Dry climates - using rocks (and/or barrier) under beds to help retain water?

(Edit: I originally only mentioned rocks, but now I’m considering if a layer of some sort of barrier to prevent soil from infiltrating the rocks would add to the affects I describe below. I know people here will frown on landscape fabric, but maybe there’s an alternative?  I’ve found many posts about Hugels in desert environments thanks to the logs that retain moisture, but I’ve never seen one describing how to prevent water from escaping into the ground beneath a hugel.)

I just posted this in the soil forum here https://permies.com/t/190774/Soil-Structure-important-add-inorganic#1573514

I was searching for what I should add to my very sandy soil to prepare to build some Hugelkulturs.  I was lead to this realization about water retention that I was aware of in container gardening.  Please let me cut/paste it here to explain since this would be specifically for Hugelkulturs:


Hi everyone.  I just learned something else one this website - https://www.smilinggardener.com/lessons/how-to-improve-clay-soil-and-sandy-soil/

I believe they are simply pushing compost but it's all worth considering.  However, the greatest takeaway was in something I already have learned about water retention (hinted at early when I mentioned my container gardening).  A user named Al (aka Tapla) on Houzz container gardening forum (formally Gardenweb) is nothing short of a guru when it comes to container gardening, and it's all about the soil medium.  Here is a link if you want to learn everything you would ever need to know, his thesis is "Water Retention". https://www.houzz.com/discussions/6167237/container-soils-water-movement-and-retention-8-26-21
He first stated this thesis probably over a decade ago but it's so important to understand.  He updates it all the time.  In short, no particles in a container should be smaller than 1/8" so that there is no water retention issues and there is plenty of air between each particle (very much simplifying).  He also busts the myth about placing rocks on the bottom of containers in order to help drainage.  This does the exact opposite by raising the perched water table.

Back to my first link.  They mention how golf courses lay down a layer of gravel before putting in the topsoil for the grass, and the purpose of this is to provide more moisture to the short grass roots.  What they are doing is creating a perched water table!  It won't be 100% like it would be in a container because it is still effectively tied to the earth below, but it the gravel is dense enough I can see it would certainly slow.  And if this is true about golf courses it must work.

I know many of you don't live in tremendously dry areas.  It's been recommended to me in the past to use my rocks as mulch, but I'm wondering if using rocks under beds would help slow the water from draining away?

I can attest to the fact that the sun shining on rocks creates water.

I know I have placed rocks in pots for drainage.

In "Dry climates - using rocks under beds to help retain water?"

An experiment to prove this would be interesting.

Anne Miller wrote:I can attest to the fact that the sun shining on rocks creates water.

I know I have placed rocks in pots for drainage.

In "Dry climates - using rocks under beds to help retain water?"

An experiment to prove this would be interesting.

You should read the thesis I linked. It’s very informative and will show you exactly why rocks in pots exasperate the issue it’s meant to solve. It completely changed your approach to using rocks as a way to increase drainage.

I would love to test the use of rocks under beds as a way to retain water, but don’t have heavy machinery and it would be very difficult to “undo” if I’m wrong.

Has anyone put something anything under a bed to help it retain water? Something impermeable or anything else?

I have taken an entire sub irrigated planters and buried it next to a tree as a water reservoir.
It was basically a perforated ~3 gallon bucket inverted into a 5 gallon bucket, with wicking soil packed between the walls of the two buckets.

I was thinking big rocks on top of the soil will cool at night,
store that coolness and condense water on them during the day.

I read the first post you linked to, but didn't have the patience to find the rest of his posts to continue his thesis. My takeaway is that he's talking about plants in containers and ensuring proper drainage and aeration. If you're planting in the ground, keep in mind that the layer of rocks under your bed will break the capillary action from below and prevent deep moisture from getting up into your beds. So however deep your beds are, that's all the water they'll have access to.

Another thing I think worth considering is that you have sandy soil which doesn't hold water well. (I feel your pain 😁) No matter what the layer of rocks under your bed does or doesn't do, your soil can only hold so much water. Once it's saturated, the water will drip through the rock layer and be lost, since there's no longer any way to get water up from under the rocks. So before your rock layer can do much good, you need to increase the water retention of your soil.

If the rock layer works the way he says it does, by holding water closer to the surface, rather than letting in drain away, you'll essentially be growing in a big planter, rather than in the ground. I have some 50gal planters that I have sunk into the ground. Anything I grow in those dries out way faster than stuff planted in the ground right next to them.

Our driveway is about half a kilometre. Parts of it are built on silt, parts on sand, and parts on big rocks.  Some of the rocks are almost as big as me, down to ones about the size of my head. The road on top of these materials is just whatever dirt was next to the road base during construction, so it's anything from silt to silty sand with lots of fist sized and smaller rocks. Regardless of what the road is made of, the sections built on top of rock dry out waaaaay faster in the spring than the sections built on sand or silt.

Something I've learned to do is site my gardens where the soil is siltiest. My first gardens here were better located for sun, biut they were in a gravelly area. I dug trenches and filled them with some of the siltier soil and lots of organic material to hold water. The water pissed out of those beds and they were very dry all the time. My gardens built in the silty areas hold water much better.

