swale headaches

Hello Permies !

My wife and I started our half-hectare plot in Transylvania, Romania, about a year ago, and I'd like to share some of my experiences so far, particularly with regard to certain permaculture methods such as tree planting along swales.

The plot is on a gently-sloped south-west facing hill side. 100m long, 50m wide. The soil is heavy clay. The plot was sown with alfalfa (lucerne) 7 years ago, and when I bought the plot, the alfalfa was still being harvested 3-4 times a year. It is now in its 8th year and is dying back slowly in some parts of the plot, but still growing strongly in most parts.

The plan was to (a) plant a hedge of mixed species all around the property; (b) plant a "forest" of mainly chestnut and hazelnut in the upper 1/3 of the plot; (c) plant a mixed orchard in the mid 1/3; (d) the house, outbuildings etc. and kitchen garden in the lower 1/3. (B) and (c) were to be planted mainly along 4 contour swales. The interswale in the "orchard" section was to be used for annual, mainly staple, crops.

We have completed steps (a), (b) and (c) - planted several hundred trees last autumn, some more plantings are planned for October this year.

The way we went about it followed the permaculture recommendations for establishing tree systems:

- improve water retention in the soil with contour swales (we made 4 swales)
- vegetate the swales with nitrogen fixing legumes (this was not necessary as the legume - the alfalfa - came back in full force through the disturbed soil; however, we also sowed some clover, with limited success)
- plant trees immediately along the swale berm - done in October last year

The whole thing looked great after the planting, and looked extremely promising in April / May of this year. Nearly 100% of the trees and shrubs survived the winter and started growing fast in spring. A wet summer also helped a lot. I was a bit disappointed that the swales never seemed to collect any standing water, despite the abundant rainstorms and the poor percolation rate of our clay soil, but that was ok.

But what happened during the summer got me a bit worried about the manageability of the system. Alfalfa grows very fast, it has to be mowed 3-4 times a year, and on this big an area it has to be done with machines. And that's fine in the inter-swale, where the terrain is suitable for machinery - but on the swale berm, I had to mow it with a scythe. Leaving the swale berm un-mowed would have led to trees being swamped by alfalfa and other herbaceous plants (weeds). Besides, because of a high vole population that threatens the young trees, we have to keep the herbaceous layer mowed.

So... between May and August, I did did A LOT of scything. We thought about inviting neighbours to bring their animals (cows? sheep?) to graze, but (a) browsing by animals would have damaged the trees (b) it would have led to compaction of the swale berm, which is a no-no.

Short of better ideas, we'll keep doing this until the trees are big enough to (hopefully) out-shade most of the herbaceous plants. Very labour intensive, and a long way from the no-work garden that we dreamed of.

Oh and by the way - in other projects we have used mulching of trees very successfully, so we thought that we would use it here as well, especially as alfalfa hay is an excellent mulch. But sadly we had to give up on this idea very quickly - voles love digging holes under the mulch... ! So contrary to what we used to swear by until not long ago, we are now cleaning the ground with a rake right after mowing...

That's all for now, thoughts, comments and ideas are welcome !

Levente

Have you tried any living mulches around the trees like comfrey, hostas, or nasturtium?

I've found that water collects in the swales with horrible soils but not in rich, black soil.

Pics?

Hi,
For what it's worth, Patrick Whitefield hates swales for cold wet climates and prefers ponds instead. I disagree (for now). I don't think it's one or the other.

http://patrickwhitefield.co.uk/one-permacultures-holy-cows-death-swale/

As for your problem (alfalfa on the swale berm), could you swap for clover? Clover grows happily. The other thing we're trying is planting annuals on the berm. It keeps the berm uncompacted and gives us a second reason for weeding. A moterized weed-eater is nice too. I took down 30 sq meters in about 10 minutes. I don't have a good sythe, so this is faster for me.

William

Cj Verde wrote:Have you tried any living mulches around the trees like comfrey, hostas, or nasturtium?

I've found that water collects in the swales with horrible soils but not in rich, black soil.

Pics?

Comfrey and dandelion grows freely all over, including on the swale berms. I have tried to establish self-seeding annuals like phacelia, with very limited success. Not hostas - apparently they are gourmet food for the voles ...

The soil in the swales varies from black, clay-rich soil to yellow, nearly pure clay. Percolation rate is very poor, I would have expected the swale to hold water for a while.

How do you insert photos?

