Deep Mulch Gardening

Back to Integrated Closed Loop Production Systems


In permaculture we try to observe the interaction between elements of a system and then place the elements in relation to each other in a way that will enhance the productivity of the system.


Key Hole BedIn the case of our No Weed, No Water, No Till, Deep Mulch, Drip Irrigated Gardening System, we are considering (1) the needs of the gardener, (2) the needs of the plants we want to grow, and (3) the characteristics of the drip line we are using to deliver water.


The basic design of a key hole bed that we learn from the permaculture literature is derived from the interaction of 1 and 2.  1) Plants need a root zone that delivers the nutrients and water they require to grow and 2) the maximum distance the gardener can reach without stepping on the beds (and compacting the root zone) is approximately four feet.  We then bend the four foot wide planting bed into a circle, giving the gardener access to the entire planting area from one short narrow path way, in order to maximize planting area relative to path way.


In 2009 we built standard key hole beds and then overlayed that design with drip line with preinstalled emitters on 12" centers rated at a flow of .6 gallons per hour.  According to the literature, each emitter will water a hemisphere approximately 18" in radius depending on how long the water is on and the type of soil.  We had no problem with lack of water in any of our sheet mulched beds but the configuration of the bed in relation to the drip line meant that we were watering areas between beds that were not intended for planting in the original design.





Observing the interaction between all three elements, we suggest a modification to the standard key hole design where drip irrigation is to be used.  We call this the double key hole pattern.


Double Key Hole PatternIn this design all sides of the bed can be accessed placing most plants within a two foot reach of the gardener and making it more convenient for those with physical limitations.  Also, the rectangular planting area results in no break between beds so that the drip line continues from bed to bed without a gap in planting area.




If we are building according to Toby Hemenway's formula in Gaia's Garden, here are the materials we will need:


One cubic yard is 3'X3'X3'. The foot print is 3'X3' and if we want to spread it 3” deep, as we do with the manure, we have 12 layers (36”/3”=12 layers). That means we can do 12 of the 3'X3' foot prints and cover 108 square feet (3'X3'X12 = 108 square feet) or approximately one key hole bed (10'x10' = 100 square feet). The formula calls for 1/2” to 1” on the bottom two layers. If we want to skimp, or if we are short of manure, we could stretch it to 162 square feet or about 1 and ½ key hole beds per yard.


Hay is harder to calculate because it comes in bales that are more compressed than we want. The best I can tell you is that we use about 1/3rd of a 3'X3'X8' bale per 100 square foot key hole bed.


If you are using wood chips, they come by the cubic yard. In that case, where we want to make the mulch 12” deep, we have three layers in each foot print, so each yard will cover 27 square feet 12” deep. That means, we will need about 3.7 cubic yards to cover each key hole bed that deep.




Laying out drip line requires a calculation of how much water we can deliver over a given area.  The variables are 1) the volume of water available at the source of supply, 2) the size of the hose through which the water will be delivered, and 3) the amount of water coming out of the emitters installed on the hose.  The variable that allows the least amount of water in the calculation determines the area that can be watered in one 'zone'.  Then we can plan multiple zones to run at different times.


For example, we are preparing to build a garden at the Crescent Grange.  We went to the water source which is a spigot on the outside of the building.  We opened the spigot wide to fill a five gallon bucket.  It took a full minute to deliver 5 gallons which means that the maximum amount of water that we can deliver to one zone is 300 gallons per hour.  (5 gallons x 60 minutes).


We will design for a 3/4 inch supply line that can easily deliver 300 gallons per hour (a 1/2 line will max out at 220 gallons per hour) so our limiting factor will be the length of drip line that we can run without exceeding the 300 gallon per hour available supply.  In this case, we are using .6 gallon per hour emitters spaced at 12" along the line so we can have up to 500 feet of drip line in a zone.  (300/.6)  In the configuration shown, there is approximately 40' of drip line per bed - or 40 emitters per bed delivering .6 gallons per hour each or 24 gallons per hour per bed.  300 gallons per hour divided by 24 gallons per hour per bed gives us 12.5 beds - which we will round down to 12 beds maximum receiveing water at any one time.


In our Broomfield Garden in 2009 we ran the one zone we had there for one hour each day.  That was plenty of water even during the dryest of weather so we can set an upper limit of 24 zones from each water source.





Garden LayoutWhen laying out the garden we consider all other uses with which our garden needs to be compatible.  The Grange in particular is a multi use facility.  In the diagram at right, all of the blank area in the upper left is used for parking.  The area between the building and the gardens on the right is designated for picnicking.  We were also asked to leave room for a vehicle to be driven around the edges of the garden.  In addition to those 'facility requirements' we are mindful of the permaculture prinicples of zones - where we want those things that require the most attention to be closest to the area of most activity.


