Open water is a unique element, unlike anything else you can put in the garden and you should have at least one water feature of some kind (and preferably more). You might even have a series of linked water features, starting at the house. This could begin with rainwater running off of the roof down rain chains into a basin, then trickling out into the pond (this gives you nice sound) and then through a rill to another pond, then to flow out of the pond and end as irrigation water for your crop plants.
Water brings life and increases biological diversity by providing a greater variety of habitats, such as ponds, marshes, bogs and more. This is particularly obvious in dry climates where it becomes a magnet for wildlife. Water attracts nearly everything: dragonflies, frogs, birds and many other creatures (I’ve often been buzzed by Hummingbirds while hand watering). One morning after accidentally running an overhead sprinkler overnight (this was before I invested in a timer) my vegetable garden was alive with dozens on Butterflies.
Water also has an instinctive appeal for humans, especially children. Of course they won’t be satisfied with just looking at water, they find it an irresistible plaything and will want to play with it for hours.
A pond is an attractive, interesting and productive element and really brings the garden to life. Every garden should have a pool of some kind, whether it is only a few square feet, or large enough for boating. The magic of a pond is so powerful that it will be beautiful, useful and productive even if it looks completely artificial. The benefits of a pond include:
· Ponds are so beautiful they automatically become a focal point. They have movement, reflected light, tranquility and a luxuriant growth of plants.
· Ponds have psychological value because humans instinctively like to be near water. Witness the child’s fascination with playing in water, or the calming effect of splashing water.
· There are more utilitarian values to ponds as they were once incorporated into kitchen gardens to keep ducks or fish (and as a source of irrigation water).
· I love ponds because they give me the opportunity to grow all kinds of fascinating new plants, especially edible water vegetables.
· Large ponds can be useful for fire prevention and suppression. Obviously a fire won’t burn where there is water, so a pond can be an important part of your defensible space. They also hold water in a convenient place for fire fighting. To be useful for this, a pond should hold at least 5000 gallons of water or more and should be close to the buildings it is to protect. It should also be accessible by a good road. There should also be a sump so water can be extracted easily. This should be free of any vegetation that may clog a pump.
· A pond can be an effective water storage system, so long as evaporation isn’t too high (deeper is better in this regard – though potentially more dangerous). For this it should be placed at the bottom of a catchment area (this could be a hillside, a roof or a clean paved driveway). Of course a fluctuating reservoir pond isn’t very attractive.
· A pond absorbs heat during the day and releases it at night and so affects the microclimate around it.
· In warm weather a large pond can be used for cooling off, or even swimming, without all the hassle involved in maintaining a swimming pool.
· Pond mud is an excellent fertilizer, as are the various pondweeds and algae.
· Ponds increase the diversity of life in the garden. Last spring I dug a pond and my daughter introduced some rescued tadpoles. This spring the garden is full of croaking frogs. They don’t care that it is completely artificial, they consider it their birthplace.
Problems with ponds
Water can be a hazard to very small children because it is so fascinating to them. They will play around with it until something unexpected happens and have been known to drown themselves in a few inches of water, or even a bucket, so be aware and take precautions (see below).
Water also attracts less welcome visitors such as raccoons (they love to mess about in it as much as children), deer (they like to drink it) and mosquitoes (they like to breed in it). In warm weather Mosquito larvae will appear in any still water that stands around for more than a few days. In such cases you may have to take preventive measures, such as adding Mosquito fish.
A pond comes a close third behind the internet and television as a time waster. I don’t need any more distractions, but my pond attracts me like nothing else in the garden. I often just sit and gaze at the water instead of getting anything done.
Don’t build a pond if you don’t like the sound of frogs. They can be astonishingly noisy on warm spring nights.
Ponds are addictive, so one probably won’t be enough.
Ponds as ecosystems
Many modern ponds are treated like swimming pools and have pumps, skimmers, aerators, sterilizers and biological filters (these need to run constantly so can be quite expensive.). However all this technology isn’t really necessary. If you use the right plants in the right numbers and maintain it regularly, your pond can remain clean and healthy all by itself.
Plants are the key to keeping a pond healthy naturally. The submerged oxygenators are particularly important because they release oxygen into the water (they also absorb soluble nutrients from the water). Floating plants are useful for removing the suspended nutrients that cause the algae growth that make ponds murky. All plants provide food and habitat for fish and other aquatic creatures.
