Zai holes, roof gardens, circle gardens
Topics include: the modified or de luxe zai hole, Zai pits, Zai holes, Tassa, Tumbukiza, greenroofs, roof gardens, bag gardens, circle gardens, sorghum, millet, the “three sisters”, woodlots, reforestation, green walls, permaculture, human manure, human urine, degraded soils, soil improvement, plant shelter.
Zai holes or Zai pits are used to greatly increase the productivity of grains such as sorghum and millet, particularly in the hot, dry Sahel region of West Africa, and now for growing other plants including trees in many other places around the world.
For more information, see the excellent sites:
the excellent ECHO technical note 78 (Motis et al 2013, search for “Zai pits” and click on “Zai pit system – ECHO community org”), which also has additional references.
The De luxe Zai hole
The diagram below shows some possible modifications to increase the productivity of Zai holes in various soils and climates, for various purposes. See also the “Arborloo” page.
Zai holes are an effective but labour-intensive technique used in the drier zones of West Africa, especially if the soil surface is compacted or has a hard surface crust, mainly to grow grains such as millet and sorghum, in infertile sandy soils and harsh climatic conditions. In these circumstances, Zai holes should also help trees to grow better in reforestation projects. Zai holes/Zai pits basically involve breaking through the surface crust, by digging a pit/hole, and mixing some animal manure (and sometimes trees grow from the seeds that were in the manure) or compost (and/or biochar) into the soil. In addition, some of the subsoil (or stones) is then used to make a surrounding ridge or rim to hold water (on flat ground), or on the downslope edge on sloping ground, in order to catch more water runoff from the paths in between the zai holes.
The same or similar techniques have been used extensively and successfully by the author in suburban ornamental landscaping, and tree planting on farms and degraded land in Australia. For tree planting/reforestation projects on degraded sites, it would help if a few groundcover plants are also planted with the tree, (or in separate holes) to cover, mulch, and slowly improve the soil between the Zai holes. Ideally groundcovers would be species that root at the nodes (e.g. Ipomoea spp. and stoloniferous grasses), with at least some nitrogen-fixing species used.
A possible strategy or sequence for longterm improvement of degraded soils to increase crop productivity, using Zai holes:
Initially trees could be grown in zai holes to form a woodlot (for fuelwood for heating and cooking fires, or fodder trees, or other purposes). In the Sahel region, for example, suitable trees could be Acacia holosericea, A. torulosa and A. tumida, and perhaps Cajanus cajun. These trees and shrubs also provide edible seeds. In slightly wetter regions, some other possibilities include Acacia auriculiformis, Gliricidia sepium, and Sesbania sesban. After around 2-10 years, the woodlot trees could be removed, having improved the soil by adding organic matter and nitrogen to the soil, as well as a surface mulch of leaf litter and twigs. Following this, Faidherbia albida (or perhaps Prosopis cineraria in India, and other Prosopis, Acacia and Parkia species in various parts of the world) trees could then be established from seeds or planted in Zai holes, at approx. 10 metre spacing. At around year 5 – 10, the soil should have been improved even more by the Faidherbia (or other trees) and crops could be grown under the canopy of the Faidherbia trees, with greatly increased yield (probably twice as much and perhaps much more with Faidherbia trees).
Zai holes: Size and shape
Zai holes can be made in the shape of a circle or a square, or could be a rectangular trench dug horizontally along the contour to catch water which runs off from the paths. In very dry areas, there are usually larger paths (a greater surface area to catch more runoff), and fewer Zai holes. Over time, the manure which has been mixed in is usually further distributed through the surrounding soil by termites, and/or earthworms. Other appropriate amendments could include compost, vermicompost, fertilizers, biochar, rock dusts, burnt bone, clay in sandy soils, etc.
A similar system to Zai pits is used in East Africa, called Tumbukiza, where goat or cow manure is mixed into the holes, and is commonly used to grow Napier grass, Pennisetum purpureum, (or Andropogon gayanus, or Panicum maximum) in cut-and-carry fodder systems for stall-fed dairy cows.
The width and the depth of the holes varies considerably, and does not appear to make a great difference to plant growth, however larger holes would store more water, and be more suitable for larger plants, including trees, or a greater number of individual plants. In Western Kenya, a depth and diameter of 60 cm (2 feet) is popular, with paths of the same size between the plantings (Orodho, 2006). Zai holes can be as little as 20 cm in diameter and 10 cm deep, while Tumbukiza used to grow the very large Napier grass in Kenya, can be as much as 90 cm deep, and up to 90 cm long. See the Napier grass photo below.
