Design guidelines for street verge gardens
Story and photos by Russ Grayson, April 2010
This guide to edible street verge planting has been pulled together from discussions with local government staff, sustainability educators, and street verge gardeners.
FARMING THE FOOTPATH—it’s been going on for some time in our cities but the last few years have brought an upsurge of interest. It’s one of those ideas that is now capturing the public imagination and we are starting to see more and more street verge gardens, many of them growing food.
Most verge plantings have so far been created by gardeners who know what they are doing, but the recent burst of popularity suggests that a little thought before acting might be a good thing. There is a concern in local government, which is responsible for public footpaths, that street verge gardens might be planted to inappropriate species and could interfere with underground services such as water, gas, and sewage pipes or block easy access to and from the street. There are design solutions to these reservations.
It can be confusing for local government when they are approached by people wanting to make a verge garden or who have already turned their nature strip to citrus and cabbage, nuts or natives. Rather than think how this could be done well, there have been incidents where councils have ordered the removal of verge gardens or removed them themselves.
However, for councils willing to creatively engage with citizens in this new use of public land, a little design thinking can ensure that planted street verges—edible and otherwise—are made to a high standard of safety, access, and finish. Where councils decide to go with the flow of public interest and enable street verge plantings, the publication of a set of design and planting guidelines can be a great help.
This article introduces current thinking on street verge gardening.
An established practice
Street verge gardening is the practice of growing ornamental, native or edible plants on the footpath. The rise in popularity of edible gardens has brought the planting of fruits, herbs and vegetables, sometimes mixed with flowers and native plants, to our street verges. The practice is another means of returning food production to our cities and is attracting attention and support in professional design circles.
Edible verge gardening in Australian cities can be traced back to the days of mass immigration in the 1950s, especially to immigrants from Mediterranean countries. Take a walk around the suburbs where the immigrants of that time made their homes and you find the olive trees they planted on the footpaths are now fully grown and laden with fruit in season. In older parts of sydney, the loquat with its bight yellow fruit is occasionally found on footpaths, but more commonly in gardens, however this is not so good a choice as it attracts fruit fly. This is another consideration in selecting fruits for the street verge.
Unknowingly, some councils have made their own contribution to edible streetscapes. Take a walk along a certain street in Stanmore, in Sydney’s Inner West, and you encounter the Australian bush food tree, the Illawarra Plum (Podocarpus elatus). This strange, plum-red fruit with its seed on the outside can be picked and eaten raw or made into a sauce by those with culinary savvy. Walk down a particular street in Windsor, Brisbane, and you encounter another Australian bushfood serving as a verge planting, the macadamia nut. Then there are numerous species of lillypilly, the Syzygiums, that have been established as street trees and that yield edible fruit. Some of these species are to be found in city parks too, a fact not lost on gleaners.
Councils take a proactive approach
Some local governments have taken a procactive approach. Recognising that citizens want to beautify their neighbourhoods and to turn poorly used land, such as the grassy strips along the footpath to productive use, a number of councils have written the opportunity for street verge gardening into policy.
Where a number of households on a street are involved, the City of Sydney covers verge gardening within its Community Garden Policy. In 2011, the City incorporated the Myrtle Street, Chippendale, street verge gardens into its Sustainable Streets Demonstration Project when it decided to support a trial of verge gardening, community composting, Michael Mobbs’ retrofitted sustainable demonstration house, and other local initiatives already underway among local people.
The verges of Myrtle Street, which is dominated by two-level Victorian era terrace houses, has been planted to a mix of fruit trees, vegetables, natives, and ornamentals. Early 2011 saw local people enjoy their first harvest of green pawpaw which they grated into a salad. Parts of adjacent streets have been planted and several espaliered citrus grow along a trellis in Peace Park at the end of Myrtle Street. The community composting trial in the park is being restarted, monitored, and evaluated, a maintenance plan developed, and workshops offered to local people in managing the system. The community compost supplies fertiliser to the street verge gardens.
Other councils have a verge garden approval process that requires gardeners to submit a plan for their garden and levies a charge for considering the proposal.
Council staff and local people may find justification for verge gardening in local government city, environment and urban greening plans. People planning to approach councils for permission to farm their street verge would do well to research these plans and to make the link to them in their application, pointing out how their verge plantings would implement aspects of the planning documents. Linkages might include:
- opportunities in neigbourhood beautification
- increasing biodiversity
- food security
- urban regreening
- visual amenity
- global warming amelioration through carbon sequestration in garden soils
- reduction of the urban heat island effect that raises air temperature in cities
- developing social capital and civic engagement.