A pond isn’t static, it goes through distinct stages as organisms get established and nature takes over (there is often a stage of excessive algal growth in the first year). Small ponds are particularly vulnerable to minor changes having big effects.
I built my first pond as a learning experience, with the idea that it would allow me to learn about how ponds work, while growing and accumulating some interesting new plants (and other pond life). It worked out even better than I expected (and really does attract children).
The pond site should ideally be in full sun (6 hours a day minimum) and sheltered from strong winds that can cause excessive evaporation. It should also be free of overhanging trees, which cast shade, drop leaves into the water and perhaps damage the pond liner with their roots. Avoid Bamboos too, as their newly emerging shoots can penetrate the liner. In hot climates a pond should have a mix of shade and sun to prevent it getting too hot.
The bottom of a slope is a good place for a pond, because that is where water would naturally accumulate (though beware of frost pockets). Slopes lend themselves to running water features, such as waterfalls and streams and you could have several ponds connected by a stream.
A pond is such an attractive feature it is usually placed fairly close to the house, where it can best be appreciated. A large pond creates a flat area in the garden, which can be visually similar to a lawn (though more interesting).
The pond should be in scale with the garden. A formal pond for a small garden might be as small as 2-3 feet in diameter. Bigger is generally better with ponds however and you might think of 8 feet as a minimum width and 3 feet as a minimum depth. Marginal water plants will spread out into deeper water and soon make the pond appear smaller. A natural pond in a very large garden could be big enough for swimming.
A pond could be composed of straight lines and have a formal feel, or it might be informal, with random curves to make it look like a natural pond. A very small pond doesn’t provide much scope for using your imagination, as there isn’t enough space for graceful curves. A large pond should have an irregular shoreline, with bays and promontories to create productive edge. It might also have at least one island (or a floating island platform) to create more edge and to provide a refuge for birds.
The larger the surface area of a pond, the greater the potential for heat gain and evaporation, so big ponds need to be deeper than shallow ones. The temperature of shallow ponds fluctuates too much for fish to be comfortable (warm water is not good for fish because it contains less oxygen). A deep pool also contains more water, so temperatures are more stable and there is less fluctuation in water level and quality. If your pond is fairly shallow you should have at least one deep area to give fish access to cool water in summer and provide a refuge from predators. This is also good place for deep-rooted aquatic plants.
The depth of a pond is usually determined by its size and the climate. In mild climates 18” is the minimum depth for a small pond (up to 50 square feet). In colder climates it should be at least 24-30 inches deep (this also works for a pond up to 300 square feet). If you want to keep fish such as Koi then it will need to be at least 36 inches deep (this is good for ponds over 300 square feet).
If you are growing plants in containers (usually plastic baskets) then the bottom of the pond should be level so the containers can sit flat (you vary the height of the plants by supporting them on bricks).
A pond should have an overflow (spillway) to channel excess water from rainfall to where you want it (perhaps to a bog garden, a watercress bed or a swale). This spillway only needs to be 12 inches wide and an inch or two lower than the rest of the edge You could also have a L shaped pipe as an outlet.
| Reducing the risk to children |
If you have a pond there is always the possibility of some small child drowning in it. Statistics show that children under 3 are most at risk and that drowning most often occurs in other peoples ponds (which brings potential liability to add to the nightmare). Fortunately you can reduce the risk considerably by careful design. To minimize the risk you should put your pond where it won’t be seen from most places (if children don’t know it’s there, it won’t attract them) It should be visible from the house however, so you can see if anyone is near it. You should have a secure fence around the pond to keep unsupervised children out (this is actually required by law in most places). This fence should be unobtrusive, so that it is a part of the garden rather than just a security fence. If well designed it may also help to keep raccoons from raiding your pond periodically. The pond should consist of a series of shallow steps or shelves, so if a child falls into the water they can easily scramble out (there should be no steeply sloping sides). The shallow water zone (2” – 8” deep) is the most productive area of the pond anyway (the edge effect again). Unfortunately this also helps raccoons to get at your fish (fill it with plants to help prevent this). Ponds are at risk from children to some extent. If you have children the edging and plants will have to be fairly robust or they will get loved to death.
In hot dry climates the water may evaporate so quickly the water level recedes visibly each day. It’s important that you have an adequate supply of water for your pond, so you can replace this regularly. If you have to re-fill your pond every few days and water is very limited, this might be a problem. In such cases you should evaluate whether you have enough water for a pond before you build it.