The size should take into account what is ergonomically easy for management, such as reaching down or across for watering and weeding, and that the roots of most vegetables only go down to a depth of around 30 cm (12″), although maize roots can grow down to a metre (yard) or deeper. Many plant species absorb most of their water and nutrients from the top 60 cm (2 feet) of soil. Larger holes require more work, which is a consideration where many holes are to be dug, and in Africa where the farmer may be female and perhaps not as strong as they were in their younger years. One idea to reduce labour is to use a draft animal/s to rip lines that criss-cross the area, so that zai holes can be more easily dug where the rip lines intersect. Generally hoes or mattocks ar used to make the holes, but in some soils (but not very sandy soils), and for some people, a garden fork may be easier, since you are using your body weight to push the fork into the ground, and levering the fork upwards (using the principle of the lever), rather than the relatively hard work of lifting a hoe above your head, and then striking the ground to make holes.
If four stakes are to be used to enclose each hole with clear plastic or non-woven polypropylene fleece, then fewer, larger holes would be more economical, since four stakes (and other materials) are needed for each hole. If stakes and a covering of plastic or fleece are to be used, then a large diameter of about one metre (one yard), and a soil depth of 30 cm (1 foot) for smaller herbs and vegetables, and a depth of 60 – 90 cm (2 – 3 feet) for larger grain crops, should work well.
The size and shape of paths should allow easy access for people, and possibly wheelbarrows, mowers and trolleys. In dry areas, the most important function of paths may be to provide a large and impermeable surface area to collect water. In wetter areas, and/or where space is limited, the area covered by paths should be as small as possible. Plants could also be grown on paths, to be mown and produce mulch for example, (perhaps subterranean clover, Biserrula pelecinus in Mediterranean climates, or other nitrogen-fixer), or to attract beneficial insects such as bees (e.g. many groundcovers in the Asteraceae family, legumes, Phyla/Lippia nodiflora). However, in drier areas, plants grown between the holes would reduce runoff into the holes.
Zai hole productivity increases
Reported increases in plant productivity for Zai holes and Tumbukiza range from 40% through to 600%, and should be further increased by the modifications suggested for the “De luxe Zai hole”. The higher figures would be from results in dry, infertile soils, often with a hard surface crust. In good soils with favourable climates, a lower response should be expected.
Some more productivity figures for zai holes, and microdosing of fertilizer from Motis et al 2013 (ECHO technical note 78):
Microdosing of fertilizer: “Smallholder farmers – about 25,000 of them – in West Africa have seen 44% – 120% increases in sorghum yields and 50% – 130% increases in family incomes (ICRISAT 2009).
Zai holes in a high annual rainfall (1300 mm) area of the Ethiopian highlands, combined with nitrogen fertilizer: “Zai pits, in combination with additions of nitrogen, increased potato yields 500% – 2000%; bean yield increased by 250% with zai pits. The pits themselves contributed more to yield increases than the nitrogen inputs, indicating the significant role that water can have on crop growth and the use of existing soil nutrients. Importantly, farmers earned 20 times more income than the labor required to dig the pits”.
Zai in West Africa: “yield increases of 300 – 400 kg/ha in a year of low rainfall and as much as 1500 kg/ha in a year with good rainfall were observed by Kabore and Reij, 2004. With rainfall amounts ranging from 359 – 449 mm, over three rainy seasons, sorghum yields in farmer’s fields increased from 319 – 642 kg/ha without zai to 975 – 1600 kg/ha with zai (Sawadogo 2011)”.
Shelter to increase plant growth
In the modified or de luxe design illustrated above, the clear plastic (polythene) in cold conditions, or polypropylene garden/horticultural fleece/frost protection balnket (see photo on the left) in hot conditions, would provide protection from hot or cold winds, sand blast (especially young seedlings), extremes of temperature, low humidity, and high solar radiation, where needed. To gain an appreciation of what shelter can do in colder climates (the U.K.), the shelter/greenhouse effect of plastic tree guards increased the growth of young oak trees in the first three years, growing five times taller and 6.2 times the stem volume compared with unsheltered trees (Tuley, 1985). Note that this effect should be in addition to the figures of 40 – 600% productivity increase given
above for an unsheltered Zai hole (at least in colder climates or seasons).
