Vernal pond/crater garden

Hi Zach,

Thanks so much for taking time to answer questions for us. I have created a smallish ovular 'crater garden' type hole in a suburban lot. It is about 15 feet wide and 25 feet long, and around 6 feet deep at the very bottom. I live in a mediterranean climate so our winters are wet (supposedly- major drought this year) and our summers are dry. The crater garden is at the base of a large hillside, so during heavy rainstorms it fills quickly once the hillside is saturated and starts shedding water. The native soil is very high in clay, so it holds water fairly well, though I haven't done anything in particular to seal it.

Are crater gardens meant to ever fill up with water completely like this? Last year the crater garden was about half the size, and was really just a retention basin. It drained completely by around May and I didn't do anything to it until this winter when we were able do increase its size. I have read that earthworks are all supposed to be somewhat leaky, that there isn't really such thing as a completely water tight natural pond. Is this the case? Or should I really expect to be able to create something natural at the bottom of the crater garden that will hold water year round? This year I decided to bury the liner of a hot tub to the very bottom of the crater garden/retention basin. It is in the middle and holds about 300 gallons of water. In this way I'm hoping to keep a water ecosystem year round at the bottom of the garden so that when the entire garden floods again the following winter I will have an ecosystem in place ready to colonize the new water to prevent mosquitoes from breeding. Then in the summer when the majority of the crater garden is dry I'm planning to use it as a sun trap and fill it with annuals such as melons/sunflowers, etc?

Any thoughts? I will try and get some photos to post.

Did you listen to Paul's podcast about the earthworks in San Diego? It sounds like this pond eventually drained out during the summer as well. If anyone reading this is familiar with that project I'm curious to know if they intend to find a way to keep water in that space year round, or if the best thing to do is to let the water just gently leak out?

In a small pond like this that isn't regularly fed by a stream what would you recommend to keep the water alive/oxygenated? A small solar bubbler/fountain? Reeds/aquatic plants?

Zach- do you have an photos of crater gardens in action? I've just seen pictures of recently constructed ones that have bare soil.

Cheers! I'm reading a copy of Desert or Paradise from the library right now and would love to have a copy to keep on my coffee table to indoctrinate visitors with. Thanks again for joining us on the forums this week!

Hi Luke - your post caught my eye so I thought I'd try answering some of your questions - I would take these answers as thoughts on the subject, rather than edicts, btw...

"Are crater gardens meant to ever fill up with water completely like this?"
By whom? I won't speak to what Sepp wants out of them, but I thought I'd let you know that before the Central Valley of California was flattened by Green Revolutionists, it was pocked all over with such things. In the natural world, they are called vernal pools. They are basically a tragically endangered habitat that provides for a very unique variety of native plants. We can emulate them ("crater gardens" ), and put plants of our own choosing in place of the natives. As such, an "artificial vernal pool" would certainly hold water for a time, and would exhibit concentric rings of different plant species, the rings shaped by the differing water needs of the plants used.

An interesting idea might be to dig a REALLY DEEP hole, then fill a lot of it with wood, then top it with soil. So, you have a concave hugelkultur bed. I think this might be of use in very dry environments.

"I have read that earthworks are all supposed to be somewhat leaky, that there isn't really such thing as a completely water tight natural pond. Is this the case?"
"supposed" is really a matter of who wants what. However, the short answer is yes, this is the case. Even if you did manage to completely seal the UNDERSIDE of a pond, it would be tricky to completely seal the top!

"Or should I really expect to be able to create something natural at the bottom of the crater garden that will hold water year round?"
In a Med environment, where for most of the year, evaporation is greater by far than precipitation, creating a pond (small year-round body of water) is difficult and uses up a lot of water. But ponds are pleasant; one possibility is to seal a small portion of the very bottom of a depression; you could end up with a "vernal pool" of sorts, that had a lot of space that was seasonally flooded, with a small area in the middle that always stayed wet. Again, though; maintaining water exposed to air in a Med environment can be water-expensive.

