The Trench Garden

There are two reasons for making a trench garden lined with a sheet of plastic. One is to filter water before releasing it so it does not pollute the surrounding area and the ground water. The other is to use every drop of that water as efficiently as possible. If water is a limited resource then the most can be made of the water you do have with what is sort of an outdoor version of a flood and drain hydroponic system. The trench can be used as a flood and drain garden if it's being used to catch rain water or using irrigation water and when it's used for waste water treatment the water just keeps moving through it like an under ground stream. Plants love it. The average western household uses a lot of water and used properly that water can make a lot of garden.

Above. A trench has been dug 40cm deep and lined with plastic. Slotted drain pipe has been placed in the bottom of the trench and then the trench has been back filled with the sand that was dug out. The trench can be made to over a meter wide and can change width on the course of it's journey through the garden.

The water first passes through a composting worm farm filter to remove all solids then the water can flow freely down the pipe and fills the plastic lined trench.

It's possible to get at least 20 meters of grey water garden for every person in the house.

Such a garden should be well planted with as many plants as possible to make use of the water and should also be well mulched to encourage worms to aerate the soil. 

Below. The slotted drain pipe. This is the magic that makes it all work. It introduces both water and air to the plants roots so they are well watered without drowning. 


1. Preparation

If you have not made a garden before, any method of garden preparation will take a lot more work than you expect. Some people just want to jump in and plant. It is far better to get all your preparation done first. This way you are not still trying to build the garden while you are looking after plants, which is a lot of work in itself. This increases your chance of success after you plant. The easiest way for you to get started may be with some pots. Look to the chapters on pot culture. 

For making a large garden, it is best to start preparation work a long way ahead of your planting time by at least one season. Use the dry season to prepare for planting for the wet season. There is no point trying to grow a garden during the dry season until next year when you should end the wet season with a full tank. You will only break your back trying to keep your garden alive in that first dry season. It's also easier to get the digging done without the hassle of heavy rain and mud. 

2. The Site 

This garden can be made in remote places and can be dug on a slope and near the top of a ridge to get the full benefit of the sun. Slope in the site is very helpful for collecting rain water and draining the garden by siphon into an Earth tank. Hill slopes and ridge tops are most likely to be solid clay or full of rock, as that is what holds them together, so digging here is going to be difficult but rock and clay are excellent materials for building terraces and tanks.

3. The Method

The trench garden is basically an outdoor version of flood and drain hydroponics. A level trench is made across a slope and lined with plastic to save water from draining into the Earth. A drainage pipe full of holes is placed on the plastic at the bottom of the trench garden. This plastic lined trench is filled with soil. A plastic sheet is placed on the ground on the slope above the trench. Any rain that falls will drain from the sheet into the garden. A siphon hose can be used to drain excess water from the drainage pipe in the garden. The excess water is drained into an 'Earth Tank' dug below the garden. Water can be bucketed or pumped from the tank back into the garden. This method makes it possible to grow in dry locations by taking advantage of intermittent and seasonal rains and mountain mist.


4. Levels

A 1.2m - 4ft carpenters level is a very handy tool. A fiberglass level is relatively cheap, strong, light and easy to carry. However they can be a little tedious to use when marking out a long line on the ground. For longer distances, a sight level can be used or a water level can be made with a 15m - 50ft piece of clear vinyl pipe, filled with water. Ensure that there are no bubbles in the pipe. Two black plastic taps, one at each end, can be turned off and will hold the water in the pipe when you are moving the water level around. The ends of the pipe are taped on to 2, 185cm - 6ft staff's, with measurements marked on them in centimetres or inches. Two of the above mentioned fiberglass levels will stand in well for this job, they often have measurements marked on them. 120cm wooden T-squares also work well for this job and are cheap. 

The staff's are held upright, either by tripods, made from branches with forks left on the end, or by two people, which is much easier. The two people read off the measurements indicated by the water level against the measuring sticks to each other. When the measurements are the same, you have found the level. This device can also be used to accurately measure the rise and fall of the land. 

5. The Trench

Use a level to mark out a level line on the ground and dig a trench below the line 19m - 62 ft long, 40cm - 16 in's deep, 50cm - 20 in's wide at the bottom and 100cm - 40 in's wide at ground level. It is important that the bottom of the trench is as close to level as possible for maximum efficiency. At both ends of the trench, dig a V shaped sump 15cm - 6 inches below the base of the trench. Of course this trench can be any length. I have made them 5m - 16ft long but the longer it is, the more efficient it becomes. When you water one plant, you water the whole garden. 