William James wrote:Hi,
For what it's worth, Patrick Whitefield hates swales for cold wet climates and prefers ponds instead. I disagree (for now). I don't think it's one or the other.

http://patrickwhitefield.co.uk/one-permacultures-holy-cows-death-swale/

As for your problem (alfalfa on the swale berm), could you swap for clover? Clover grows happily. The other thing we're trying is planting annuals on the berm. It keeps the berm uncompacted and gives us a second reason for weeding. A moterized weed-eater is nice too. I took down 30 sq meters in about 10 minutes. I don't have a good sythe, so this is faster for me.

William

I agree with your first point, it's not one or the other. In fact, in order to have a pond you might need a swale.

Anyhow, I did weigh the pros and cons of swales on my property.

Pros were the acute need for soaking / keeping more moisture in the soil. The climate here is temperate continental, with hot and potentially dry summers. The last 2 years precipitation has been quite evenly distributed throughout the year, but prior to that, for 2-3 years in a row we had no rain from April to October. Everything was bone dry. I would think swales to be very helpful in such a situation.

Cons were the cost of making the swales, and the lack of experience and expertise locally. We marked out the swales ourselves and directed the backhoe operator who dug them. With hindsight, another downside of swales is that voles love to dig tunnels in them; they help keep the berm's soil nice and loose, but if they could have their way I believe they would flatten the berm within a few years.

I do have the intention to sow white & red clover on the swales, which will hopefully outcompete annual weeds and alfalfa.

As for annual vegetables on the swale berm - we have not started growing annuals yet; when we do start, I suspect swale berms will not be the first place that we'll try, as vegetables growing on swales are too exposed to vole attacks.

Levente

you will probably dislike my answer but i will say it anyway, make friends with the voles and keep mulching as much as you can.

i know the voles, gopher and other underground tunnel diggers are really annoying and i have certainly lost a bunch of stuff due to them, with the approach of just letting them be. but really all in all i havent lost that many plants to tunnelers, and some losses are just to be expected as i see it, and over planting helps.

but it occured to me that they actually work for the earth too, and that we re on the same team, much as they can cause a lot of problems in the short term for establishing plants. they are definitely on team earth =) and have beneficial relationship with the soil in particular.

their holes and tunnels are actually beneficial to soil, although i know this is hard sell to a gardener! but they loosen up the soil, their tunnels eventually fill with water and help with absorption and work against compaction.

actually thats a good way to annoy them, cause even if i try to be friends with them i cant help but wish they would go away! put a hose in their tunnel and use it like a watering system. which annoys them and might make them move on to a non garden bed are if their tunnels get too wet. sometimes i follow their tunnel and turn them instead into open trenches, which helps keep the ground lumpy and absorbing water better.

trying to google some info, and this was the best link i found(besides all the dozens of ways to kill them!), although its on gopher, just as hated by gardeners, but with many of the same benefits to an ecosystem:

http://latimesblogs.latimes.com/home_blog/2011/11/gophers-get-rid-of.html

this looks like a pretty good link:

http://thesagebutterfly.blogspot.com/2012/03/vole-in-garden-control-methods.html

"Environmental Benefits

Although voles can be pests, they do have a role in nature. Their habits and behaviors ensure that nutrients are spread and integrated into the upper layers of soil. Mycorrhizal fungi, a soil nutrient, is dispersed by the vole.

Voles are a food source for many predatory birds, snakes, foxes, and cats."

Here's the pic of a swale from Patrick Whitefield post:

He doesn't appear to have used the swale for any purpose, other than maybe stopping, soaking, spreading. Geoff Lawton stresses that swales are tree planting systems. I don't see any trees and it doesn't really look like there's a berm to create a micro climate.

Levente, I believe you are working much larger areas than I am so my advice may not apply, but our climates must be fairly similar as I too was ravaged by voles when I used straw type mulches, to the point where I gave up on carrots and other root crops in my annual garden for a couple seasons. My solution for my hugel berm/swale plantings is two fold; I use wood chips for mulch around the trees and areas where I want to do some mild weed suppression. This is short term, until my cover crops and annuals and such produce enough veg matter to provide a deep cover of litter from chop and drop and natural die back.