"This year I decided to bury the liner of a hot tub to the very bottom of the crater garden/retention basin. It is in the middle and holds about 300 gallons of water. In this way I'm hoping to keep a water ecosystem year round at the bottom of the garden so that when the entire garden floods again the following winter I will have an ecosystem in place ready to colonize the new water to prevent mosquitoes from breeding. Then in the summer when the majority of the crater garden is dry I'm planning to use it as a sun trap and fill it with annuals such as melons/sunflowers, etc?"
This sounds like a pretty cool idea, to me - clever way to deal with the mosquito issue, & a small water area like that, with depth & not as much breadth, will cost you less water to maintain in the dry season. Really, yes, I am excited about this idea.

" Did you listen to Paul's podcast about the earthworks in San Diego? It sounds like this pond eventually drained out during the summer as well. If anyone reading this is familiar with that project I'm curious to know if they intend to find a way to keep water in that space year round, or if the best thing to do is to let the water just gently leak out?"
I was in the workshop, but I can't recall if the desire was that the pond portion hold water all year. I - yes, it WAS; I recall we spoke a lot about sealing, and there was tractor work compacting the inside. As I mentioned, though; keeping water exposed to air in habitats where evap > precip is fundamentally wasteful (er, unless you are going to set up a water-based food system the production of which justifies the water use...) - Personally, I don't remember getting the impression that there was a plan to make seriously productive use of the standing water; I think it was mostly a matter of how we most all of us love to have water bodies around. I may be remembering this wrong, however. Also personally, just for the sake of sharing a slightly different perspective on the installation, I would have wanted a pond, too, and expected that I might have to give that up, and also, I would have preferred the pond to be more central to the property, rather than right by the road. So I would have put the swale in off the road, & the pond at the end of the swale. I see a few benefits to this: If it worked out as a pond, it would be visible from several points on the property. It would draw in animals to the center of the property as well - I like animals! The center of the property was also a shallow ridge, so I would have created a situation that soaks more water into the soil, right on the potentially driest portion of the property (that I could get the water to; elevation is an issue... as is where the pond is placed relative to other structures - in case the wall blows out!).
but I digress. I also think that as time passes, the clay in the base of the pond as is may soak, swell, & settle more, & biotic material will be growing & rotting in the water... I think it will be better sealed in a few years than it is now; it may well end up holding water a lot longer, with minimal input. Finally, when it comes to conserving water in such a dry environment, letting it soak through the pond wall into the surrounding soil is great.

"In a small pond like this that isn't regularly fed by a stream what would you recommend to keep the water alive/oxygenated? A small solar bubbler/fountain? Reeds/aquatic plants? "
All good ideas. The problem with a solar fountain is it doesn't work at night, so you need one that can run via battery at night. Likewise, plants tend to release less oxygen at night, so free O2 can drop seriously at night. Of course, if you are low on fish et. al., this may not be a problem...

Really just scratching the surface here; I hope a few bits of this are helpful...

Hi Luke,

It sounds like you've gotten off to a great start on your project; starting small, observing the results and improving and expanding the project as you go! Krater Gardens are indeed designed to store water from the wet season for the dry season. A vernal pool a very good natural example of this type of technique. How sealed the garden is, how much it fills up, how much water it stores, and for how long it has water are all ultimately up to the steward and their goals for the project. When possible Sepp tries to create aquaculture systems that hold water year round. Water is what all life comes from, the more water you can store the more productive your landscape will be. That said vernal pools also help store water for the dry season and are valuable and productive ecotones as well. Usually Krater Gardens have drier zones as well, so as to not completely inundate some of the growing areas. This provides ideal micro-climates for fruit trees and the like, species that can't tolerate complete inundation (all roots under water) but benefit from the extra soil moisture. The flood level during the wet season is determined by the elevation of your spillway. Once the water fills your retention area ithe excess will flow out the spillway. It is important to allow a substantial flood zone for your climatic features.