Ponds shouldn’t receive runoff directly from roads, driveways or chemically fertilized lawns, because they may contain pollutants such as heavy metals, organic chemicals and excessive amounts of fertilizers (which will lead to algae growth). Wetland areas such as Reed beds can help to purify the runoff water before it enters a pond, and can be an important buffer zone. Frogs are a great indicator of water quality, if you have frogs your water is good.
Building a pond
Digging the pond
Before you start digging make sure there are no buried utility lines (gas, electric, water) in the way. Also decide where to put the excavated soil (this should be separated into topsoil and subsoil). Most often it is placed around the pond to give some variation in height, but you might want it elsewhere. If you are creating a pond on a slope you may have to build a berm, which is a good use of the excavated soil. Make it fairly wide or it won’t look very natural.
Any pond, no matter how big, can be dug by hand, it just takes time. Mark out the outline of the pond with wood ashes or ground limestone and get digging. Dig for a half hour a day and it will eventually be done. Start by removing all of the topsoil and put it in one place, then dig out the subsoil. If the pond has several levels, you should dig out one level at a time, finishing each one completely before going down to the next. If you are renting a backhoe for other purposes, then by all means dig the pond at the same time.
Water is always level, so make sure the perimeter of the pond is level too, otherwise it can be visually disconcerting. Level the perimeter by laying a long straight 2 x 4 from side to side and put a level on it. In a very large pond put a stake in the ground in the center of the pond at the correct level and use a long 2 x 4 with a level to check the perimeter.
Ponds are sometimes lined with clay, concrete, rigid fiberglass or plastic, but the most common are flexible liners of rubber, EPDM or vinyl. Avoid polyethylene sheet, except for a temporary pond, as any part that are exposed to sunlight will disintegrate in a few months.
It is not a bad idea to put a liner under the liner to prevent it getting punctured. Commercial liners are available but old discarded carpet padding, available from carpet shops works well. You might also use sand, cardboard or newspaper.
|Estimating liner size The length of the liner should be the length of the hole plus twice the depth plus 2 feet. The width of the liner should be the width of the hole plus twice the depth plus 2 feet. For example if the hole is 8 feet wide x 12 feet long x 3 feet deep, the liner should be: Length 12 + 6 + 2 = 20 ft. Width 8 + 6 +2 = 16 ft.|
A large pond may require more than one piece of pond liner. Separate pieces are connected with special double sided tape. You have to do this very carefully though, otherwise it may leak
Lay the liner in the completed hole and weight it down at the edges with rocks. Try and keep the liner as wrinkle free as possible. Then simply start filling with water. The weight of water will pull the liner down into position better than you could. The edge of the liner is then pinned down with wire pegs (use old coat hangers) and cut to shape. Don’t trim the liner until the pond is filled with water and everything else is finished, you don’t want to cut it too small.
I’ve never had it happen but apparently gophers have been known to bite through liners. If you worry about this you should lay down gopher wire underneath the liner (this is another expense though).
Clay as a sealer
Bentonite clay can be used as a natural pond sealer as it swells when it gets wet, making a waterproof seal. You will need from 2 – 8 lb. of dry clay per square foot, depending upon the soil. You scatter this over the entire surface of the excavation and work it lightly into the soil. Then you fill the pond with water and hope. Gravel is sometimes put over the clay to protect it from waders.
For ultimate convenience you can now get clay blankets where the clay is sandwiched between layers of landscape fabric. For no convenience at all you could dig clay from your own land and used it for lining the pond.
|How much water? To find out how many gallons of water there are in your pond, time how long it takes to fill up with water. Then time how long it takes the same hose to fill a 5 gallon bucket. Finally divide the first number by the second one. If you are mathematically inclined, you can calculate the volume of the pond in cubic feet and then multiply by 7.5 to get the number of gallons.|
A well made concrete pond is tough, durable (you never have to worry about gophers), easy to clean without damage and fairly easy to repair. It is also quite a bit more work to build.
A rectangular concrete pond is fairly straightforward to make; you just build a form, add reinforcing wire and rebar and pour. You can even use concrete blocks as the form.
