how close to a tree line can I make a swale?

The best location for swales is on the higher contours of a property, of which mine has mature trees. The concern is damaging the root systems and either damaging or killing the trees. Would appreciate some input on this from others' knowledge and experience.

Alas, I only have basic knowledge and no experience, I'm sure someone will come along with the more experience. I have just finished a couple of PDC chapters which had lots of information on trees, rainfall, Swales and run-off. The mature trees are going to absorb 95% of rain fall, either through recycling, absorption in the leaf litter, bacterial gels, mycelia etc. I doubt there's going to be very much runoff for the woods under normal conditions. (If you're thinking of Swales next to your trees, then maybe that's not your experience). What do you want to use the Swales for? If it's for building up ground water and planting trees on the raised soil, then how big would the new trees be in 20 years? You don't want to be too close the existing trees. What kind of mature trees do you have? Does that species have surface roots that extend out beyond the crown?

Before you go down the route of swales, do you actually need them in that area? They are often viewed as a tool for getting trees established, as they concentrate water into the root zones of young trees. If you already have established trees then they probably don't need the swales, and there is unlikely to be surface water flows to intercept. My experience has been that the areas around trees tend to be very good at sinking rainwater anyway - probably due to the higher soil carbon content than eg grassy areas.

Do you observe surface water flowing in that area at times of high rainfall?

I was thinking I could just post a straightforward question and get a straightforward answer, but maybe not! Lol

Goal: I'm formulating my plans for swales from information for Bill Mollison's online PDC, which I'm currently taking. Yes, they are considered tree growing systems, but also (especially in combination with ponds) they are a means of hydrating the land. According to Bill, swales can rehydrate soil up to 100 meters downhill of the swale. The area I want hydrated are my pastures, so that's what I'm focusing on here.

Climate and regional factors: I live in the southeastern US, where we have horrifically hot summers (typically in the upper-90s from May through August), often with long dry spells, sometimes lasting 6 weeks or more without rain. My soil is Cecil sandy loam, which is a light brown sandy topsoil with red clay subsoil. So, I have topsoil that has low water holding capacity, coupled with hot dry summers that quickly evaporate any remaining moisture out of the soil. Consequently, my pastures dry out badly in summer, with large surface cracks often forming. I'm finding it difficult to establish and maintain good forage for my goats.

What's been a challenge, is that our property is kind of the opposite of the examples in my PDC. The advice is to start catching water as high as possible, and hold it as long as possible, through a series of swales and ponds, so that the water can be gravity fed for house, livestock, irrigation, etc. Our house and outbuildings are at the highest elevation points. Currently, we have about 4300 gallons of rainwater catchment, most of which is used for irrigation during our dry spells and watering livestock. But we still lose a lot of the roof runoff. Last week we got 8 inches of rain over six days, most of which ran off the property.

An aerial shot of our place in winter. The yellow Xs are the highest points, with elevation lines are every 4 feet. Pastures are the cleared areas surrounding zone 1.

In that photo you can see what we're working with. The trees surrounding the house and outbuildings are all mature, mostly pecans, a few oaks, and a patch of figs. The green contour line outlining the treeline is where we'd like to put the topmost swale. Roof runoff that we can't catch, could be directed into it. We're planning to start with the pasture on the righthand side of the photo, and would like that swale to be as high up as possible without damaging the trees.

I have experience in this area.
If the swale is too high it may not catch much water, what is the neighbours land fall like, any benefit to you?
Irrigating from tanks is not smart unless you have 200,000L of water, otherwise it runs out quickly.
200,ooo L tanks cost about $A16000
But you certainly need to have more tanks on the house for domestic water and I would recommend 20,000L tanks only.
They are reasonable value of $ per 1000L
If you run first flush filter systems you will have great water

Having an aerator roller for the pasture will help a lot images of aerator rollers
They open the ground allowing air and water to penetrate, thus promoting pasture growth.

You can also berm on contour to slow and infiltrate water. A swale doesn't have to be 2ft deep. Just 6inches also work.

I forgot about berms, but look at the Yowmans Keyline contour work.
Its a process developed by an Australian whereby the land was ploughed along contour lines to keep all water that lands on the land by helping it stay and go into the soil.
Natural Sequence Farming is another process.
It involves slowing water down so it soaks into the soil.

What size is your property?

John, we definitely plan to add more rain catchment tanks as budget allows. Budget is (always) our confining factor. On the other hand, if we can direct roof runoff from the house and outbuildings into our top swales, I think we'll do a lot to hydrate the land. The first swale will be above the garden (right side of the house in my photo). It may take a few years, but if what Bill Mollison says is true, then we can decrease the need to use rain tank water for the garden during dry spells.

