Which Garden Mulch Is Best At Saving Water?
Did you know that by covering your soil with a layer of mulch, you can increase the amount of water stored in the ground or in containers by 80%?
But which mulch is best at stopping your soil from drying out?
To find out I ran an experiment and measured the water content in soil over several days to find out which mulch is best to use in your garden. And the results were very surprising…
The best mulch is hemp animal bedding which managed to retain 80% of the added water over the three day experiment. While this expensive mulch won first place, the cheapest mulch, grass clippings, came second and kept 73% of the water. Bark was the third best mulch, retaining 16% of the added water.
In this experiment, both the uncovered soil and compost lost more water than was added, demonstrating that compost is an ineffective mulch in top-watering systems.
You can watch the full experiment in the video below or read on to find out why each mulch performed how it did and what on Earth happened to the compost.
How I Tested Each Mulch
To find out which mulch is best, I dug up some soil from a small area of a garden bed and put it in clear glass bowls, aiming for roughly the same weight of soil in each bowl (within 10 grams) and I tried to create a flat and level surface about an inch from the top of the bowl.
Then I used a measuring jug to add half a litre of water to each bowl which meant that the soil was approximately at saturation.
Then I covered the soil in each bowl with about an inch of mulch so that the bowls were completely filled and I left one bowl uncovered as a control test. I tested an organic all-purpose compost, dried grass clippings (similar to hay), landscaping bark (similar to wood chips) and hemp animal bedding.
Then I measured the weight of each bowl a few times a day and by subtracting the original weights, we can calculate how much water has evaporated from each bowl.
And over three very hot, sunny and breezy days on a summer solstice weekend — the perfect conditions for evaporation and therefore the ultimate test for any mulch — the differences were large enough to determine which mulch is best at stopping our soil from drying out.
And during the experiment it became clear that some really interesting and surprising things were happening…
No Mulch Means No Water
The first result is that the no-mulch bowl dried out super-quickly. In fact, by the end of the day the scales had already fallen along way from the start and by the following evening, only about 30 hours later, the bowl had already lost 500g which means that the half a litre of water that I added had already evaporated.
This demonstrates exactly why it’s so important to mulch raised beds and pots. If you don’t you’ll need to water them every single day.
Over the next day or two, the moisture that was already present in the uncovered soil had started to evaporate too. Which is why the graph of water loss actually went below 500ml, it’s why the soil in the bottom of the bowl looks grainy and dry, and it’s why the moisture sensor doesn’t detect much water.
Compost Is A Bad Mulch & Dries Out Fast
But what was surprising was just how quickly the water evaporated in the compost. It’s almost like the water didn’t see a barrier, wicked it’s way to the surface and then evaporated like it did in the bare soil.
It looked to me like it held onto more water and for longer than the bare soil too, but the data would suggest that compost is a completely ineffective mulch and you shouldn’t rely on it to keep moisture in your garden beds.
Some people have suggested that this might also be due to the colour of the material – it was very black and therefore would have absorbed more heat which would allow evaporation to happen faster. I think this is true but also the material itself really is very similar to soil so I would expect similar results and as my soil contains a lot of clay, I would expect it to hold on to water better than lighter soils or compost.
Bark Will Keep Moisture In The Soil
The next best at retaining water was bark. And while the surface did dry out, it managed to trap some of the water in the soil with about 80ml of the initial 500ml still in there.
It can be a little expensive but as well as increasing the amount of water available to plants, bark and wood chip can also add nutrients to soil as they break down.
It can be a little more difficult to get hold of organic bark but if you can find some, it’s definitely worth using it on your garden beds or in the paths that run in between.
Hemp Is Exceptional At Trapping Water In Soil But…
Next up is hemp. I normally use it in my chicken coop as a bedding because it’s really absorbent so I was curious about how it would behave as a mulch. It is very expensive but it’s easy to get hold of and the manufacturer claim that they use no pesticides.
As you saw in the graph at the top of the article and in the moisture sensor reading, it did a seriously good job, outperforming the rest and only losing 20% of the water that I added over three days.
The soil underneath is still moist and judging by the data, I could probably not water this for a week or two. So if you want to cut your water usage, especially if you are growing in containers, then mulching with hemp is definitely worth a try.
Grass Clippings Are The Most Cost Effective Mulch
The last mulch I tried was grass clippings which is totally free and because it comes from my own garden I know there’s no pesticides. So if I wanted any of these to do well, it was going to be this one.
The experimental data suggests grass clippings performed extremely well – almost keeping up with the hemp over the three days and only losing 30ml more by the end of the experiment. That means grass clippings only lost 27% of the water which closely compares to the hemp which lost 20%.
However the price difference could not be more stark between the two. Grass clippings are literally free and are a by-product of an unavoidable task: mowing the garden. It’s a no-brainer to put them on your veg beds rather than on the compost heap or in your green waste bin.
This is a massive win for home-growers.
The main drawback with using grass clippings is the risk of scattering seed in your garden beds. But if you cut the grass while it’s short, before the seed has had a chance to form, it really shouldn’t be an issue.
What Is The Best Mulch?
It’s very clear from this experiment that the best mulch to use in your garden is grass clippings. While hemp outperformed grass by 7%, it costs a lot of money while grass is completely free. It doesn’t require transport, making it the best environmental option, and it contains zero chemicals.
Water Underneath The Mulch To Save Even More Water
While using a mulch will reduce how much water you need to use, the issue is that when you water your garden, you’re still mostly watering the surface of the mulch and still wasting loads of water to evaporation.
To stop this and to water your garden as efficiently as possible, you should read the article below, where I install soaker hoses in my garden beds. They directly water the soil underneath the mulch, protecting it from fast evaporation and it happens on a timer while I’m still tucked away in bed.
Click below to find out more and, as always…
Happy Gardening!