A free form concrete pond is a little more complex. You have to excavate to the required shape, then lay down a layer of 4 inch reinforcing wire mesh. This must be carefully shaped to follow the contours of the pond and raised 2” in the air with small blocks of concrete called dobies.
To pour a free form pond you put stakes into the ground at various intervals and mark them at 4 inch depth. As you place the concrete you fill to the 4 inch mark to ensure an even depth of concrete (you could also use a stick marked at 4 inches). Remove these stakes as you go and fill in the holes. The concrete should be stiff enough to stick to the sides of the pond. Start by filling the bottom and then work up the sides (work from a board spanning the pond if necessary). If any wire touches the soil or sticks out of the concrete bend it back in as needed. Finish by troweling the concrete to a smooth finish.
Edging is usually considered a separate process and is done once the pond is fully cured.
The main reason most garden ponds look so artificial is because there is a clear demarcation between garden and pond, usually a ring of stones. In a natural pond there is no such clear boundary, plants grow in shallow water, mud or moist soil, advancing and retreating as the water level fluctuates. This edge zone is a very productive area.
Until I built a pond I always though the ring of edging stones to be a result of a lack of imagination. Now I have built one I realize it is simply the easiest way to keep the water and soil separate when using a pond liner.
If you don’t like the stone edging you can disguise it in several ways. The simplest is to hide some of it with plants. You can also use more rocks and vary their size, putting some bigger ones on large shelves with the liner going up behind them, so the rock sits out in the water. Have these at various intervals, with plants and smaller rock in between.
The pond edge must be stable so the stones don’t get dislodged and fall into the pond. This could create a mess, not to mention being potentially hazardous. Mortar is commonly used to hold the edging stones in place, though densely rooted plants can also work.
The ring of stones doesn’t have to go all the way around the pond. You could have a bog area on one side, or you could create a beach of gravel (perhaps as part of the overflow). This would be a good place for children to play and provides easy access to the pond for small animals
As with other elements a little ornamentation can turn an interesting pool into a garden jewel. You could use colored glass, tiles, sculptures and more.
If your pond is big enough for an island you can make one out of a pile of sandbags. Fill the lower sandbags with sand (it doesn’t leach nutrients into the water) and the top bags with not very fertile soil. Then just pile them up so the top is just above the water surface and plant with marginal plants. The sandbags will rot in a few years, but by then plant roots should bind the whole thing together.
Coarse sand or gravel may be put into the pond to hide the liner and make it look more natural. This also makes it easier for plants to get rooted. Don’t use soil though, as it contains too many nutrients and would stimulate the growth of algae. It would also make the water muddy every time it gets stirred up.
As I mentioned earlier a fence is often legally required for safety. The best way to do this is to fence the whole pond garden room.
Any kind of fish will help to bring your pond to life. Even the small, rather drab Mosquito Fish are beautiful and fascinating to watch as groups of them weave around the pool. You may want to introduce larger, more interesting fish species too. Goldfish will eat insect larvae, algae and some plants and don’t require much attention. They can live up to 12 years and grow to 16” in length. The best deal on fish are the feeder fish available for 19 cents each in your local pet store.
Having bigger fish means extra work to meet their needs. They need oxygen which can be provided by a waterfall or fountain (solar powered ideally). Dead leaves should be kept out of the water as their decomposition can consume oxygen. Fish also prefer some shade, so water plants should cover some of the water surface.
Bigger fish will attract the attention of raccoons, cats and herons, so will need a deep water refuge from predators. A simple refuge is a section of drainpipe in the deepest part of the pond. Terra-cotta work best, though plastic would also work if weighed down.
You could also cover the entire surface of the pond with black plastic netting (this also keeps leaves out), though this isn’t very attractive. You might also have a more ornamental grid of bamboo or metal (this can work quite well in a formal pool).
If you have a really big pond and are motivated enough you could try growing fish to eat (grow your own sushi). As this involves caring for live animals it is a lot more work than raising plants (and beyond my area of expertise).
In warm weather any standing water will become a Mosquito breeding area. Within a couple of weeks of filling my new pond it was teeming with clouds of wriggling black larvae. The Mosquito control officer brought me some Mosquito Fish and they consumed them all in short order. I can’t imagine how they managed to eat them all, or what they’ve been eating since. These fish can reproduce very quickly if conditions are right, though many die off over the winter.
It’s said that Mosquito Fern (Azolla filiformis) can cover the surface so completely that mosquitoes won’t breed in it. Of course you then have something that looks more like a lawn than a pond.