Something else Bill discussed in one of his lectures is the Wallace plow, which does similar to the keylines you mention. I cannot find anything like a Wallace plow in the US, but my husband bought a sub-soiler (soil ripper) and made a series of cuts on contour in our various paddocks. The sub-soiler isn't as elegant as the Wallace, but I tossed in some seed, and now we're observing that the soil in these furrows remains moist even when the surface soil is drying out. This is very happy progress for us.

S. Bengi, good point about depth. I feel that we at least need to get into the clay subsoil, but our topsoil at the highest contours isn't very deep.

Can I throw another suggestion into the mix?

You are establishing pasture for goats, and it sounds like part of the problem is the low soil carbon and moisture holding capacity of your soil.

Have you ever looked into vetiver grass? When planted as a dense hedge on contour it acts rather like a swale. It slows and sinks surface water flows, and traps sediment on the uphill side, building soil. The vetiver trimmings can be used as animal fodder, bedding, or mulch. You are right on the edge of the usual growing region, but you might be able to make it work. It is a tropical grass, but is grown successfully from zone 7 to zone 11.



Here you can see the soil build up uphill of a vetiver hedge due to it trapping and slowing water, carrying sediment.

USDA Plant Hardiness Zone Map for Vetiver Grass Plants

The USDA considers vetiver adapted to Zones 9 and higher. It may be used in warmer zone 8 locations with heavy mulching and  additional care during winter.
From here.

I so very much wish it grew in zone 7.

You might look into the Miscanthus grasses. There is at least one sterile variety out there, if invasiveness is one of your concerns. I want to  establish some here, for mulch production.  I like the 5 to 7 foot height range.
webpage

Michael and Joylynn, the idea of using clump type grasses like this definitely has some merit for possibly controlling  and directing rain runoff. We have some native grasses that might suit, if I can establish them in the right places. Since this will be in pasture, the goats will keep them under control, assuming they don't eat it to death! I'm still going to plan on swales, but this might help in strategic locations.

Joylynn Hardesty wrote:
I so very much wish it grew in zone 7.
webpage

I have seen varying reports on it's hardiness, and I know that there are people growing it here in Europe. I enquired of a european vetiver facebook group after I saw your comment. Specifically, I asked if they knew of successful projects in the UK.

The feedback I got was that it CAN work in our climate, but the difficulty is getting it sufficiently well established before the first frost. I think that timing of the initially planting is key. Get this right and you will be dealing with reasonably large plants with well established roots by the autumn frost. If you then do a cut in late autumn and use the cuttings to mulch the row you can add a bit of further frost protection.

I think it would be worth an experiment. If it works, we learn something.

Might be worth keeping a nursery pot going as well, so you can move a parent plant into a sheltered green house etc. in the winter.

My understanding is that vetiver can survive freezing temperatures, but struggles with prolonged periods with frozen ground.

How about laying branches on contour with brush, woodchips, etc on the uphill side. If your have any erosion it'll catch on the branches and make a mini terrace. Alternatively if you have any digging projects you might have soil left over that you could make swales out of. A lot of work, though.

Also, it seems like you could use a pond to catch the overflow from your cistern.

Here is some very good info about Keyline and how it helps the soil
How sub soil is improved with keyline ploughing

Michael Cox wrote:I think it would be worth an experiment. If it works, we learn something. . . . My understanding is that vetiver can survive freezing temperatures, but struggles with prolonged periods with frozen ground.

I'm all for experimenting. Apparently, vetiver grass must propagated with transplants. So for me, it would boil down to availability and price. A quick search led me to a site that has a limited supply of plants for sale: 5 plants for ~ 20 USD with free shipping and bigger discounts for larger quantities.

Daniel Kaplan wrote:How about laying branches on contour with brush, woodchips, etc on the uphill side. . . .
Also, it seems like you could use a pond to catch the overflow from your cistern.

Daniel, yes to both of those suggestions. I've been using branches, etc., at the bottom of small ridges to form humus beds like Sepp Holzer does.

Of a combination of swales and ponds, Bill Mollison thinks that by devoting 15% of one's land mass to these, that one can completely drought-proof one's property in several year's time.

John C Daley wrote:Here is some very good info about Keyline and how it helps the soil
How sub soil is improved with keyline ploughing

John, thank you for the link! Lots of good information there. I feel like this is the least we can do.

Mollison's context was the Australian conditions - hugely variable nationwide, but water conservation is so important. It's important to balance his general recommendations with the specifics of your situation. I don't know anything about the OP's climate.