In this case, we are working with a small area around the building which we will plant to perennials and ornamentals to entice people down toward the garden.  The division between the parking area and the picnic area will serve a similar purpose.  In the area desginated for the garden proper, we have layed out three zones of 12 beds each.  Each of the zones starts near the end of the divider beds so that we can place all our zone valves in one place and minimize the run of hose (and hose friction loss) to the start of each zone.




To build a garden based on a double key hole pattern, mark off an area 14 feet wide and some multiple of 10 feet long.     The area marked off need not be a straight line or any particular length.  Just adjust the pattern to fit the space available and work with the requirements of your drip zones.


Within the area marked off, mark off two feet along each side along the length of the bed and an additional two feet at each end.  Water the entire area thoroughly.  Spread an organic fertilizer at the recommended rate on the 10 foot wide section and then cover the entire 14 foot wide area with 1/2 inch of newspaper and cardboard (weed barrier) being sure to overlap the seams often (otherwise bind weed will find its way through).  Water the weed barrier thoroughly.  Remark the two feet wide strips on both sides and the end, then mark the center along the length of the area.  Spread an additional layer of fertilizer* on the center 10 feet of the area.


We now want to mark the center of each 'key hole path way'.  The path ways go into the center of the beds at five feet apart and then out to the edge alternating from one side to the other.  It is useful to permanently mark the centers.  You can use anything available but wood blocks with a nail to hold them in place work well.  To make that kind of marker cut off scrap 2X4s at 3 1/2 inches making square blocks.  Mark a line from corner to corner making an X on the block and locating the center.  Drill a hole in the center and drive a nail through the hole.  Push the nail into the weed barrier marking a permanent location for each pathway terminus.


Now make the path ways by spreading wood chips along the two feet on each edge.  Make the path ways into the beds 1 foot wide to the center mark and then a 1 foot radius around the center mark.  Spread the wood chips at least 1 1/2 inches deep but do not hide the center markers.  Now add 12 inches of organic matter to the entire planting area (hay, straw, leaves, etc.**)  and cover that with 1/2 inch of compost or top soil to keep the organic matter from blowing away.  Water the organic matter and covering thoroughly.  See the Sheet Mulching Diagram at the right.


Now run your drip line.  Run the 3/4 inch line along the edge of the beds that has the first and the last pathway to the center.  At one end attach two drip lines, one 12" in from the outside edge and another 18" in from that.  Wind the drip line around each path way to the center as shown in the diagram.  As the lines pass through the center, an even spacing will put the lines 12" apart and 12" from the center path.  As the lines run along the edge, one should be 12" in from the edge and the other 18" in from that leaving another 18" to the center path.  At the end of each drip line run reconnect the drip line to the 3/4 inch supply line creating a continuous loop and equalizing the pressure through the drip line.  (The drip line we are using is 1/2" in diameter and can run up to 400' with 350 emitters - although I like runs of 3 or 4 beds at a time for even water distribution or +/- 80 feet.  1/4 " line should not run more than 30' from the supply line.  Check the specifications for the product you are using).


Building the Beds - Updated

Sheet Mulch DiagramWhen we began this work we were using hay.  Hay is expensive and 12" of hay will decompose in as little as one year in a healthy garden soil.  The hay must then be renewed every year.  For those reasons, we built up our mulch around the pathways.


We have since discovered, thanks to Leo Kacenjar at the Digital Garden on Leetsdale, that wood chips work just as well as hay in a sheet mulch.  (except a possible delay of a couple of months to get the decomposition going).  Wood chips can be obtained for free and take multiple years to decompose.  When we are building with wood chips we can just sheet mulch the whole area and then mark our pathways on top.  Don't put on the last 2" of seedless mulch in Hemenway's formula and use wood chips to mark the pathways on top of the last layer of manure.


Drip Line Options

The double keyhole pattern is particularly good for a specific type of 1/2" drip line with preinstalled emitters on 12" centers (agrifim).  It is designed for long runs (up to 200') and few connections (a connection at each end of the run).  Another option is 1/4" line with preinstalled emitters on 12" centers (6" centers seems too close to me).  1/4" line has a maximum run of 30' and you still need a connection at the end of each run.  If you are doing a limited area that is not too big a problem . . . the connectors punch through the 3/4" supply line and insert into the 1/4" line with emitters.  3 "U" shaped loops will be sufficient for a typical bed and will be about 43' of drip line with six connections.  In 2011 the 1/4" line was going for $23.00 for a 100 foot roll.