Mulches of plant materials (organic mulches) on the surface of the soil generally keep the soil cooler, insulated against temperature extremes, and moister for longer. These conditions usually result in a proliferation of earthworms (which increase water infiltration, soil aeration and nutrient availability); and promote the growth of mychorrhizal fungi, which can greatly improve the efficiency of water and nutrient uptake by plant roots. Mulches usually reduce weeds (which compete for light, water and nutrients, and so reduce plant growth) and need to be topped up as they decompose.
Mulches can greatly increase plant growth. For example, a mulch of wild grasses and banana leaves increased yields of coffee in Ethiopia by 22% – 93% (Troeh et al, 1980). In my experience, mulches of pea straw or alfalfa (lucerne) hay are particularly good for growing vegetables, and encouraging a population explosion of earthworms, and hence their beneficial activity. The leaves (preferably small leaves) and twigs of most nitrogen-fixing plants are likely to be good, e.g. from legumes, casuarinas, alders and Azolla (however Azolla decomposes quickly and is probably best used as fodder or to make compost). Gliricidia sepium for example, is tried and proven. Some Acacia species with large phyllodes (“leaves”) may contain tannins and may not be as good, or will require partial composting before use (this would also apply to most wood chips, barks, and sawdust, which may contain toxins and require some composting with additional nitrogen). The leaves of Tithonia species are high in both nitrogen and phosphorus (and the related Montanoa and other daisy family plants may be similar) and should make a good mulch. Tagetes minuta or khaki weed is high in potassium (2%). Tithonia, Montanoa and Tagetes are all in the daisy family Asteraceae, so it is reasonable to assume that other plants in the daisy family may have leaves high in NPK. Some grasses used as mulch also seem to be good with vegetables, such as Guinea grass and sugar cane leaves/bagasse. Leaves from fig trees and Grevillea robusta may work well, and the cladodes (“leaves”) of casuarina are high in silica and long-lasting as a mulch layered over the top of other mulches or compost, and allow for easy infiltration of water.
Mulch from plant materials should be kept away from plant stems as this may promote rotting of the stems, however, some plants such as maize and tomatoes, will benefit by growing additional adventitious roots if mulch or compost is piled up against the stem, later in the growing season.
Black plastic mulch (polyethylene/polythene) can be a very useful mulch in colder climates or seasons.
In a trial undertaken by the University of Arkansas the use of black plastic mulch (compared with no mulch) resulted in an average increase in vegetable yield of 90%. Bushbeans, cucumbers and straight necked squash exceeded 100%, while purple hull peas only increased by 16%. Crooked neck squash had a 77% increase in yield, and sweet corn produced an increase of 83% in the number of ears. These very high figures should porbably be viewed as a best case scenario.
In some climates, where it is cold in winter and hot in summer, it may pay to use black plastic mulch initially to warm up the soil, which could then be removed and replaced with an organic mulch as the weather and soils get warmer.
See also the “Earthworms, Mulch, Compost” page.
Carbon dioxide enrichment
If the hole is enclosed on all sides by clear plastic (and preferably on top), partly decomposed compost (preferably made of leaves and twigs, to replicate the mulch found in a rain forest) could be used as a mulch. This would give off carbon dioxide, which is heavier than air, which would pool at the base, and should help to get low-growing seedlings/young plants off to a good start. An indication of the likely amount of carbon dioxide is suggested by research in a rain forest in Puerto Rico, where “At the ground surface, respiration maintained carbon dioxide concentrations of 400-620 ppm compared with 320 ppm in the free air above the forest” (Odum et al 1970, in Richards 1998). See also the article: vegsand , page 6 and 7.
Partially decomposed compost should preferrably not be in direct contact with the soil, as this may cause nitrogen draw-down, where micro-organisms outcompete plants for the nitrogen they use in breaking down plant materials (and so plant growth is reduced due to nitrogen deficiency). It would be best to place fully decomposed compost onto the soil first, and partially decomposed compost on top of this, preferably with additional nitrogen, which could come from human urine. Once the partially decomposed compost has fully decomposed, more could be added on top to keep up the carbon dioxide concentration, but results are likely to be diminished as the plants grow taller, and become adapted to CO2 enrichment. CO2 could also be produced by a solution of water, a sugar of some sort and brewer’s yeast, in shallow containers placed around young seedlings.