Food security and council planting policy
The global food price rises of recent years, the food crisis of 2007-2008 and recent natural disasters have highlighted the value of cities retaining a high food production capacity on their rural fringe and within the suburbs. Community food gardens are a response to this as would be edible street verges though the volume of food that verges could produce is limited.
Urban agriculture and food security advocates now propose that those councils that have urban greening plans for the establishment of street trees consider fruit and nut trees as part of those plans. Nut and edible fruit species as street and park trees would provide the same environmental services as other street trees, including native species, in terms of shading, biodiversity, air filtration and visual amenity but, unlike commonly planted street and park trees, they also provide an edible yield.
These people are not suggesting mass planting of edible species all over the city, as councils seldom have horticultural staff to maintain the plantings, management for which would rely on community organistions. They propose that where there is support that edibles be established as street trees and shrubs. An example is the citrus planted between eucalypts on the street verge adjacent to Glandore Community Centre in Adelaide, the Myrtle Street plantings in inner Sydney and the community nut trees established in Totnes in the UK. In Adelaide, the citrus provide an understorey to the taller eucalypts in a linear mimicry of the natural forest. The association of both species provides a pleasant and productive streetscape and contributes to a varied urban canopy.
Food security policy is being considered by few local governments in Australia at present although South Sydney City Council introduced what is believed to be Australia’s first in 1997 and the City of Maryibyrnong in Victoria has produced a policy. Although only limited amounts of food could be produced on the verge, it has the potential to form a supplement to family diets.
The beneficial functions of verge gardens
Let’s turn now to the functions of kerbside gardens. Functions describe the indirect benefits of the plantings. They do not refer to their direct value of the plantings to people, such as their yield of food.
The critical question here is this: How can we maintain and increase, where appropriate, the beneficial functions of our verge planting?
Function 1: Provision of environmental services
Like any ecosystem, that of an edible plant association established in a verge garden provides the environmental services commonly associated with plants:
- filtration of air
- shading of footpaths in summer (and access to sun in winter where deciduous species are selected)
- shading of footpaths and streets to reduce the urban heat island effect that raises neighbourhood air temperature in summer
- slowing of rainfall runoff and assisting it infiltrate as soil water rather than be lost to the stormwater drain, thus obtaining a use from it before it returns to the water cycle
- provision of habitat for insects, birds and small reptiles
- carbon sequestration in organically-rich soils.
This requires establishing a diversity of plant types.
Funciton 2: Making productive use of urban land
Kerbside gardening makes productive use of land in the city. It puts to practical use small patches of land that are otherwise neglected or planted to simplified plant communities—such as lawn verges—that are unproductive or that may consume excessive water and fossil fuels in their maintenance. This is an important point for councils seeking to reduce their carbon footprint and verge gardens offer a ready solution where there is interest in creating them.
Edible kerbside plantings value urban land more than alternative uses because they are multifunctional.
Function 3: Boosting biodiversity
As mixed edible plantings, verge gardens attract insects and other small animal species that interact through food webs. This is the basis of their biodiversity value. Flowering species attract bees, providing habitat for pollinators in the city.
Biodiversity functions can be enhanced where open pollinated, non-hybrid vegetable and herb species or rare varieties of fruit tree are established. These can become a seed source for distribution to other gardeners, spreading the availability of species that make up our agricultural biodiversity, a type of biodiversity as threatened as that of native species, if not more so.
The gardens can contribute to the preservation of the biodiversity of non-edible species such as local native or heritage exotic plants where these are included in plantings. Verge gardens can blend edibles, natives and exotics.
Function 4: New ways to engage with public space
A further function of kerbside plantings is less to do with plants and more to do with people. It is this: taking responsibility for a kerbside garden provides a new means for people to engage with public space. It is a means of assuming greater responsibility for a neighbourhood and encourages the role of ‘engaged citizen’.
Public space is often viewed as the sole responsibility of local government. Citizens make minimal use of the space and often feel no responsibility for its care even though some councils expect people to mow the verge on their property boundary. Thus, local government adopts a managerial attitude as a service provider and sees little potential for a public role in open space management. Engaging with the interested public in working out how to make verge gardens work well provides a more modern and participatory approach.