Connecting timers and zone valves are beyond the scope of this garden layout page.


Planting TechniquesBuilding beds can be done any time of year but if you build them right before planting time then you will need to use some specialized planting techniques for planting in newly sheet mulched garden beds.

*We are using two applications of fertilizer.  The first application is under the weed barrier and the second is on top of the weed barrier.  Recommended application rates for fertilizer are designed to deliver adequate nitrogen to your plants in a traditional tilled garden.  In this system, in the first year, we also need enough nitrogen for the decomposition of the mulch you are using.  Too much nitrogen can 'burn' your plants - but, with a deep mulch, a lot of nitrogen will be tied up in the decomposition process.  We are recommending 2 times the recommended application rate but some experimentation is called for.

**I prefer grass hay because it has retained nitrogen from when it was green.  Some people object to the possibility of grass seeds in the hay but I have not found that to be a problem.  If you are using straw or leaves you will need to add additional fertilizer because those materials have no nitrogen left in them.  Also, most straw contains wheat seed which will also sprout.  I don't find that to be a problem either as the wheat sprouts can be easily pulled for additional, high nitrogen, mulch.

Selecting Materials

Drawing PathsPotential materials for a sheet mulch include leaves, straw, hay, saw dust, wood shavings, wood chips and sticks and branches (Hugelkultur).  We have always been concerned with the balance of nitrogen and carbon and the possibility that all the nitrogen will be tied up in the decomposition process.  We do not have research on sheet mulching, but we do have research on composting.

A compost pile wants to be 30 carbon to 1 nitrogen and moist, to heat up properly and rapidly decompose.  Fresh cut hay starts out at 30:1.  In 2010, at the Digital Garden on Leetsdale, Leo built his beds with wood chips.  A wood chip is around 500:1.  However, Leo's beds performed as well as the beds we built with hay, with no additional manure.  Subsequent research revealed that a problem in rapid decomposition in composting is the "bio availability" of the carbon.  In other words, we cannot count the carbon "inside" the wood chip in our calculations.  That is bad for composting but good for a soil ecosystem producing a continuous flow of nutrients.  It also explains the long term fertility described in the Hugelkultur literature.  The advantage to wood chips and branches is that they can be obtained free or produced on site.  A healthy soil ecosystem can eat 12 inches of hay a year and therefore that much hay has to be added each year.  We are testing how long these other materials can last.


The No Weed, Deep Mulch Part


Please remember that this is a deep mulch system. In the years after the initial sheet mulch, the idea is that there will be three things to do each year:  plant - mulch - harvest . . . or, if we plan of planting in the top of the mulch:  mulch - plant - harvest . . .

The mulch part begins before you plant, or, as soon as additional mulch will not smother your plants - the middle of June and after.  Our goal is to keep a continuous supply of organic matter on top of the soil as a continuing food supply for the soil organisms that are producing the nutrients our plants need to thrive. Theoretically, the system can retain the nitrogen so that additional manure is not necessary - but look out for yellow, slow growing plants, or plants vulnerable to insect attach - insufficient nitrogen may be the cause.  Also, any time you have a particularly agressive plant in your beds, we can add additional newspaper/cardboard and mulch over that.

Now, to obtain the mulch we need for our soil organisms . . . the most valuable source is those plants growing in your bed that you did not intend to grow there. Those plants are right where you need them and when you pull the roots out of the mulch, or separate the top from the root, the new mulch will be as high in what your soil needs as anything you can find.  Since, by definition, a weed is a plant in a place it is not wanted, and because you want this mulch, these plants are not weeds.

After you have applied this free and convenient source, you will want additional mulch to make as much as an additional 12" of mulch on your bed. Do not put on so much mulch that it smothers your crops - but as much as possible. You can go ahead and use purchased hay or straw but, we like to maintain "natural" areas around the garden where we can obtain additional organic matter for mulch.  Here in Colorado natural areas are grass lands (see, Living in Place) and if you cut the grass once a year (use a scyth or string trimmer***) and go out with a pitch fork and gather it, you can use that grass for mulch. Because it is fresh cut it holds more of what your soil needs and because it is still green it has the nitrogen to rapidly decompose.

A side benefit of the deep mulch is that most of the "weeds" that have plagued gardeners and farmers for millennia are "pioneer plants", that is, plants that do well in bare soil. Since we do not have bare soil, the mulch you will be growing in place is generally slower growing and easier to deal with.


*** a mower makes grass clippings, not hay, and grass clippings will mat together, prevent water penetration, and potentially decompose anaerobically creating a smelly mess.


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