CO2 enrichment should increase growth by 10% or considerably more – click here for an example of the results that can be achieved with young vegetable growth in a CO2 enriched environment (52 days at 1250 ppm) :Plant response to Co2 enrichment
Water barriers to potentially increase plant growth:
The optional water barrier of newspaper should be beneficial in dry situations, in free-draining sandy soils, or for vegetables/grains/plants that have a high water requirement. Around twenty sheets of newspaper in a bowl shape at the bottom of the hole (e.g. around 30 – 60 cm deep/1 – 2 feet deep) form a slowly permeable barrier, which should prevent water from draining away for six or more hours (up to 12 hours – I tested it). This will give the plants time to use most of the water, while excess water will still drain away eventually, avoiding waterlogging, which most vegetables do not tolerate well and can result in root rot. For plants that grow best in very wet conditions, an impermeable layer of plastic sheet or similar could be used. See the article “Growing vegetables in sandy soils”, vegsand page 5 and 6. Water barriers in dry, sandy soil should increase productivity by at least 35%, and possibly a lot more for plants that normally have a high water demand.
Zai holes are designed to catch water runoff, but additional watering is possible and may increase growth. Drip irrigation is an option of course, but buried plastic bottles, waxed cardboard milk cartons or tin cans with added small holes in them can be filled to irrigate the plants, and could include soluble fertilizer such as human urine. Clay/terracota pots or pipes could also be used (Garrity pers.com.), partly or largely buried, with a moveable lid such as a plastic plate on top (to reduce evaporation losses – see photo below).
The photo above, and the quote below, come from the article “Mini forest garden greenhouse”, by Maddy Harland, Editor, Permaculture Magazine no 77, Autumn 2013, pgs. 59-60:
“Inside, I have adapted ‘Ollas’, a 4,000 year-old irrigation system originating in Africa. It comprises a 25.5mm (10 inch) buried porous terracotta pot (above) with plates for lids. In Africa these would be beautiful terracotta bottles, but pots work fine. The drainage holes are plugged with a wine cork and the pots are filled with water. As the surrounding area dries out, water is sucked through the terracotta into the soil and the roots of the plants wrap themselves around the pots. This appears to be working well and I intend to add to the collection next year. The system is a simple and cheap way to help irrigate the greenhouse and it can also be used in any outdoor veggie bed. I hope it will prove especially useful when I am away on holiday for a few days.”
“In summary, the zai system allows farmers to concentrate both fertility and moisture close to crop roots” Motis et al 2013.
Micro doses of fertilizer (typically a bottle cap full per plant/hole) can be placed precisely in the root zone in Zai holes, which is very cost effective, potentially promoting the growth of crops, and/or early growth of trees, see http://www.icrisat.org/impacts/impact-stories/icrisat-is-fertilizer-microdosing.pdf . A newspaper barrier (about 20 pages) beneath the roots of vegetables or grains (but not trees, where the newspaper could impede downward growth of roots, which may be a major problem for some species such as Faidherbia albida) should also reduce leaching losses of expensive fertilizer/nutrients, and could be very effective in well-drained sandy soils.
Various chemical and organic fertilizers, rock dusts etc., could be used, but the increased nutrient availability of vermicompost (compost made using earthworms) should give excellent results. Vermicompost can be enriched with small amounts of commercial fertilizer (or rock dusts, or burnt bone, wood ash), so that the vermicompost has a higher nutrient content, as well as higher nutrient avilability, and since the fertilizer should form a complex with humus in vermicasts, leaching losses of fertilizer should be reduced. For more info on “enriched vermicompost” some research has been done in India, click here: http://www.jtropag.in/index.php/ojs/article/viewFile/80/75 , or visit the “Earthworm” page – click on the button in the menu bar at the top of this page. See also “Human manure and urine” below.
Some plant combinations for zai holes
Modified Zai holes should suit the growing of the “three sisters” of maize, climbing beans and squash/pumpkins/zucchini (a compact form of pumpkin may be better), a complementary plant combination developed by native Americans. The maize (or perhaps sorghum or millet) provides support for the beans to climb, and the beans add nitrogen to the soil, which aids the growth of the grains and pumpkins. This is also a nutritious combination, with carbohydrate from the maize, protein from the beans, and carotene from the orange-fleshed pumpkins. The grains, pumpkins, and some types of beans can also be stored, providing food security.