It is in this sense that the gardens enhance citizen and community engagement with public lands. Local government might choose to see this as developing the capacity of communities to take a more proactive role in public space management.
Function 5: Enhancing urban amenity
Urban amenity is the deriving of often intangible benefits from the built environment.
Kerbside food production increases the amenity of urban areas through the provision of:
- foods to supplement a household’s diet
- habitat and environmental services
- urban revegetation and the development of the urban tree canopy and understorey
- improved visual aesthetics of streetscapes
- improved food security for households and, if adopted on a larger scale, of the city.
Understanding council concerns
Street verge gardens are often spontaneous installations constructed without the approval of local government and often without the knowledge that councils might require notification of a proposal to plant the footpath and that their approval may be needed.
Advocates of the edible planting of public space would do well to understand the concerns of councils, for whom it can come down to a question of public safety and council liability for accidents. Councils, after all, are responsible for plantings in public places and for footpaths.
Discussing the topic of verge planting, a council officer mentioned the potential issue of fruit falling from trees onto parked cars, or of pedestrians slipping on fruit left lying on the footpath and injuring themselves.
This, of course, is already a risk with the seed pods, ornamental fruit and heavy, seasonal leaf fall of some ornamental street trees. Whether what falls from street trees is edible or not doesn’t change the risk much at all and it remains a consideration.
The question of maintaining and harvesting
Someone working in the parks section of a western Sydney council said that he is not opposed to planting edible street trees, the question is who maintains them? His suggestion was that councils could plant edible trees were they requested to do so by a community group prepared to care for and harvest them. He pointed out that most council grounds staff have no training in the maintenance of fruit and nut trees or skills such as pruning, pest management and harvesting.
The best solution is to glean the fruit and nuts before they fall. Gleaners are already at work in our cities with some harvesting unwanted fruit for exchange at food swaps, such as Melbourne’s and Adelaide’s Urban Orchard. There is also the potential for community organisations such as rare fruit enthusiasts and community permaculture associations to take on the voluntary jobs of maintaining the trees and collecting the harvest. Of course, as plantings on public land, anyone can harvest from edible street verge trees.
What about abandoned gardens?
The potential for gardeners to abandon their verge plantings is something that plays on council minds. What happens when the householder moves home, more than one council staffer has asked?
It’s a reasonable question because there is no guarantee that the new occupant will be interested in maintaining the verge garden. One solution would be for the departing resident to return the verge to lawn, and this is a solution favoured by some council staff.
The question refers only to verge gardens established by individual householders on the footpath immediately outside their property boundary. Where the verge garden is a community garden maintained by a team of local people the question is less relevant because such verge gardens are maintained collectively.
The realities of verge gardens
There are a few things the would-be verge cultivator might contemplate before turning the footpath turf. The items that follow are all drawn from experience and are worth thinking about.
Reality 1: Road verges are on public land and produce might be taken
A gardener in Sydney’s inner west who has long maintained an edible verge of low-maintenance vegetables, herbs and a solitary, dwarf orange tree watched over the months its one and only piece of fruit turned from green to bright orange… and then disappear.
That didn’t faze her—she had expected it and was prepared to share her abundant verge that otherwise would support only a biologically un-diverse monoculture of lawn. What the incident demonstrates is the reality that the street verge cultivator has no control over people seeing the produce as public property and has no property rights to what is grown on the verge. The verge is accessible to anyone and nothing can be enforced to stop the public helping themselves to what is grown there. The verge might be thought of as an extension of the home garden in planting terms, however it is not an extension of the home garden in legal terms because it is on public land.
Most verge gardeners are happy to share what they grow and expect that people will take some. Perhaps a little sign suggesting people take edible leaves or fruit when ripe but not pick the entire plant would go some way to minimising damage.
Some street verge gardeners regard their plantings as ‘forage gardens’ where people are free to take some of what is growing. In cases like this the street verge garden is regarded as edible landscaping.
Reality 2: Neighbours and passers-by may complain
Not everyone will like your turning footpath lawn into footpath food. They may complain to council about the presence of the garden or parts of it. Often, this stems from the shock of the new, the unorthodoxy of putting street verges to productive use.
One case I know of was a complaint about the clumping grass, Lomandra, overhanging a Sydney Inner West footpath. The householder was told by council to remove the plant. Yet, in Manly where I used to live, a householder had planted the verge to the native Malaleuca (tea tree) and some of the branches protruded at head height and blocked access to parked vehicles. It is a wonder that nobody complained about that. It would have been a proactive move to prune the offending branches.