Water loving dogs don’t mix well with ornamental ponds, especially those with fish (some dogs will catch slow tame fish).
Raccoons have been nuisance in my pond, even without any large edible fish. They think it was made for them and wade around, knocking over any container plants that aren’t firmly fixed in place (it’s fortunate that water plants are so resilient). I can’t imagine the damage they would do if they were pursuing fish as well.
Planting the pond
For me one of the biggest attraction of ponds is the unique growing conditions they provide. They give you an opportunity to try out some entirely new kinds of plants: submerged plants, free floating plants, emergent plants, marginal plants, bog plants and other moisture lovers.
The aquatics are some of the easiest plants to deal with. They never need watering, they transplant very easily (with some plants its just a matter of physically moving them) and many root readily from cuttings. The biggest problem is often controlling their excessive vigor.
Though water gardens are usually planted for ornamental purposes, there are a lot of useful aquatic plants. In fact some of the most useful of all edible plants are adapted to this habitat. See Water Plants for more on these.
Local conditions will dictate what species can and cannot be planted. Look around to see what grows locally before you attempt to introduce anything new to the pond. Also be careful what you introduce, as a number of aquatic plants have become pests when introduced into alien environments. Some (such as Water Spinach and Water Hyacinth) are actually illegal in areas where they can survive the winter.
As with any other garden design you need to ensure that the plants you choose are the right size for the growing conditions (in this case the pond). In warm weather many aquatics grow very quickly as they always have an abundance of water. Some can spread relentlessly and threaten to take over the entire pond. You could always create a larger pond at a later date to accommodate them, or give them their own water garden.
Most aquatic plants prefer fairly neutral water, not too acidic or alkaline and full sun (though many will take light shade). They also need adequate nutrients if they are to maintain their rapid growth.
Most ornamental pond gardeners grow their plants in containers because they are easier to maintain (you just take the container out of the water). Dividing a plant in a container is a lot easier than dividing one that is rooted under two feet of water. This applies even more to plants with edible roots; you can just tip them out of the container on to the ground and pick out the roots. Using containers also makes it easy to keep aggressive plants under control.
The best container for aquatic plants is a wide plastic basket, which allows roots to grow freely and water to get into the growing medium. Water plants don’t do well in conventional plastic pots because there is little exchange of water and gases between the soil in the pot and the pond. This lack of oxygen can encourage anaerobic bacteria, which give off gases that are toxic to plant roots. As these gases build up, the plants cease to grow well and may even start to decline. You can smell this if you take the plant out of its pot.
Fill the baskets with a sifted, not-too-fertile soil (I’m too cheap to use the specially formulated commercial mixes). Put the plants in the container and then cover the soil surface with a layer of gravel to hold it in place. Put the basket on bricks or flat rocks to bring it up to the desired depth from the surface.
It is also possible to use woven plastic mesh bags for planting (or squares of this material tied up in a bundle). These are good for edible tuberous plants that do better in soil, such as Arrowhead. You can also plant into sausages made from the legs of a pair of tights.
Some creeping pond plants do better if they able to root into a substrate of gravel, sand or subsoil. This allows them to creep as they like. To plant into the bottom of a pond you can put a plant in a square of burlap filled with a suitable soil mix. Fold up the edges of the burlap and tie together to make a small package. Soak the bag in water to exclude air before putting in the pond, so it will sink. The roots will come out of the bag and root into the gravel, while the burlap will slowly rot.
Water plants have no limits to growth in that they get all of the water, nutrients and light they need. They may just keep growing until it gets too cold or they run out of room, which means they have the potential for getting out of control. If they get too numerous you will have to start removing excess plants. The floaters such as Water Hyacinth, Parrots feather and Azolla are particularly inclined to do this if given the opportunity. The best way to control them is by harvesting, either for food, mulch, green manure or compost.
You can prevent the spread of marginal plants by varying the depth of the water. Plants that grow in marshy conditions can’t grow in deeper water.
Algae is a common problem in new ponds, when there are too many nutrients in the water and too much sunlight on the surface (the more the surface is shaded by plants the less algae you will have). Algae is unsightly and can clog fountains and pumps, but it isn’t necessarily bad. It provides oxygen just like other submerged aquatics. It only really becomes a problem if it decays in the pond as it then consumes oxygen, so remove any excess algae and use it as fertilizer.
