A garden that waters itself
It’s just the sort of garden we need in the drying summer heat of a warm temperate climate, when we’re not around to water our plants every couple days.
The challenge: Construct a minimum-maintenance training garden in a regional park where gardens dry rapidly in the heat of a Sydney summer.
The solution: A self-watering garden that could be left for days or longer without watering.
The problem: Designing and building it.
The permaculture principle: Design from patterns to details.
The pattern was set by environmental conditions, proximity to the teaching centre building and the movement of people through the public open space. This was determined by applying the permaculture principle of observe and interact to assess where the self-watering garden might be built. Shelter from hostile wind, access to year-round sunlight and other factors were considerations in deciding where to build the garden.
However self-watering garden beds are constructed they follow the same principle: moisture to irrigate plants moves from a reservoir below into the growing medium above from which it is separated by a permeable membrane.
Self-watering gardens are often known as ‘wicking beds’ because water ‘wicks’ into the growing medium from the water reservoir below.
Self-watering gardens consist of a container with a:
- lower reservoir giving structure to support the growing medium above with a coil of 100mm polypipe or with gravel, and filled with water through a vertical tube of 50 or100mm polypipe capped against dust, leaf litter and snails that would block it; an outlet consisting of a short length of 25mm pipe which allows excess water to drain from the reservoir
- layer of permeable geotextile through which water can move but which prevents soil particles moving into the reservoir below and blocking it
- an upper growing medium such as soil or a growing mix in which plants are established.
Geotextile is a synthetic fabric which has the durability needed of a membrane. Natural fibres would degrade and soil fill the reservoir.
The Training Garden
The garden was constructed as a training facility for Randwick Council sustainability educator’s:
- seven week part-time organic gardening course, Growing Big in Small Spaces
- her Introduction to Permaculture courses
- as a place for the Permabee community volunteer program set up to provide social contact and reduce social isolation (an application of ‘social permaculture’ design) while teaching a little horticulture.
The self-watering garden containers are part of Randwick Sustainable Hub, the community resilience program located at Randwick Community Centre in eastern Sydney.
They are adjacent to a large keyhole and two rectangular garden beds that are part of the Training Garden, a hive for colonial native bees on the adjacent Habitat Trail and the foot track around the ephemeral wetland in Randwick Environment Park.
Funding came through council’s environment levy.
Fiona Campbell, then council’s sustainability educator. Fiona is a permaculture educator and project manager who ran permaculture design courses through PacificEdge and taught permaculture design at TAFE and community colleges.
Six raised, self-watering garden beds were built.
- four clean intermediate bulk containers — IBCs — cut in half as garden beds
- reused, salvaged hardwood to clad the containers
- fastenings for assembling the garden beds
- imagination, design, skill and planning.
IBCs are reusable, stackable containers used to transport and store liquids. They consist of a cube made of high-density polyethylene (HDPE) held inside a cage of galvanised steel tubing.
Caution: If using used IBCs to make self-watering gardens, first find out what they have stored. Best to avoid those used for liquids that are toxic to plant life. Make sure they are thoroughly washed with a detergent.
Design and construction process
Identify the need
Identifying a need is the first thing in any design process. A design team should clarify the need to make sure they have it right.
A garden for use in the community resiliency courses and for the community Permabee volunteers to use.
If we are making something on land that is not ours, or on public land, we need the go-ahead of the landowner or manager.
The training garden was okayed by council management.
A site analysis was made to determine the best location for the garden.
The training garden has:
- access to year-round sunlight
- access to water for irrigation
- access to and from the teaching centre classroom
- as sheltered from wind as is possible on the open, windy site that is the regional park; potentially damaging southerly to westerly winds come mainly in winter
- sandy soil across whole region (agricultural grade soil was brought in to fill the IBCs)
- existing vegetation of weedy grasses on the edge of the remnant bushland of Randwick Environment Park
- wildlife — an assortment of native and exotic birds; blue-tongue lizards; an occasional black marsh snake.
Plan and construct
Planning and construction followed a logical, sequential progression:
- Measure the site.
- Conduct a site (soils, existing features etc) and sector (sunlight, winds etc) analysis.
- Plan location of raised container beds.
- Clear and prepare area and level the places where the container gardens will be placed.
- Install IBCs.
- Construct hardwood cladding.
- Install wicking bed fittings — polypipe in reservoir, filling tube, geotextile, growing medium.
100mm polypipe laid in the base of the IBCs was used as the structural element in the reservoirs. Gravel is an alternative. Polypipe is easier to handle than gravel. The geotextile drapes over the polypipe, providing greater contact area for water transfer when the growing medium is installed. The coiled polypipe is also a structural element. It holds up the growing medium.
The raised container gardens were clad in recycled hardwood because they are in a public place where aesthetics are important. The self-watering garden would work without cladding, however the sharp tops of the galvanised steel cage would need capping where they are cut to prevent injury. The cladding was finished with the addition of flat hardwood strips on the top to serve as seating.
The garden is in a public place and risk to the public must be minimised.
Councils and designers planning works on public land or on private sites visited by the public or students have a duty of care to their safety.
Risk may be due to:
- trip hazards
- sharp edges
- rough surfaces
- poor construction
Minimising risk is part of the maintenance cycle.
The design and construction of the wicking beds ensured:
- paths between raised beds are smooth and useable by people with impaired mobility
- no sharp or pointed edges
- timber is smooth with no rough surfaces which would injure with splinters
- sturdy construction.
How it works
A brief excursion into science shows how wicking beds work by making use of osmosis and capilliary action.
Moisture moves from the reservoir below the growing medium, through a geotextile membrane permeable to water but not to soil particles, and into the growing medium, commonly soil or a specialised growing blend.
