you can see the shroud of death emerging from the bark mulch (which has no means of staying in place, especially on a slope).

And even close you can see the soil that’s blown in, along with the bark and other organic matter. Just add water, and you get weeds!

Weeds, weeds, weeds! Lots of weeds. Sunny weeds!

And shady weeds!

Border weeds!

Rocky weeds!

The weeds are thriving – but the trees are not. The crowns are dying…

…and the trunks are suckering.

But you’ll note that the trees in the first photo outside of the beds are thriving.

And it’s all because of that “weed control fabric.” Which is working so well that this landscape had to be treated with herbicide the day I was there – to control the weeds.