Earth dug from the trench is placed in front of the trench and packed down to create the front wall and help make the trench deeper. The front wall should be level with the back of the trench, which is on the up slope side of the trench. The front wall should also be kept flat and level to make a path that can be walked on. To pack the soil tighter, it helps to sieve the soil through a 20mm - 1 inch sieve. Pack the Earth down tightly with your hands and even stomp down with your boots. Some moisture in the Earth will help it compact more tightly but don't make it muddy. 

The Earth and rock may be kept piled up on 'woven weed mat' until it is to be used. 'Weed Mat' is very strong and comes in fifty meter rolls, six feet wide and can be purchased by the meter. Cover loose Earth with plastic in case it rains before you have time to use it. Line the completed trench with a single piece of concreters plastic, no holes or joins. If you are using a number of pieces of plastic together, then a number of overlapping layers will seal together well. 

6. The Slotted Drain Pipe

Place 20m - 66 ft, of 55mm - 2 in', agricultural drain pipe in the trench. This pipe is the magic ingredient that makes the garden work. Normally a garden that is lined with plastic will become water logged and starved of air, suffocating the soil and plants. The pipe allows you to drain water easily, and introduces fresh air, right down to the lower layers of soil. Tie plastic bags on either end of the pipe before you begin, to prevent soil getting down the pipe while you work. 

Place rocks on the drain pipe at either end to force the pipe to bend down into the bottom of the sumps. Then fill in the trench with the best soil you can get. 

You can now dig out the rocks holding down the pipe. Both ends of the drain pipe should be exposed by about 10cm to 15cm - 4 to 6 in's. Cover the ends with old cans to prevent the pipe being eaten by rats and to prevent soil, rats, frogs getting down the pipe. The pipe has hundreds of slot shaped holes cut into each metre. Because they are slots, they cannot block up and air and water can always drain through the pipe. A stick can be inserted in the pipe to check for the presence and depth of water in the garden. 

If you do not have access to plastic, the trench can be lined with clay to seal it. If you also do not have access to slotted drain pipe, then tiles, bricks or rocks can be used to create a space in the bottom of the trench for water to flow along the garden. If you do this, then one side of the base of the trench should be left free so that the soil can still reach the bottom of the trench, so that the soil can soak up water. Where the sump goes, you can put a small well from which to drain the water. 

7. The Soil

You may be lucky enough to have good topsoil nearby but don't be fooled, even good looking forest soil can be quite poor. Perhaps take a sample to a nursery suppliers to test first. If there is absolutely no usable soil in the vicinity, you may need to purchase five or six cubic meters of free draining top soil. This is also a good opportunity to mix your choice of fertilizer through the soil as it goes in the trench, as you can easily mix it right through from the bottom to the top. This also avoids possible damage to the plastic liner that can occur by using a spade to turn the soil over once it is in the trench. I prefer to use my own kitchen compost for this job. There is nothing better, except for mulch over the ground, which should be used as well as compost. I like to sieve the garden soil through a 10mm - half inch sieve. This breaks up all clods and clay and helps mix the soil thoroughly. Use larger sieves, roughly 50cm - 20in' across, for all garden work. 

8. Collecting Water 

On the slope above the trench, lay a large sheet of concreter's plastic. It is usually 4m - 12ft wide, folded in half on a roll, and can be purchased by the metre. Lay the lower edge of this sheet over the plastic liner that emerges on the upper level of the trench garden and dig it down a little under the soil. Any rainfall will flow off the catch sheet and into the garden. 1cm - 0.4in's of rain will add up to 10L - 2.2gal of water per one square metre - one square yard. Excess water can be siphoned from the garden and stored in a tank below the garden. 

If rainfall becomes excessive then the plastic sheet may be removed or the lower edge of the catchment plastic may be tucked under the edge of the plastic liner of the trench. The water will then flow under the trench sealer and soak into the ground. 

Cloud mist can be caught with this method if gardening in the mountains. Day light may evaporate the mist before it has a chance to flow down the plastic but at night droplets of water will accumulate and flow into the garden. I found that an 8m x 4m piece of plastic could collect more than one hundred litres a night. The tank slowly filled up and kept the garden going through the whole dry season, purely on cloud mist. 

9. The Earth Tank

On the slope below the garden trench, dig a rectangular hole, 1m - 3.3 ft deep, 2m - 6.6 ft wide and 3m - 10 ft long. Sieve the Earth through a 10mm - 0.5 inch sieve and loosely pile up the rock around the hole and incorporate the rock removed from the trench. Use a level to ensure that the rock is level all the way around the top edge of the tank. 