Secondly, and the thing that really made the largest difference for me, is I abandoned my annual garden for the time being, and am doing all of my annual production in the swales around the new trees. That way, my weeding and mulching and watering efforts are focused where I really need them. After a season or two on each swale, I plan on moving on to develop another new planting, and hopefully these will be established and need much less attention as they mature. I have a bit of success to look back on in my old annual garden, where I did develop the lower hugel/berm/swales, albeit with less foresight, and now have an incredibly vigorous raspberry/currant patch with some lovely hazelnuts and plums rising above them. I haven't watered or weeded once down there and it is definitely not reverting back to nettles and willow. If anything, it's all threatening to spread into a food thicket.

At any rate, that's my solution: use your annual plantings to establish your perennials. I made a long rambling video that shows my new plantings, perhaps it will give you some ideas. It's long, I had a lot I wanted to document so I could look back in a couple years, so you will probably want to skip around. It's here....

Cj Verde wrote:Here's the pic of a swale from Patrick Whitefield post
... I don't see any trees and it doesn't really look like there's a berm to create a micro climate.

Yeah! That's one of my beefs with the article. If you're not growing trees, why would you make swales in the first place? Plus "mounding" is a thoroughly acceptable practice in wet, waterlogged soils - for planting trees. A swale is a mound!

The other problem I have is that he says his "swales in brittain" paragraph basically is huge support for the use of swales, listing all types of benefits.

So, for me, the holy cow lives on.

William

edit: that pic might have been taken before planting it up. He says the water inside is rainwater since it was just dug. That being said, the word 'tree' is nowhere in the article. Strange that.

Here' Ben Falk extolling the benefits of swales in a cold, humid environment:

leila hamaya wrote:this looks like a pretty good link:

http://thesagebutterfly.blogspot.com/2012/03/vole-in-garden-control-methods.html

"Environmental Benefits

Although voles can be pests, they do have a role in nature. Their habits and behaviors ensure that nutrients are spread and integrated into the upper layers of soil. Mycorrhizal fungi, a soil nutrient, is dispersed by the vole.

Voles are a food source for many predatory birds, snakes, foxes, and cats."

Thanks for the suggestions. I try to be as relaxed about the voles as possible. And I do believe that one should not be afraid of over-planting - although the locals have already criticised me for this, as they have also for planting non-fruit - i.e., "useless" - trees in the hedge.

Here are a couple of pictures with which I try to illustrate the issue

More to come...

More pictures

Looks beautiful to me. The way i understand it, what you want growing around trees are deep rooted things that so they do not compete with the trees which get nutrients from the top 20% of soil. Alfalfa is deep rooted (mines deeper soil for minerals) and fixes nitrogen and is attractive to beneficial insects. I love the stuff and always let it grow. Alfalfa-good, grass-bad. I would guess voles and a lot of other pests would like alfalfa (better than trees). Looks too me like you are doing great, I'm jealous.

I agree it looks great. You could put a few comfrey around each tree to do extra grass suppression & living mulch. When the trees get bigger it'll be less scary for the weeds to be so high. My nasturtium & comfrey are as tall as my newly planted fruit trees.

Thanks everyone for your feedback and ideas.

Just to sum up the main issue:

Management of herbaceous vegetation around trees growing on a swale system

The choices that I can see at the moment:

- Mow the swales regularly to keep the herbaceous layer from swamping the trees, and reduce the hiding places for rodents that may harm the trees. It is labour intensive, and I wonder what I would do if my swales were longer than they are (total length is about 170 metres / 560 feet).

- Let nature take its course. Trees fending for themselves - survival of the fittest. No mowing, perhaps just some weeding around the trees / chop & drop. Definitely labour-saving. Potentially lower tree survival rate? Lower growth speed? Tall, woody weeds (especially towards end of summer) will be an eyesore for my neighbours...

I've retained suggestions regarding plants for living mulch - comfrey could be particularly good for my system, as it might outcompete some of the taller weeds.

L_

Another choice since you have tree guards on is to borrow some sheep (not goats) for a few days. Or maybe geese.

Cj Verde wrote:Another choice since you have tree guards on is to borrow some sheep (not goats) for a few days. Or maybe geese.

Wouldn't the sheep trample the swale berm, and flatten or compact it?

Some shrubs (hazels) haven't got protection...

My sheep are on the small side so that didn't concern me as much as them eating unprotected plants. Supposedly foraging animals don't like hazelnut leaves but my sheep ate 'em. If you have a friend with sheep & portable electric maybe try a small section? Geese certainly wont compact the berm.

I took some pics this morning with this thread in mind. These trees & friends were planted on a small swale I made this spring. The fruit trees have trunk protection. There are lots of weeds but no grass and growth has been good. The comfrey, horseradish, nasturtium look great. The only thing that did get girdled was a beautiful dill plant but the tree is fine.