The way I look at it there are 3 ways that the water leaves the pond. The first is through the air (either evaporation or transpiration). The second is wicking through the earth's body. The third is leaks in the bottom of the pond. Leaks in dams should be avoided at all costs, this is a guaranteed catastrophe!

These are all beneficial to the landscape as a whole in one way or another; the idea is to store the water in the landscape so that it can be used for longer, never to separate the two entirely. Type one and two are the primary ways that water retention landscapes will benefit your growing areas. You want to encourage these both as much as possible. Type 3 on the other hand will lead to the water running away before it's time. Keep in mind this creates streams and in ground reservoirs down slope from you, but it won't benefit the growing areas right around the pond.

You want to encourage wicking from the water retention to the soil body as much as possible. It is the leaks that you want to avoid, particularly in cases of seasonal rainfall where you won't have a steady flow feeding the pond. This is the way that earthworks are meant to distribute the water. Liner ponds do not release their water into the surrounding landscape, eventually leading to stagnant water that can result in all kinds of other problems. You need the exchanges that happen between the water soil and plants to keep the water vital. It sounds like that with a decent size excavator you could do a better job of sealing the bottom of the krater, while also encouraging this water to wick through the surrounding soil.

If you are hoping to keep a year round aquaculture system adding some terraces to the slope and making sure there is both tree and ground cover would provide an enormous benefit. I can't say for sure that this would lead to a year round aquatic system, but at the very least it will extend how long the water lasts into the dry season. The flood zone is one of the most productive parts of these ecosystems, and provides a unique opportunity for cultivating plants that thrive in this kind of landscape. Using the drier areas to cultivate melons, sunflowers, etc is a great plan. When a Krater Garden has both drier zones and water retention it results in a drier wet season and a wetter dry season. It's one of the best ways I know to balance the extremes of climate for a more year round productivity.

I have listened to Paul's podcast about the earthworks in San Diego, but I was not there, so I really can't provide any of the answers to this question. Each situation, climate, and landscape is unique.

In small ponds that aren't regularly fed with a stream having some air exchange and a healthy diversity of plants is essential. Here is a picture of a Krater Garden we built this past spring/summer. It wasn't planted till mid summer, yet significantly out performed my expectations. I am very excited for this year, as it will receive all of the spring moisture and the perennials have now had some time to establish their roots. This past year we actually had to pump a huge volume of water out of the Krater Garden in order to be able to finish it. The HugelKultur in the bottom didn't have time to absorb any moisture due to this. This year we will be able to keep all of that water. I will be eventually start a thread on this project and update it throughout the year. It is also important that this Krater Garden is in a climate that is considered "extremely difficult" to build a pond in, with twice as much evaporation as precipitation each year.


After Completion, Mulched and Annuals Starting to Sprout


About 1 Month Later, No Rain Since Completion

In this garden there is no spring or anything like that feeding the garden, it is only fed by the rainwater from the site. We have a LONG dry season and very little overall rainfall. Every year at the right time there are severe fires all around and the air is thick with smoke. I was very concerned about the apparent lack of oxygen that this water would receive, yet it always remained a productive aquatic system full of insects and aquatic plants. There are wind powered water aerators made for ponds, I have a design for a more natural, passive, and cheaper aeration system that I plan to implement this summer.

Is there a good source to learn more about crater gardens? There doesn't seem to be much information published about them. I feel like I have a general understanding of them, but I tend to like to study up on things, so if there are good sites or resources which highlight them, it would be appreciated if you could direct me toward them.

The garden you show is quite beautiful and almost calming in it's fluidity of design. It will be interesting to see it as it develops and progresses through various stages. Thank you, Zach, for all of the effort in explanations you give and for being so open to sharing what you know so that we all might better succeed. Your passion is apparent and infectious too!

I love the flow created by paths mounds & terraces there Zach. I find the shapes soothing. Cover seems to be coming in well; do you have an idea how much water was run through the upslope irrigation system during initial establishment? Where is this project?