The fines, dirt passing the sieve, can be mixed with water into a mud and thrown into the rock wall and smoothed down by hand. When dry, this will be very strong and stable.


10. The Tank Liner

At each corner of the tank, place a star picket fence post and stretch fencing wire between them, level with, or just below and outside, the top edge of the tank. Concreters plastic will not be strong enough to line the tank, as it will not stand up to daily use, as one often needs to get in the tank while it is still dry. A large tarpaulin 9m x 9m - 30 x 30 feet, folded in half is ideal. Then use at least 30 strong fold back clips to clip the tarp to the wires.


11. Siphon

If possible, make the top level of the tank lower than the bottom of the sumps in the garden trench. This way, all excess water can be siphoned from the garden into the tank. Sediment, mainly caused by worms, will creep through the pipe and collect in the sumps. This sediment will block a 1/2 in' hose preventing it from siphoning. 3/4 in' poly-pipe is best. Install an elbow in the pipe where it emerges from the drain pipe, so it can easily turn down to the tank at ground level. 

Unlike a 1/2 inch pipe, a 3/4 in' pipe is impossible to start with lung power. A very handy device is a '12 volt Submersible, Conga in-line pump' which can be found on the net and at good nursery suppliers. A small 12 volt battery can easily be carried to your garden and can be recharged with a solar cell. 

Pump some water from the tank into the garden and when you no longer hear any bubbles, stop and remove the pump and the siphon will start immediately. Allow the first of the back flow to drain into a 20L - 5gal bucket to catch the sediment, which is mostly worm castings and sometimes worms as well. The water will run clear before the bucket is full, you can then let the pipe drain into the tank. Pour the bucket of dark water back on the garden. 

12. Watering - Flood and Drain

When the garden needs water, you can bucket water out of the tank, be careful to avoid damage to the tarp. Make life easy and use your 'Submersible, Conga in-line pump' to water the garden. Pour the water on the garden, not down the slotted drain pipe. 

Just as with flood and drain hydroponics, you can completely fill the garden with water and drain it by siphon back into the tank as described already. You will hear bubbles clicking in the soil as the garden drains. Flood and drain ensures that every cubic centimetre of soil in the garden is wet and therefore available to root growth. If you are using totally organic fertiliser, the water in the tank will get smelly but you will not have to worry about a buildup of salts in the water being recycled. 

13. Oxygen

By removing water from the garden this way, air is being sucked down into the soil and the drain pipe itself will hold a column of air under the garden, which will make oxygen available to the roots. Roots need oxygen because they are white and do not photosynthesize. This is why flood and drain and D.W.C. deep water culture hydroponic systems, work so well. Plants just love growing this way. They produce deep, thickly matted roots.


14. Automatic Siphoning

A 3/4 inch pipe removes the water very quickly and the soil will remain very damp. This remainder will slowly drain to the bottom, filling the sumps. With the sediment removed from the sumps by the larger siphon, a 1/2 inch clear vinyl hose can then be used to drain the rest. You can use the hose of the 'Water level' for this purpose. 

An interesting phenomenon of automatic siphoning can be exploited by simply bending the pipe down the slope, 30cm - 12in's below the ground level of the garden, where it emerges from the drain pipe. Then turn the pipe back up to the level of the front wall and follow the path to where it then flows down to the tank. Get the siphon effect going and ensure that the lower end is submerged in the tank. Once the water drains from the sump, an air bubble will enter the hose causing the flow to slow right down and even to stop. Surprisingly the pipe will remain full of water as long as the lower end of the pipe remains submerged in the tank. When rain starts filling the garden again, pressure from the raised water level in the garden will overcome the blockage caused by the bubble and the siphon will resume flowing again. 

The flow will be slower because of the presence of the bubble restricting the flow of water. However, even at this slow pace, the garden will still be fully drained by the time you get back. And the siphon will maintain itself, should rain continue spasmodically. This is very useful during wet weather when you have to leave the garden unattended for some time. 

15. Grey Water Gardens

A house with plumbing has clean water running into it and grey water running out of it. This waste water may be unfit for drinking or cleaning but that is no reason to throw it away. It is perfect for use in the garden and when it is looked at in this light the house with plumbing and waste water running out of it becomes a spring and the possibilities for a garden become apparent. 

There are a number of different kinds of grey water garden. There is a filter system which filters the water in biologically active tanks. The water can then be pumped from there onto the garden. This method can also be used to generate methane gas for use as a lighting and cooking fuel. 