The best way to remove excess algae is to wind it up on a forked stick. Leave the algae by the pond so any creatures caught inside can get back to the water. You can also use Barley straw as an algaecide (you can buy this in the garden center for $8.00 a bag – or you could save a bit of a winter cover crop). Apparently this can’t be used if you have fish though.
Planting around the pond
A pond shouldn’t exist in isolation and it’s important to pay attention to the land around it. Have a diversity of habitats surrounding the pond, each merging into the pond (and creating a variety of edge effects). An overflow can give you an area of wet soil adjacent to the pond (especially if you bury plastic to impede drainage) which can become a useful bog habitat.
The emergent plants around the margins of the pond can help protect it from pollution by absorbing nitrates and phosphates (and even killing pathogenic bacteria) before they enter the pond. Such plants need to be in a band at least 6 feet wide to have any effect on a large scale. The same plants can also be used for treating gray water.
A pond is one of the higher maintenance garden elements and even after it is filled and planted you
still have to experiment and refine it to make it work well. Fortunately this isn’t really work, in fact it is the kind of job your children (or grandchildren) may do willingly.
The most important aspect of maintenance is keeping the water clear (though this is by no means essential, I’ve seen some really cool ponds where the water looked like green pea soup). One of the commonest reasons for excessive algal growth (and murky water) is using garden soil in your planting containers. This contains a lot of nutrients and these leach into the water and stimulate algae growth (you will often see algae growing right onto the soil).
Water quality can be improved by growing submerged and floating plants, as they will absorb most of the nutrients in the water. There is a fine line here however, as too few suspended nutrients will means poor plant growth. It may sometimes even be necessary to add fertilizers to increase growth. If you are growing edible plants you won’t want to add any kind of fertilizer that might contain pathogens (such as manure tea).
Pond plants can grow so rapidly in the right conditions that there is always the potential for one or more to get out of hand and threaten to fill the whole pond. Don’t panic if this happens, just remove a proportion of the plants periodically and use them for mulch or compost. They are full of plant nutrients (Mosquito Fern even fixes nitrogen). In my pond the algae disappeared almost by magic in the spring of its second year. Ultimately you are always in control of what happens, because as a last resort you could empty the pond.
The working pond
A garden pond is not a natural pond, no matter how much you try and disguise it. To me it makes sense to concentrate on making your pond work as an efficient water plant growing system and not worry about its appearance too much (water and plants are beautiful no matter what you do). You might use a series of stepped concrete boxes, to create areas of even depth for planting with minimal shifting or tipping. This could have parts sectioned off for growing specific plants, as well as bog areas, deep water areas and more. There could even be channels or canals to take water to separate areas, perhaps ending in a sump at the bottom, from which irrigation water could be drawn.
The food pond is a pond specifically designed for growing edible aquatic plants. It is one of the best examples of edible landscaping in that it has all of the ornamental value of a pond, but can also produce a lot of food. Growing water vegetables can be quite fascinating and gives you another opportunity to increase your production of food with relatively little work.
A surprising number of the plants we grow as aquatic ornamentals are actually edible. In Asia a number of these are important vegetable crops and are widely grown in small ponds. These include Arrowhead, Lotus, Taro and Water Spinach.
Water vegetables can be useful as salad materials through the hotter summer months, when many traditional salad plants don’t do well. This is almost foolproof vegetable gardening; as long as there is water in the pond they will grow.
The patio is a good place for a food pond, not because the pond needs much attention, but because it is such an interesting feature. This also makes it more convenient for harvesting.
Water vegetables can be grown in anything that will hold water, from a half barrel to a childs’ paddling pool to a custom made concrete pond. A plastic paddling pool is one of the quickest and easiest ways to get started, if not the most attractive. This can be set up almost anywhere and could even be taken into the greenhouse for the winter (drain the water out before moving it).
You can create a pond bed for the vegetable garden without any digging, simply by using a 2 x 4 frame and a sheet of pond liner. Make sure this is perfectly flat so it will hold an equal depth of water all over. It doesn’t need to be very deep as you only need a couple of inches of water to grow the plants. The water should be able to flow so it doesn’t go stagnant. This means having an inflow (from a hose) and an outflow (exiting water can be used for irrigation).
If you get really serious about this you could make shallow concrete pond beds.