Another plant combination which should work would be maize and pigeon pea Cajanus cajun. In an experiment growing maize and pigeon pea separately, and mixed together, produced interesting results, where a combination of the two produced more grain than either by itself:
Total grain harvest after 24 weeks (maize, pigeon pea, maize/pigeon pea mix)
maize (corn) grown in rows by itself: 3139 kg/ha
pigeon pea grown in rows by itself: 1871 kg/ha
maize and pigeon pea (plants mixed together within each row, a mixed intercrop):
maize and pigeon pea (grown separately in alternating rows, i.e. a row intercrop):
Oelsligle et al, 1976, in Tivey 1990.
More possible plant combinations for zai holes:
“In an ECHO zai trial in South Africa, combined grain yield of cowpea+sorghum was as much as threefold higher than sorghum alone”. The two species were grown in separate holes in alternate rows, which has the added benefit of allowing for crop rotation, plus, nutritionally, the cowpeas are higher in protein. Cowpeas may also provide a harvest of food before the sorghum harvest. Lablab beans may provide a later harvest than the sorghum (Motis et al 2013). Therefore, alternate rows of sorghum, then cowpea, then sorghum, then lablab, all repeated, could spread out the harvest and provide food security. The lablab and cowpeas also produce organic matter (lablab up to 10 t/ha, and cowpea, 3 t/ha), which could be used to make compost, used as mulch, or livestock could feed on the crop residues, as long as they are kept there long enough to redeposit the crop residues as manure on the same field.
The greater biodiversity (using three different plant species in the same area) could also help by providing habitat for beneficial birds and insects, and this may reduce pest and disease problems. A couple more possibilities for rotation and increasing biodiversity, and food security, could be okra (or roselle), yacon and cassava.
Faidherbia trees could be established at around 10 metre spacing, perhaps with seeds sown in zai holes that have had one season of lablab growing in them. The lablab topgrowth could provide a surface mulch, and the downward growth of the roots of the Faidherbia trees may be facilitated and hastened by growing where the decoposed lablab roots have paved the way.
Human manure and urine
In places where famine is a recurring problem, and/or where organic matter and nutrients are unavailable or not affordable, human manure could be used in Zai holes or circle gardens, or human manure could simply be used for the satisfaction of recycling a problem waste product into something of value. It takes about a year to ensure that all pathogens have died off (two years would be better – Jenkins 1999), so green manure/fallow plants could be grown instead of human food plants, to further improve the soil for a year or two, with the tops of the plants cut off at or just below soil level, and then used as mulch or dug into the soil for newly planted vegetables/grains. Another option would be to grow cut-and-carry fodder plants for animals, or fuelwood (e.g. Sesbania sesban, pigeon pea – Cajanus cajan, Moringa oleifera, grasses), for the first year or two.
Alternatively, dry leaves, paper and kindling (small sticks) could be placed in the hole, with human manure on top, and then more kindling on top of the manure, and then burnt to sterilize the manure. Ideally the hole would be covered to induce pyrolysis. This would also add beneficial charcoal (and perhaps burnt bone which has a high phophorus content and availablity) and ash (usually containing potassium, calcium, magnesium) to the Zai hole (see also the “Charborloo” on the “Arborloo” page, and the “Biochar, Terra preta” page), before planting vegetables or grains.
Human manure could be deposited into a plastic bowl, the base of which has been lined with about twenty sheets of newspaper. After each visit, the manure is covered with soil, saw dust, rice hulls or similar for hygiene purposes and to prevent odour. The manure and newspaper would then be carefully placed (wearing rubber gloves specifically for the purpose), newspaper down, into the bottom of a Zai hole which has a concave base.
A couple of litres of water can be heated (to 80-100 degrees C) in a solar cooker or on a fire, and the hot water poured on to the manure to kill harmful organisms. The Zai hole would then be filled with soil which has added compost, rock dust etc., and planted. Another option would be to heat treat the bowl of manure in a solar cooker which is kept specifically for that purpose.
Diluted human urine could be used on Zai holes as a free and available fertilizer, usually diluted 1 part urine to 5-10 parts water. See photo below:
Zai holes for producing mulch, compost and fodder
Zai holes can potentially be planted with plants to produce mulch, material for composting, and fodder for livestock. Two species could be combined, for example, a fast growing grass e.g. Napier or Guinea grass, plus a legume e.g. pigeon pea, or Gliricidia or a sesbania species.
