I don't know what kind of neighborhood you live in, but wherever it is, chances are that patches of bare dirt are frowned upon. In California, a wild, endless drought has rendered most lawns brown, and people are searching for new and exciting ways to cover the dust bowls around their home.

Chips of tree bark, sometimes dyed red or black, are a cheap option. This black bark mulch claims to keep moisture inside the soil, but I had doubts. The black bark would absorb a lot of heat from the sun. Couldn't that actually increase the amount of moisture being lost to the air?

Without delay, I set up a test. I used two identical plastic bins and filled them with potting soil so that they both weighed exactly 16 lbs.

Potting soil probably doesn't accurately represent the hard clay I have in my backyard, but I needed a homoegenous dirt to keep the test fair. I used a bag from the Kellogg dirt factory.

Next, I added a layer of black bark mulch to the top of one of the bins. I added four pounds of bark, to be precise. The other bin of dirt would remain topless.

The final step of preparation was to add one gallon of water to each bin. This increased the weight by 8 lbs. 6 ounces (3.8 Kg).

In the hot sun, both bins would heat up, and both bins would lose all of that water to evaporation. But would the bark help save water, or help lose water.

I wouldn't find out for 17 days.

At first, the bin with the black bark lost weight a bit faster. I think that was because the bark itself was damp, and the surface water burned off in the first afternoon.

In the next few days, the amount of water being lost by the plain soil began to overtake the water being lost by the mulch-protected bin.

In the end, the mulch worked! Despite having a hotter top layer, the wooden cover did its job, helping protect the water in the soil from evaporating.

Seventeen days after I started, the plain soil had lost the entire 3.8 Kg of water. It was completely dry. The mulched soil, on the other hand, had retained about 18 ounces of its water weight.

Summary
Black bark mulch is not a great moisture barrier, only about 14% better than nothing, but it is surely more attractive than bare dirt, crushed rock, or a loosely tethered sheet of black plastic.