Speaking of upslope, that's something I've been wondering about re ponds / Krater gardens: I was thinking of Sepp's spring terrace design & it occurred to me that even in arid environments, a pond established at the bottom of well-managed slope might pick up water from underground flow from upslope, which would help to stabilize the above-ground water level. Nifty.

Another question: when you say the hugelkultur in the bottom didn't have time to pick up water - are you referring to the mounds around the pond (I'm guessing they are hugelkulur mounds), or was wood actually buried under the pond area?

Thanks Zach and others for the responses. When I have access to a better camera I will post some updates. I really love the idea of using earthworks to create wetter areas during the dry season and dryer areas during the wet season. The krater garden we built has a raised berm that is about 2 -3 feet higher than the spillway and has trees planted in it. I'm in the bay area and have heavy clay soils. The tendency is to worry about getting enough moisture but I've heard from many fruit tree growers around here that the majority of home orchard trees which bite the dust actually die from getting water logged during one of our very wet winters which happen on occasion. By creating even a very small berm the tree's most vulnerable roots can be protected from drowning. The krater garden is a perfect solution. Not only are the trees protected from inundation but the moisture from the depressed 'pond' area is still retained and seems to be carried against gravity- wicking up into the berms as the trees need it.

I'm eager for suggestions about productive plants for the areas in the krater garden that flood seasonally, yet are very dry during the summer. One thought is to focus primarily on annuals in this soil. Should I lightly dig the flooded terraces after they start to dry out and prepare them as an annual bed. Are there good perennials that will tolerate seasonal flooding and long dry summers?

I'm eager to see your updates later this season Zach! Thanks again for the thorough response.

I'd love to see more pictures of other peoples' krater gardens.

Also, for those who are curious: my krater garden filled completely shortly after we built it, thanks to a 4 inch rain event over the course of 3 days. The 'pond' area filled up very slowly, but then in the last 1/2 inch of rain it filled up very rapidly as the nearby hillside became saturated and water swiftly rain down into my pond (the krater garden wouldn't have filled hardly at all if it didn't receive the influx from the hillside). Then over the following month it slowly released about half its water. A recent couple inches of rain filled it back up somewhat- it's the short, hard rainfalls that really fill it quickly because they provide the most runoff. Imagine if the suburbs were full of rain gardens. I used to love watching rain rush down the gutters on the street, but now it really pains me to know we're letting so much of the valuable water swiftly gush away when drought is just waiting around the corner.

Some updates: This past year I buried an old water tight hot tub liner at the bottom of our suburban crater garden. We are in the San Francisco Bay Area region and finally got some decent rains this past month. The pond/kratergarden/vernal pool (whatever you want to call it) slowly filled until we'd gotten about 4-5 inches of rain, then the runoff kicked in and it quickly filled up the rest in a matter of a few hours. I think the capacity is somewhere around 6000 gallons. If more people put these sorts of systems in, we would reduce runoff significantly. Right now during any decent sized rain the water treatment plants get overwhelmed and they wind up releasing untreated sewage into the bay. Gross. If during heavy rains most of the water was retained in the landscape this problem would be averted. Even better- people would be sending most of their grey water into their gardens/reed bed systems and using composting toilets so that we would hardly even need treatment plants.

Anyhow, I finally have gotten some photos together that I wanted to share. I hope to plant some squash/watermelon/etc this summer and let it take over the dry pond bed (the clay soil does a good job of keeping the water in during the rainy season, then it slowly wicks in to the surrounding landscape). This dry area should really cook during the summer since it's so well protected from winds. I plan to keep the hot tub in the bottom topped up with water as it slowly evaporates during the summer (I don't anticipate this requiring very much irrigation- I will track it though).

Anyone else have krater garden photos to share?

Pics from after the recent downpour.

Raleigh put together a nice video on the Crater Garden at Sage Mountain Center.