I obviously haven't done any controlled experiments, so maybe I'm totally wrong to be skeptical about all this. But those are my thoughts based on observations of my property. I'd be very interested to hear of any experiments you do. I might actually do my own next year, since I'm looking for ways to exclude gophers from garden areas. If I could put a foot of rocks down and build my bed on that without losing water, I'd be all over it.

I've read of a simpler way that has been tested and found effective, using layers of paper.

Paper Lined Zai Hole gardening

Zai hole experiment results 30 July 2019

These experiments compare the yield of three crops in standard Zai holes, and in Newspaper Zai holes, where 20 sheets of newspaper are placed at the bottom of the hole in a bowl shape.

Results show that Zai pits with newspaper at the base of the hole produce more than Zai holes without newspaper at the base of the hole for all three different crops.

...Zai holes generally produce 50% or more (up to 600%) compared to conventional row cropping, and the newspaper Zai holes may produce even more (depending on the crop, soil type, climate etc.)...

...While this experiment has very limited replication, a yield increase of 300% is enormously significant and is considerably more than expected. I double-checked with Salim, and he has confirmed that the only difference is the 20 sheets of newspaper at the base of the Zai hole, and that the watering and fertilizer regimes were the same for all the Zai holes, as well as the environmental conditions. Yields are likely to be different in different soils, climates and crops, either more or less.

It seems likely that the newspaper has concentrated and held water and nutrients in the root zone for longer, but not long enough to cause waterlogging or root rot, while at the same time maintaining optimal soil aeration, all of which have greatly increased plant growth and food yield. Another factor in the success of this technique may be that evenly moist but aerated soil could have promoted the growth of of beneficial soil life such as mycorrhizal fungi (which enhances nutrient and water uptake).

Another dry place teaching and using these concepts is in Arizona through Hopi Tutskwa Permaculture.  They call them "lasagna beds" but the principle is the same. I call it desert hugelculture.  Here is a blog post showing the bed making process:

Hopi Tutskwa Permaculture, building sunken garden beds in Arizona

They have more pics, but here are a few for the concept:







We make whole garden beds with principles like this, but sans the paper. The paper might have made our beds more effective, but we decided to focus more on going deeper and using more mass of compostable plant material, thinking that more plant material will translate to a larger mass of "sponge" in the ground. But we didn't do a test plot with cardboard yet.  We have one bed left to dig, and may try cardboard there.

My take FWIW:

Rocks in the subsoil increase drainage, not moisture retention.

However, rocks ground up by glaciers and deposited in glacial lake beds, after depositing their load of sand at my place (!), will hold moisture (and nutrients) like crazy. AKA "clay." There are many different forms, some nasty and some helpful. But all retain moisture.

A recent poster presented the adage "clay on sand, money in hand." I'm going to try it. No mention of rocks though. My 2c.

In the thread "9+ Ways to make garden beds", I put a sequence of photos of how we did our desert hugel/zai pit beds that shows our method, which is similar to the above but we go deeper, didn't use paper, and don't have manure to add.  We rely on the bulk of plant matter.

Making sunken desert hugel beds to retain moisture and build soil in the ground in arid regions
(You may need to scroll down a bit to my pictures of our long beds - I replied twice in this thread linked above.)

We received more summer rain than usual this summer.  The beds all filled and progressively overflowed into the lower ones.  The fenced areas of the garden are buffered with rocks to help prevent runoff.

In our third sunken desert hugel/zai pit garden, we put a trellis with a bed going down the inside of one side of it.  The trellis created space to grow a lot of things vertically, like tomatoes and beans last year, and beans, peas, grapes and flowers this year.  Under the trellis is like a cooled microclimate, where we have plants that would otherwise cook in our extreme desert heat and drying winds. It's also nice to hang out under there.

I think there are lots of ways to do things. I hope you've been given enough ideas to try that you will find what works for you!

Regarding rocks in subsoil -

As for rocks found in the soil, in my experience with digging these beds as above - we found that rocks trapped moisture underneath them.

A great example, we would dig a bed like this and come to a long root from a hardy desert plant... follow it (digging more), and it would often end up at a rock.  Then underneath the rock the root would be circled around and around in a mass.  I can't explain why the root would curl around and around like that under a rock unless the rock trapped water...

The first time we saw this was in very sandy soil that was made of decomposed granite.  The rocks were actual granite chunks. So since the soil was decomposed granite, and the rocks were granite chunks, I don't think the roots balling up under the rocks would be for nutrients.  Other than water.

Our soil above is very sandy clay.  It has more water retention capacity than the sand/soil I just mentioned, and though we do encounter rocky areas we remove the rocks and focus on packing in the plant matter.  We know there are more rocks below, and it's so hard to dig already (digging requires a lot of water, a pick, and a shovel) that it doesn't seem useful to put more rocks in rather than take them out and make more space for plant roots to grow.

My point in the end is that I have seen examples that seem to show that rocks trap water - but the water seemed to be trapped under and or between layers of rocks.