Then there is the problem of the personal sense of aesthetics. What is a beautiful vergeside food garden to some is something inappropriate to others. Aesthetics, of course, is no basis for local government decisions on verge gardens because aesthetics allows no objective measure, however councils have to respond to complaints and they often lack any formal means by which people can appeal a decision, potentially putting those whose verge gardens are complained about at a disadvantage.
Reality 3: Your verge garden may be vandalised
This I experienced while living down by Botany Bay in Sydney’s southern suburbs. We had planted the area around the malaleuca street tree—it was a council planter that protruded into the roadway—with hardy herbs and a pineapple. Later rather than sooner, the pineapple started to fruit and this we watched as it got bigger and bigger… until, that is, a young boy with a cricket bat thought the pineapple fruit would make a fine cricket ball.
The theft of young fruit trees is something that occurs in community gardens and it happens in verge gardens too. Young fruit trees have disappeared from the Myrtle Street verge gardens.
Uncommon it might be, the possibility of vandalism is something verge gardeners have to live with.
Reality 4: Streets are dangerous places
Managing a verge garden could involve stepping out onto the street to access your planting. There are clear dangers here, especially if you are working with traffic-unaware children.
This is the sort of thing that arouses the interest of council occupational health and safety officers and although the risk of being hit by a vehicle may be small (most adults are traffic-aware and take care crossing the street) it is none-the-less a low level risk that should be kept in mind.
Design considerations for verge gardens
1. Not all street verges may be suitable
Like any garden, construction of a verge garden requires an initial site analysis to check that the plants would receive sufficient sunlight, not be damaged by strong, cold winter winds or the hot, desiccating winds of summer and whether soils would require improving by loosening and the addition of compost.
A necessary part of site analysis for street verge gardens is to assess drainage from the street and whether this would affect the garden by bringing in excessive loads of hydrocarbon contamination from spilled oil and other sources.
Testing for lead levels in the soil would be a good idea, as decades of leaded petrol use may have left excessive loads in the soil of older suburbs although lead has long ceased to be used in petrol in Australia. Lead accumulates in the body and can affect mental functioning.
Where these contaminants would present a difficulty for verge gardening, a container garden at least 500cm in height might be a solution for vegetable and herb cultivation and the growing of dwarf fruit trees as this would isolate their roots from the contaminated soil.
2. Design for pedestrian safety
One of the challenges that even supportive councils can be presented with is where street verge gardeners erect a low edge around their gardens. This can be a trip hazard and a potential source of injury claims against council. This type of edging is commonly made with timber boards or a single course of bricks to raise the garden above footpath level.
There may also be an administrative difficulty as a raised garden, even one raised a few centimetres above ground level by a low edge, may constitute a construction on public land and that could require planning permission.
The solution might be not to raise street verge gardens and leave them without an edge. This, however, leaves them open to grass invasion and the washing of mulch and the erosion of soil into the stormwater drain during rainy periods. This could be seen happening in a verge garden adjacent to a block of apartments on Gordon Avenue in Coogee where the bark chip was washed over the footpath and into the drain by rain runoff.
How do verge gardeners work around the issues of trip hazard and erosion of mulch and soil?
The example comes from Marrickville gardeners in inner urban Sydney whose garden serves as an example of thoughtful, good design.
3. Design for access to and from vehicles and the street
A verge garden that abuts the gutter may impede people getting into and out of their vehicles.
The need here is for sufficient space so that people:
- can access the street from the footpath
- can open a car door and easily get into and out of a car.
This is even more of an important consideration where those are aged people who cannot nimbly step around plantings or people who use a walking aid.
This is accomplished by:
- leaving access to the street on at least one side of the garden as a strip a minimum of 1.2 metres wide; this should be level, perhaps paved, so that people with wheelchairs or walking aids can negotiate it safely
- leave a sufficiently wide strip unplanted or left to lawn or bark chip mulch between the kerb and the streetside edge of your verge garden; this might difficult in inner urban areas where footpaths are narrow and some compromise may be needed.
4. Think before you dig
If you make a street verge garden above buried pipes or cables, what might happen when the utility company needs to service them? Your verge garden will go in the excavation of the trench to access the buried service.