- water moves through the membrane by an absorption process called osmosis
- moisture then moves into the soil by capillary action, passing through the small spaces between the soil particles; here, it is accessed by the fine feeder roots of the plants growing above
- the movement of moisture is stimulated by the moisture gradient, the difference in moisture between the water in the reservoir and the dryness of the soil above the membrane
- its movement is influenced the structure and texture of the growing medium—its coarseness—which influence its permeability to moisture
- the downward force of gravity exerted on solids and liquids by the mass of the Earth sets an upper limit to the movement of moisture through the growing medium; moisture content decreases with distance above the water reservoir and on reaching its maximum any additional depth of growing medium becomes irrelevant; there appear to be no figures for the maximum depth of soil, which would anyhow be complicated by the permeability of the growing medium used, however anecdotal advice suggests somewhere around 35cm as the deepest.
Once the growing medium is moistened, the water molecules and their mineral and nutrient load are absorbed by the fine root hairs of plants into their vascular system and transported into the plants to feed cell growth. Vascular tissue called xylem conveys water and dissolved minerals from the roots to the rest of the plant and provides physical support. We can think of it as the way moisture is diffused from reservoir to plant cell.
Most plant nutrients are obtained in this way, other than carbon which is absorbed through tiny openings in the leaves, called stomata. Plants cannot use organic compounds like compost or manures until they are broken down into their basic or ionic forms (electrically-charged molecules). The growing medium is home to microbes that do the breaking down organic compounds into basic inorganic compounds, and fungi that are important in assisting some plants take up the nutrient phosphorus.
This explains why a nutritionally-balanced growing medium is necessary to healthy plant growth. In other words, add compost to our self-watering garden.
We can also mulch the container gardens to reduce soil moisture loss in hot weather, to reduce weed growth and to provide plant nutrients as the mulch breaks down.
Applying permaculture principles
There are a number of permaculture design principles we can see in use in the training garden. They are derived from the sets of principles published by both Bill Mollison and David Holmgren, the inventors of the permaculture design system, with an additional principle of my own.
Permaculture is information and imagination-intensive
To start any project we need baseline information. Baseline is the starting information which forms the basis of design by describing what exists on site, how the site is used and how people move through it. Information is collected by:
- applying the permaculture principle of observing and interacting with the site—over a number of years on this particular site
- by measuring, which is important to detailed planning
- by research into previous landuses, which could indicate pollution or other potential problems to test or watch out for, and for information on local ecology or climatic factors.
Imagination played an important part in conceptualising the form of the garden. Imagination is free-form thinking which can make links between ideas to make more-effective design.
Design for usability
A garden designed with usability in mind is fit for purpose and more likely to be well-used.
Characteristics of usability include:
- located close to other design components used by the gardeners — relative location
- ready access
- garden dimensions scaled for easy use
- clean and orderly appearance that invites interest and use
- low risk
- minimum and easy maintenance.
Relative location is a permaculture design principle which suggests positioning components of design within convenient reach of other components used by the site users. It is about convenient connection.
In the Training Garden that means proximity to the classroom, the open space of Munda Street Reserve, the Habitat Trail and to the foot track around the ephemeral wetland of Randwick Environment Park. The garden is a short walk along a paved path from the community centre carpark, a consideration for aged course participants or those with limited mobility and visitors. This is important in design for public open space as all visitors must be designed for.
Obtain a yield
When our designs produce something of use we obtain a yield. A yield can be material, something physical like vegetables, or psychological, like learning.
The prime yield of the Training Garden is learning—knowledge gained by the course participants .The self-watering garden also produces a yield in the form of vegetables and herbs. Another yield is time. Less time is spent keeping water up to the garden than would be the situation were the garden built on the soil.
Use edges and value the marginal
The training garden is situated at the end of the Habitat Trail which links the teaching centre building to the walkway around Randwick Environment Park.
It brings into use the transition zone, the edge, between the Habitat Trail and the remnant bushland of the Environment Park, a previously-neglected space.
Produce no waste
The reuse of hardwood cladding uses recycled material that would have been discarded during building demolition.
Design from patterns to details
The pattern was derived from observation of how people used the larger site and their movement patterns. Seasonal weather was another pattern influencing the design of the Training Garden.
These larger patterns influencing the entire larger site fed the details of the garden design.
Creatively use and respond to change
The change was that to the use of the existing Permaculture Interpretive Garden and the Organic Gardening and Introduction to Permaculture courses that the sustainability educator ran on-site for a decade. The program’s success stimulated the need for additional gardening space.
A little background: the Permaculture Interpretive Garden was used for the courses as well as by the Permabee community volunteers. Permabee was an initiative the educator started to reduce the social isolation experienced by local people by creating the opportunity for social contact and at the same time the opportunity for learning a little about horticulture by developing and maintaining the garden with a horticulturally-experienced permaculture educator. The garden was also used by the Australian Conservation Volunteers’ exercise and health-through-gardening program for older people.
The permaculture principle of acting on feedback, which is derived by applying the observe and interact principle—commonly stated as apply self-regulation and accept feedback—informs the ongoing use and maintanance of the Training Garden.
What would be done differently?
The self-watering training garden has been in use for several years. What would be done differently next time? Not much. It seems to work well.
Wicking gardens are container gardens. There are a variety of ways in which the principles behind how they operate can be implemented.
In this video on a Randwick City Council YouTube link, permaculture educator and landscape architect, Steve Batley, takes us through making a self-watering garden in a half-wine barrel.
The graphic describing Steve’s process is inside Council’s Barrett House premises in Randwick. I don’t have a credit for the artist.