Back on the vetiver. I reached out to a european supplier, who has supplied for UK based projects. They consider vetiver hardy, in our conditions provided it has properly established. Some of the above ground growth usually dies back in winter, but the roots send up fresh shoots in the spring.

Michael Cox wrote:Mollison's context was the Australian conditions - hugely variable nationwide, but water conservation is so important. It's important to balance his general recommendations with the specifics of your situation. I don't know anything about the OP's climate.

Well, like I said earlier...

Leigh Tate wrote:Climate and regional factors: I live in the southeastern US, where we have horrifically hot summers (typically in the upper-90s from May through August), often with long dry spells, sometimes lasting 6 weeks or more without rain. My soil is Cecil sandy loam, which is a light brown sandy topsoil with red clay subsoil. So, I have topsoil that has low water holding capacity, coupled with hot dry summers that quickly evaporate any remaining moisture out of the soil. Consequently, my pastures dry out badly in summer, with large surface cracks often forming. I'm finding it difficult to establish and maintain good forage for my goats.

So, I think swales would apply to my context as well. The vetiver? Mmm, maybe, since I'm fringe to it's preferred growing zones. I'm willing to experiment, but not replace the concept of swales.

How deep is your soil? In my area, when soil is very close to bedrock, tree roots go further, and are more sensitive to disturbance. Sounds like you have pretty deep soil though and closer to the surface roots might already be stressed from drought and dry soil?

A 6" swale  i'd be comfortable closer to trees. Not sure how close,

A larger/deeper swale I'd want to be well out of the drip line of the trees.  Maybr 1.5 x the drip line? Trees can recover from some root disturbance but typically have roots 2-3x the radius of the drip line.

This feels like a question for walking the land with a spade. Walk, think about where you might like a swale, put the spade in the ground and hop on the top of it. Try that in a few areas. Do you hit roots? How often?

Catie George wrote:A larger/deeper swale I'd want to be well out of the drip line of the trees.  Maybr 1.5 x the drip line? Trees can recover from some root disturbance but typically have roots 2-3x the radius of the drip line.

This feels like a question for walking the land with a spade. Walk, think about where you might like a swale, put the spade in the ground and hop on the top of it. Try that in a few areas. Do you hit roots? How often?

Leigh, I have read your question several times.

I always come away with more questions.

How are you making the swales?  What equipment are you using, etc.

I really like Catie's answer as I feel it comes the closest to what I feel is the answer. Especially, the part about the drip line and walking your land.

When dear hubby makes swales he uses a hoe or something similar and the swales are about a foot from the tree and are shallow so maybe only 3" deep. I didn't get the impression that is the type of swale you are wanting.

Anne Miller wrote:How are you making the swales?  What equipment are you using, etc.

Anne, we will probably have to rent something to dig the swales. That will depend on what's available and what it costs. So, it feels like it's a good idea to gather as much information as possible and have a pretty good plan in place, to make efficient use of rental time.

I really like Catie's answer as I feel it comes the closest to what I feel is the answer. Especially, the part about the drip line and walking your land.

I agree, very helpful. Thanks Catie! Your questions are ones I need to get out in the field and answer.

Any updates on the swales?

I did not realize the age of this request.  I think the post has been answered.  Please delete if possible.

Leigh Tate wrote:The concern is damaging the root systems and either damaging or killing the trees. Would appreciate some input on this from others' knowledge and experience.

Leigh,

I believe the best guide would be the drip line of the tree.  The general rule of root growth is the feeder roots will spread to the diameter of the branches, or drip line.  Not a perfect correlation, but a good rule of thumb.

I would not be too concerned with getting closer than the drip line on mature trees.  You may get some roots in the cut, but the trees will still have a lot of support.  I am not an arborist, but would feel confident cutting in the top 12" inches of soil out to about 2/3 of the drop line of a mature tree.  

Jack, I definitely appreciate your response, even a year later. The discussions on permies are meant to be perpetual, for several reasons. One is that a new reply helps bring the thread back to the top of the forums' 'most recent' lists. This is important because the same questions arise over and over, as new folks join, or as others take on new projects. It may help someone else who recently has the same question.

Another reason is that there is never a one-size-fits-all answer to permaculture problems and questions. So much depends on one's location, weather patterns, resources, ecosystem, etc.. Considerations in one situation may or may not affect another situation.

Then too, when I find more than one person leaning toward the same solution, there's a sense of affirmation toward it. On the other hand, I appreciate unique solutions too! Permaculture is continually evolving as we incorporate it into more aspects of human life.

So, please! Never hesitate to jump into an older conversation. It's something we appreciate here on permies.

To address the original question:

A long time ago I was taught that the active root zone of a tree is approximately 50% of its height.

In practice, I find that to be consistently true. My 2c.