Where there are buried services a solution might be to make a container garden high enough to be above trip hazard height to, perhaps, a minimum 500cm; designed well, these might be movable by a lifting vehicle so as the underground services can be accessed.
When planning a street verge garden, check to see if there are any underground services.
Find out about underground pipes and cables: Dial Before You Dig is a free, online information service on underground pipes and cables anywhere in Australia— http://www.1100.com.au Phone: 1100 during business hours.
5. Select species carefully
Herbs, vegetables and shrub fruits (such as berry fruits) are not the species in question here because of their low growth form and smaller root systems. Rather, it is trees that must be thoughtfully selected for kerbside planting, such as the fruits and nuts.
As well as horticultural considerations such as planting species that are suited to climate, the selection of edible fruit or nut trees should avoid those that:
- are known to have vigourous root systems that could could lift up paved footpath and road surfaces and create irregularities that could pose a hazard
- are likely to grow tall enough to contact and damage overhead cables
- are species known to have root systems that damage buried services, such as water, gas and sewer pipes.
It’s best to avoid planting thorny species on the verge. These could lead to complaints to council were someone to take objection to being pricked by a sharp thorn. This includes roses, thorny fruit trees and cactus.
6. Prune plants so that their foliage does not overhang the footpath
Here, what is suggested is the selection of appropriate plants and the pruning of those plants so that their branches and foliage so not protrude over the footpath at head height or below. Trees branching higher overhead may be useful for casting shade onto the walkway in the heat of summer.
As the trees grow, gardeners can prune the lower branches that could intrude over the footpath or road. This can be done while the trees are young so as to ‘lift’ the canopy and encourage branching higher above the ground.
Remember that parents push strollers carrying young children along the footpath and children ride scooters and bicycles along it. The last thing they want, quite reasonably, is for their children to by brushed in the face by overhanging foliage.
Overhanging and protruding foliage may also be a deterrent to aged people, especially those using walking aids.
7. The need for care and maintenance
Gardeners of public land such as street verges have a duty of care in maintaining their plantings so that they are safe, look good and do not become vectors for the spread of fruit, vegetable and other plant pests. They must maintain their plantings. Herbs and vegetables, fruits and nuts planted on the kerbside need as much care as those grown in a home gardens.
Care for kerbside planting includes:
- regular watering
- mulching, to reduce evaporative water loss from the soil and to reduce water consumption; ensure the mulch you lay will not be washed into the stormwater system where it could block drains and pipes
- the application of compost or other organic fertiliser to stimulate healthy growth; do not overapply as rain could wash excess nutrients into the stormwater system
- monitoring and treatment of insect pest or plant disease infestation
- pruning of trees and shrubs to prevent their encroaching on pedestrian access.
8. Think about aesthetics
Irrespective of the gardener’s attitude to aesthetics, verge gardens should look good. Gardens thought to look bad or untidy are likely generate complaints to council.
Concern about neatness and appearance, in some cases over-concern, is a social reality. It’s true that people project their personal sense of aesthetics onto others, however this is something verge gardeners have to live with. What is riskier is the likelyhood that council, if it intervenes, will have no objective criteria to assess aesthetics.
9. Start small, grow incrementally
Where you have a large area of verge, do not attempt to plant the entire area unless you are confident you can keep the entire garden planted and maintained.
Better to start small, consolidate the area you start in then move on in small steps, consolidating as you go. This way, through fully consolidating what you do in your small steps, you reduce maintenance needs because things have been done properly.
Taking a measured pace allows us to use observation to assess what is working or nor working as we go and to make timely adjustments.
A model verge garden
A raised verge garden been built by householders in Marrickville demonstrates the value of thoughtful design.
The garden demonstrates recommended design criteria:
- dimensions—length 3.35m; width 1.2m; height 0.45m; the height lifts the garden above trip hazard
- constructed of recycled hardwood planks; this is a durable material
- a layer of geotextile was placed in the bottom of the garden to prevent root invasion by the eucalypts at either side of the garden
- level access to the street from either side of raised garden with a lawn groundcover— 1.2m
- offset from kerb to streetside edge of raised garden—0.8m; this allows access to vehicles and the unimpeded opening of vehicle doors; it was pointed out that it would not be possible to get a mower into this space, however the grass could be managed by whippersnipper
- herbs and vegetables are grown in the garden in a compost-enriched soil.
Edible rain gardens – potential?