Same thing with this apple tree (middle of the pic):


Lots of stinging nettle.
The real test will be next spring but I've been happy with the whole system.

Those trees & friends look really happy...

Levente Andras wrote:
Management of herbaceous vegetation around trees growing on a swale system

My strategy is to pack the place with herbaceous vegetation and use a kama to chop and drop/harvest on occasion. I try to concentrate weeding efforts on single pernicious elements you don't want (for me it's blackberry, if I see it, it's gone) and keeping grass at bay. Bushes also help.

'letting nature take it's course', at least in my situation, means lots of thorny blackberries, kudzu and other vines jumping all over the trees and pulling them down (which is just a part of the forest succession, but one which makes management a pain) and generally not getting the most out of what you've planted. Oh, it also means grass making advances if the place isn't vegetated well enough, especially if you make the mistake of mowing or scything the other, taller plants that compete with grass. I Only chop perennial weeds when they inhibit other work or when I need a path, which will end up being a grassy path, since that's how nature responds to continual chopping.

Hope that helps. Sorry if my previous posts lead the discussion astray. Thanks for getting things back on track!
William

Levente Andras wrote:Those trees & friends look really happy...

I agree. The question is would your trees look that happy if you didn't scythe? I really don't know the answer. You could do test patches and compare but it's getting late in the season. Maybe next year?

The companion plants/weeds are almost as tall as the trees and they could be spurring the trees to grow taller.

If your biggest concern are moles/voles maybe just focus the scything around the unprotected plants?

Levente Andras wrote:

Cj Verde wrote:Have you tried any living mulches around the trees like comfrey, hostas, or nasturtium?

I've found that water collects in the swales with horrible soils but not in rich, black soil.

Pics?

Comfrey and dandelion grows freely all over, including on the swale berms. I have tried to establish self-seeding annuals like phacelia, with very limited success. Not hostas - apparently they are gourmet food for the voles ...

The soil in the swales varies from black, clay-rich soil to yellow, nearly pure clay. Percolation rate is very poor, I would have expected the swale to hold water for a while.

How do you insert photos?

My soil is thick clay. It does hold the water but you can't tell until you step on it. Then you sink in. The clay absorbs it very well but it doesn't percolate down. At least not in my experience.

Hello again,

Six months have passed so I thought I should post an update.

We have had some snow over the winter - not much, the most was probably about 1 ft, at the height of the season - but nearly all of it is already gone now.

It was nice finally to see water (from snow melt, and currently frozen) accumulating in two of the 4 swales. Picture below (taken yesterday).

Update on swales:

This winter we have had a lot of snow - in mid-January our plot was covered by about 1/2 metre. Thawing came abruptly, less than a week ago, with 80% of the snow blanket gone over 3-4 days of warmer weather.

Yesterday the swales filled to the brim, and as the soil also began to thaw, I discovered another of those things that are never mentioned in the swales "owner's manual"...

... I described, in this thread and elsewhere, how voles love to dig into earth mounds such as the swale berms...  Yesterday, for the first time in their 4 years of existence, two of my (otherwise sturdy and reliable) swales were leaking through vole tunnels, with water gushing from 2-3 openings and causing erosion and destruction - in one spot, the jet of water flooded and partially washed away two raised beds in a vegetable garden a few metres downhill from the swale berm.

No need for me to dwell on the obvious hazard that a "punctured" swale can represent - basically, it can behave like a breached dam, with a relatively small hole turning into a huge gap as water gushes through it.

But I think that the very purpose of the swale (i.e., to slow down and help percolate water into the soil) can be negated even if water only gently "seeps" through the base of the berm, through these tiny tunnels.

The permaculture literature available on the subject teaches us (a) to plant and seed the berm as soon as possible so as to stabilise it through vegetation and prevent damage from extreme weather events, and (b) to protect it from trampling by animals (herbivores mainly).  But I have found no mention of the dangers posed by tiny animals that burrow...

Have you considered mulching with a roll of black plastic?  

I have a lot of respect for the way Stefan Sobkowiak has built his orchard and food forest in Quebec --- a climate that might be very similar to yours in Romania.  He uses black plastic in the orchard and the difference between where he does and where he doesn't is night and day.

http://www.permacultureorchard.com/the-farm/



I don't know how voles would respond to plastic, but it certainly would keep the grasses and other plants in the herbaceous jungle from overwhelming your growing trees.  In a couple of years, once the trees have gotten much larger, you wouldn't have to use the plastic any more.  My hunch is that if you used plastic mulch, the roots that the voles like to munch on will disappear within a season.  They'll move on to better eating elsewhere, even it that's just 10 meters away.