Then there is the biological grey water garden which imitates natural wetlands by passing the water through steps that contain different aquatic plants and animals that filter the water. 

And there is the soak trench which takes the grey water, after it has passed through a compost filter, into underground perforated drainage pipes where the water can be used by plants. It is also possible to use these methods together in series. 

This chapter will detail the soak trench garden. 

Grey water, especially raw kitchen waste water, should not be poured on the ground or through Earth or sand to filter it. 

It must pass through an organic mulch or compost filter made of leaf litter, chaff, lawn clippings and or kitchen compost. It is also important to have worms in this system. 

This filter traps larger matter as well as smaller suspended solids. These solids when poured on the ground will clog it up very rapidly causing water to pool on the surface rather than soak in. 

The mulch lets water pass through it while it grabs the food scraps. The worms eat this matter and aerate the pile and reduce it to a liquid waste that drains into the trench and into the garden. Worms love water so long as they have drainage. 

This kind of worm farm, automatically watered and fed by your daily activities, would be the premier worm farm in any organic garden.  And the soak trench is the simplest way to deliver that nutrient rich water to the garden. 

The trench garden described already can be adapted to use for grey water recycling and irrigation in your home garden. If you do this, the water from your house must not go straight down the pipe. It must first be filtered of solids that would otherwise clog the drainage pipe. Where you wish to place the compost filter, make the trench much wider. The trench is lined with plastic. Place the 55mm - 2inch, agricultural slotted drain pipe in the bottom of the trench. Put a plastic bag or can over the end to stop solids from entering the pipe. Place a large 1 to 2 inch, 3 to 5 cm metal screen over the pipe supported on four bricks. Place a layer of lawn clippings or leaf litter on the screen. 

The daily kitchen compost can also go on this screen. Always spread out all new compost in a layer as it’s added. The water that drains from your home can then be drained into the middle of this and it will be filtered by the mulch and compost. 

Worms are an essential ingredient in this and may be found in your garden and put in the compost pile. If you think it necessary, you may be able to purchase composting worms to get the system going. There are three main varieties of non-migrating, composting worms; red wrigglers, Indian blues or tiger worms (eisenia fetida). 

A well constructed grey water garden with a worm farm composting filter may even be used to recycle black water from the toilet. 

Place a weather proof cover over this, a sheet of plastic covered with U.V. resistant woven weed mat will do fine. 

Back fill the rest of the trench with the soil that was removed from the trench. If the Earth removed from the trench is poor you can fill the trench with better soil and use the dirt removed from it as a path on the lower side of the trench, should it be on sloping ground. You now have your own composting, grey water, worm farm and irrigation system. 

You can take the trench in any direction you wish on flat ground, so long as the base always stays level. To achieve this, place a permanent mark somewhere, like a wall or a stump, at a measured distance above the base of the trench so that you can easily find the level again as you continue working, and if you should desire to extend the trench in future. 

If the trench is built short to start with, then the end of the trench can be left open so excess water can drain into the Earth until you are able to extend trench. 

Unfortunately tree roots may clog the pipes. Keep the plastic liner up to the soil surface to prevent tree roots entering the trench. 

The trench garden should be well planted so that the garden draws up as much water as possible. 

You can end up with quite a large garden from this method. You'd be surprised how much water comes out of your house. The average western household uses 1100 litres a day and a little goes along way. You can make at least 20m - 60ft of trench garden for every person in the house. 

The trench should be longer than necessary so it can handle occasional large volumes of water coming in. The trench may be used to fill ponds that contain fish to eat mosquito larvae, or to fill tanks so that water may be siphoned or pumped out and used for above ground irrigation. 

This form of garden bed should be mulched over to encourage the presence of worms and beetles in the garden. They dig burrows and create an exchange of oxygen and gases in the soil that prevents the soil becoming too damp and stagnant. 

Trees and plants with tap roots should not be planted in this garden as tap roots will not be able to grow down and may puncture the lining. Otherwise you can plant many kinds of plants such as water loving plants like bananas, gingers, lilies, reeds, taro and also plants that like to watered from underneath, rather than above, like melons, pumpkins, squash, tomatoes, zucchini and whatever else you can think of. 

The simplest version of this would be to have the grey water drain out the bathroom or kitchen right into a small paddy field filled with leaf litter and worms and water loving plants.  Like a normal rice paddy it is surrounded by a bank to hold water and leaf litter. Earth should be built up under the house to keep out water. This garden is put on the lower side of the house so water is more inclined to seep away from the house. Rainwater from the roof may also be drained into this garden.

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