Local government constructs rain gardens to filter storm water from streets before the water flows through the stormwater system and into streams or into the ocean.
Rain gardens are found on street verges. Because the plants established in the gardens have to endure periods of moisture without suffering damage as well as long periods of dry soil, hardy native species adapted to local climatic conditions are usually planted. Rain gardens are connected to the stormwater drain as a way to deal with excess water during prolonged rainy periods when soils may become waterlogged.
Streetside rain gardens are generally unsuited to planting to edible species because of the pollutants washed into them from the street. The design principles of rain gardens, however, might be adapted to verge gardens growing edible and other species. A verge garden in Wilga Avenue, Dulwich Hill (Marrickville Council), built by local people, intercepts stormwater from adjacent houses for irrigation, and the Myrtle Street gardens do something similar. What is important in the design of this type of garden is to provide an outlet to the stormwater drain from the garden so that excess water has somewhere to flow to.
Rain gardens can be made as raised planters or be built in-ground. Depth is variable and is influenced by soil conditions, the space available, the presence of underground services and drainage of excess water into the storm water system.
Already a reality
Councils are not about to be over run with demands for kerbside gardens. They remain the province of the few enthusiasts. However, if present indications are correct and there is a growing interest in taking over the footpath nature strip to grow food and other plants, then the time may come when local government and community associations publish design and planting guidelines.
The sooner this happens, the better.
If you plan to retrofit the street verge as a community garden, be sure to consult widely along the street.
Expect some opposition as not all will want to see the street verge turned into garden. Avoid gardening the footpath outside the residences of those not interested.
Check with your council to see if it has any policy on verge gardening.
Find out if services such as sewage or water pipes, gas, electricity etc are located below the street verge. It may be best to avoid planting high value fruit or nut trees if there are services below as, some time, the verge (and your verge garden) may have to be excavated to do maintenance on the services.
In choosing plants, avoid :
- trees with root systems that could damage roads and footpaths
- plants that are thorny or spiky and that could injure pedestrians
- plants that grow tall enough to contact electricity and broadband cables
- plants that would overhang the footpath where they could interfere with pedestrians and children in strollers
- plants with toxic foliage, flowers, fruit or nuts.
Avoid raising low edges around verge gardens as they may become trip hazards. Use pruned, close planted, wiry shrubs such as rosemary to form living edges but do not allow the foliage to overhand the walkway. It may be better to make a raised verge garden at least 50cm in height rather than make low edges around a ground level verge garden.
Do not build up verge gardens around the trunks of street trees. This can introduce disease to the trees and weaken them. Better to make a verge garden between street trees and allow access to the street between the end of the verge garden and the street trees.
Plan for access to and from the street by aged people, those with walking aids, and disabled people in wheelchairs by leaving a minimum 1m either end of the verge garden bed. In inner urban areas with narrow footpaths and narrow property boundaries, such access at only one end of the bed should be enough. Access to the street should be flat and smooth and, possibly, paved. This is becoming more important with our aging population.
Offset the outer edge of the verge bed at least 600mm inward from the gutter so that people can open car doors and get into and out of vehicles easily. This might not leave much garden space where footpaths are narrow.
Maintain your verge garden as you would any other edible species garden. Water regularly, especially when plants are young and in the summer months. Add compost regularly to nourish the plants. Add mulch to reduce moisture loss by evaporation and to break down into organic matter to feed the plans.
If you plant fruit trees, anticipate theft. This already happens in community gardens. Grafted fruit trees are expensive and continued theft can de-motivate verge gardeners. If theft is persistent, consider anchoring the young fruit trees to some heavy, buried, difficult to move object with stainless steel cable such as is used to secure bicycles. Remember to loosen as needed to allow unrestricted trunk growth. Alternatively, grow low value trees such as pawpaw, tamarillo or babaco, according to climate.
If you have permission to divert stormwater through the garden as irrigation, ensure that excess water can flow into the stormwater drain. Once garden soils reach field capacity (full saturation), excess moisture, especially during rainy periods, will need somewhere to flow to. Otherwise, erosion, local flooding and other difficulties might occur.
If your verge garden is likely to be inundated by rainwater runoff from a busy street, such as where it is downslope of the road, consider the pollution effect on what you grow of runoff contaminated with hydrocarbons from oil on the road.
Clarification: The author is on the project control group for the Sustainable Streets Demonstration Project at City of Sydney and is the City’s community gardens and Landcare coordinator.