Stefan addresses the "its not natural" complaint in this video.   Watch at about 1:25 into the video.



My response:  most of us use plastic all over in our gardens.  PVC pipe to move water, big plastic rain barrels, tools, rope, the soles of our shoes, shade cloth for young trees . . . putting down a sheet of plastic for 3 or 4 years doesn't feel like some sort of horrible sin to me.  It doesn't build the soil like natural mulches, but your concern right now isn't accumulating biomass, it's dealing with all the biomass that you have to continually find a way to mow.

I should have done a bit of digging before I posted that last post.  

Here is a thread from this forum with Stefan answering questions specifically about his black plastic mulch.

https://permies.com/t/36586/Permie-Orchard-talk-plastic-mulch

The biggest advantage he continually comes back to is time saving.  SIGNIFICANT time saving.  In your case, I would imagine that it would amount to hundreds of hours, not to mention all that back-breaking work.

He cuts holes in the plastic sheeting and plants all sorts of stuff within it, so it's not like it's a big plastic desert with nothing growing on it.  He plants annuals and veggies in the holes and they thrive.  He shares that as they mow and blow, the mower throws grass and weeds up onto the plastic, thus cooling it and putting a layer of biomass on top of the plastic.  Over time, that breaks down and those nutrients find their way into the holes or wash off onto the soil beside the plastic sheet.  He talks about the chickens scratching through the mulch but not ripping the plastic.

The key is finding THICK (6 mil) plastic that is UV resistant.

In your climate, I would imagine that the black color would help the soil warm sooner in the spring and hold more heat later in the growing season as well.

Anyhow, check out that thread.

Marco,

I'm not against plastic mulching per se - I've used it in the past, in a different location. However...

The issues I can foresee with plastic in this particular case are:

- trees are not planted in straight rows (in fact, rows are very curvy, they follow the contour of the land); moreover, trees are planted close to the base of the swale berm, which is of irregular shape, not flat, hence fastening the plastic sheet to the ground will be problematic

- the sheet may be - arguably - able to function as mulch (hence weed suppressant) in a strip of certain width, around the base of the tree - BUT that doesn't solve my whole problem, i.e., controlling the herbaceous vegetation on the ENTIRE width (=both sides) of the swale mound.  In other words, while I can suppress weeds in a (narrow) strip around the trees, I would still have to mow (scythe) the rest of the swale berm.  And scything around the edges of a plastic sheet is not fun...

- voles WILL get under the sheet, and I may not be able to see & take action when there is activity underneath

A question mark is whether plastic sheet mulch is appropriate for heavy clay soil - I'm inclined to say no, and I would favour mulches that allow some aeration of the soil's surface (gravel, wood chip)...

In my orchard, the only good vole is a:

O. Donnelly wrote:In my orchard, the only good vole is a:

Couldn't agree more !

Levente- I've had good luck with a ring of peastone mulch around my new trees. About 3-5 inches thick. Suppresses weeds and voles don't like to dig through it.  In this picture I did no weeding the entire summer. Only a few small plants emerged. Notice the difference beyond the mulch...

You could always release a few hundred thousand snakes...but seriously I am surprised nobody is bringing up predators as the solution. It may be too late for you now, or maybe not, but Sepp Holzer uses pigs to eat voles and plough before planting. I know owls are big vole predators here, and I imagine hawks would be too. I would bet Spanish cross or wild turkeys would eat them too, as they eat mice. And of course snakes and weasels and foxes would help too. The key for those would be to have rock piles and brushy thickets respectively. Predators are the only way to control herbivores in the long term sustainably. I also concur that you should look at burrowing animals like giant earthworms, and their tunnels are going to be filled with roots like a hydroponics tube. If you are looking for water infiltration, they are helping like a key line plow.

Ben Zumeta wrote:You could always release a few hundred thousand snakes...but seriously I am surprised nobody is bringing up predators as the solution. It may be too late for you now, or maybe not, but Sepp Holzer uses pigs to eat voles and plough before planting. I know owls are big vole predators here, and I imagine hawks would be too. I would bet Spanish cross or wild turkeys would eat them too, as they eat mice. And of course snakes and weasels and foxes would help too. The key for those would be to have rock piles and brushy thickets respectively. Predators are the only way to control herbivores in the long term sustainably. I also concur that you should look at burrowing animals like giant earthworms, and their tunnels are going to be filled with roots like a hydroponics tube. If you are looking for water infiltration, they are helping like a key line plow.