See also the Arborloo page
Zai holes generally increase plant growth, and a combination of some or all of the modifications suggested to create the “De luxe Zai hole” should greatly increase productivity for a wide range of plants.
“In general, crops are limited to 25% of their potential
due to the impacts of environmental stress”
Boyer, J. S.
“If we could turn official and popular interest
away from the grandiose projects
and to the real needs of the poor,
the battle could be won”
E. F. Schumacher. “Small is Beautiful”.
For more info:
Jenkins, Joseph C. (1999). The Humanure Handbook. Jenkins Publishing. ISBN 0-9644258-9-0. Pgs. 164, 194.
Miles, Tom (2007). “Microcatchment rainwater harvesting systems: Zai planting holes.” http://biochar.bioenergylists.org/nepad19zai2004
Orodho, Apollo B. (2006) “Tumbukiza technology: an alternative method of Napier grass production.”
Richards, Paul W. (1998). The tropical rain forest: an ecological study. ISBN 0 521 42194 2. Pge. 214.
Rinaudo, Tony. “Zai holes harness termites to increase crop yields.” See “Dryland techniques and mulches”, in the book “From Amaranth to Zai holes”, www.echonet.org
Tivey, J. (1990). Agricultural Ecology. Longman Scientific & Technical. ISBN 0-582-30163-7. Pge. 112, ref, pge. 273.
Troeh et al. (1980). Pge. 676.
Tuley, G. (1985). “The growth of young oak trees in shelters”. Forestry, Vol. 58, No. 2, 1985. www.forestry.oxfordjournals.org.
A simple design for a hessian/burlap bag garden on a sloping roof
In this (untested) design, a hessian bag or old pillowcase is filled with good quality potting mix, or compost with some good topsoil (which could be pasteurised before use). The bags are placed near the bottom of the roof for easy access for maintenance, and to provide a large water catchment area above the bags. Clay, cement, or polyurethane foam ridges direct additional water to the bags when it rains. Watering could be by means of plastic bottles or tin cans with holes in the base, or clay pots, and fertilizer could include human urine. The bags could be placed on a sheet of plastic, to protect the roof. On top of this, an old blanket or towel would collect fine particles and keep the roof cleaner. However, the bag may slide on the plastic, and the bag may need to be fixed into position.
Roofs are a tough environment for plants, with high solar radiation, hot and/or cold winds, frost etc. Only the toughest, most adaptable plants should be used, and only low productivity should be expected, however, something is still better than nothing. Plants could be chosen not only for food, but also perhaps fodder, nectar for bees (honey), to attract beneficial wildlife, medicines, ornamentation etc. Groundcovers on a roof can also keep a hot house cooler, and provide insulation against the cold. See also the article “Growing vegetables in sandy soils” – click here vegsand. For more ideas on growing food in limited space, click on the “Stacked gardens” button above, and the “Arborloo” button.
Moringa oleifera cultivars PKM-1, PKM-2, or other smaller varieties, should grow well in Zai holes, circle gardens, bag gardens (sack gardens) and possibly roof gardens. This very useful, tough and fast-growing plant produces edible “drumsticks” (like bean pods), and nutritious edible leaves, animal fodder, and nectar for bees. It can be grown from seed, or from large cuttings, around 10 -15 cm diameter, placed directly in the ground, early in the wet season.
An incidental roof garden
These plants have established themselves naturally on a roof at Paradise Palms golf course, Palm Cove, Cairns. The roof has collected some leaf litter, and the seeds have presumably arrived from birds depositing them. Pipturus argenteus (centre) and Homalanthus novoguineensis (left) are two classic colonising/pioneer/regrowth plants in the rain forests of North Queensland. How the sugar cane (right) got there is anyone’s guess – perhaps someone was eating the sugar cane and then threw it up onto the roof.
Vertical gardens/Green walls
It is definitely worth seeing Patrick Blanc’s amazing work and book:
Blanc, Patrick. 2011. The vertical garden: from nature to the city. W. W. Norton & Company. ISBN 978-0-393-73379-2.
A green wall (mostly bromeliads) on the side of a farm shed, Mount Uncle, North Queensland, Australia.
“No individual has any right to come into the world
and go out of it
without leaving something behind”
George Washington Carver