...My plot is visited occasionally by foxes, weasels, owls, hawks, snakes, and stray cats.  Just the normal mix of wild / semi-wild animals that you would expect in a rural human habitat in my region. They do occasionally kill a vole or two - I have found bodies of rodents half eaten (probably by a cat) and vole nests dug up  (probably by a fox or a weasel).  So... no shortage of helpful predators.

However: normally, predators don't exterminate the entire population of their prey.  They may prevent rodent populations from exploding, but won't make them disappear completely.  Whereas what I need is to be rid of them altogether. In 3 years their population has dropped significantly, but even in reduced numbers, they are waging a very effective guerrilla war against my trees.

O. Donnelly wrote:Levente- I've had good luck with a ring of peastone mulch around my new trees. About 3-5 inches thick. Suppresses weeds and voles don't like to dig through it.  In this picture I did no weeding the entire summer. Only a few small plants emerged. Notice the difference beyond the mulch...

I'm using pea gravel myself, with good results in terms of weed suppression and moisture management around the plant. Not so effective against voles - they still burrow down to the roots, starting outside the gravel and working their way underneath the gravel-covered area...

... Anyhow, the focus of my latest update was actually this type of 'swale failure' of which I have not read or heard elsewhere before experiencing it first-hand.

I'd still be keen to have Permies' feedback / opinions / ideas with respect to this.

Levente Andras wrote:Update on swales:

[...]

Yesterday the swales filled to the brim, and as the soil also began to thaw, I discovered another of those things that are never mentioned in the swales "owner's manual"...

... I described, in this thread and elsewhere, how voles love to dig into earth mounds such as the swale berms...  Yesterday, for the first time in their 4 years of existence, two of my (otherwise sturdy and reliable) swales were leaking through vole tunnels, with water gushing from 2-3 openings and causing erosion and destruction - in one spot, the jet of water flooded and partially washed away two raised beds in a vegetable garden a few metres downhill from the swale berm.

No need for me to dwell on the obvious hazard that a "punctured" swale can represent - basically, it can behave like a breached dam, with a relatively small hole turning into a huge gap as water gushes through it.

[...]

The permaculture literature available on the subject teaches us (a) to plant and seed the berm as soon as possible so as to stabilise it through vegetation and prevent damage from extreme weather events, and (b) to protect it from trampling by animals (herbivores mainly).  But I have found no mention of the dangers posed by tiny animals that burrow...

This may be a completely ignorant question - but how does temperate food forest herbaceous layer differ from subtropical herbaceous layer?  I see above in the thread people continually battling the herbaceous layer, whereas Geoff Lawton in his subtropical food forest, seems to welcome a particular kind of herbaceous layer which he terms "control through rampancy."  Is there not a way to replace the plastic layer with a desirable herbaceous layer?  Is the difference that the above examples are of a "permaculture orchard" and not a food forest, so there is no interest (or no need) for the different layers?

Levente Andras wrote:

However: normally, predators don't exterminate the entire population of their prey.  They may prevent rodent populations from exploding, but won't make them disappear completely.

This is exactly correct.  The usual question is whether that "reduced" population is compatible with the chosen agricultural system.  In your case, I would say that it is not, as you're suffering catastrophic failure of your earthworks.

Perhaps a temporary fix is to create a controlled breach in all of your swales, so that if you have another major flooding episode the deluge will not destroy other parts of your garden.  Or build some kind of "flood gate" that  you could open at will.  Maybe divert it into a pond so that you could use it later?  Swale would loose its ability to capture precipitation in these types of events but would still function under normal conditions.  Then you need to get rid of the voles.  Then seal their tunnels.  Perhaps something like a bentonite slurry would work?  Bentonite, aka montmarilonite, is a "shrink swell clay".  when saturated it will swell to many times its original volume.  Well drillers use it to seal off well casing.  it also works at the bottom of new ponds.  If you have a local well drilling supply shop you may be able to get bags fairly cheap.  Would have to think a bit of how you would get the material in.

I feel for you my man.  it sucks to put in so much effort and then to have something like this happen.  Thanks for posting.  its really helpful experience for someone to hear prior to putting in all that hard labor digging a bunch of swales...