Drought Tolerance: Should Spacing be closer together or further apart?

In my journey of planting a food forest I've often had a very hard time figuring out the rules of tree and shrub spacing. What I've encountered in my research is pretty all over the place. One point of conflict has to do with drought tolerance, and I'd like to hear people's thoughts.

Context for me: I live in western Washington. We get a fair amount of rain but half the year is very dry and hot where I am.The soil dries out and a lot of trees have a hard time without supplemental watering. I'm trying to figure out ways to manage a food forest so that it doesn't need irrigation after establishment. I'm planting into a six acre field that is silt loam. I can't sheet mulch the whole field (too big) but I mulch thickly around trees and am building hugels throughout it to increase water retention.

Some sources say that you should plant trees and shrubs closer together, because that way they will shade eachother better and provide more leaf drop. They say this will help drought tolerance.

Some sources say that you should plant trees and shrubs further apart so that they don't compete too much for the same resources, including water. I know that dryland farming has greater spacing for annuals, so maybe this applies here.

What are your thoughts and experiences, especially if you are in an area that is even drier than me?

Planting different species close together would seem like the natural response.
Low plants shading the roots of tall ones, fruit trees established in the shade of nursery trees, etc.
Then again,  what is nature's response?
Is there a local forest you could model your forest garden after?
Also,  what's your slope like?
Are these rains running off along the top of the soil,   or do they sink in but bake off during dry season?
Could some earthworks help?
I'm guess you can't get wood chips delivered  for free where you are,  or you probably would have done so.
I'm wondering if ponds would help, both as a water storage method and a way to grow mulch.
The lack of clay on the land might make gleying any ponds a real challenge.
Ducks and pigs both are given credit for gleying ponds with their poop,  but ducks seem easier  to deal with.
They also would deal well with any rainy season surge of slugs or snails.

Hi William, thanks for the thoughts.

The local forests are pretty dense once they get going. It's kind of hard to say because it varies, and the forests around here have largely been logged and replanted into fir trees.

I don't have a slope at all. It's pretty much a flat field. The winter water table is high but lowers dramatically in summer. It might not even be the true water table, just the rain taking its sweet time infiltrating the soil. The water pretty much all infiltrates and I'm at the bottom of a bowl of hills, if that makes sense. So that definitely helps keep my property wetter.

I'm hoping to do some ponds, but regulations in Washington are pretty strict. I've been approved for one in the corner of my field near my chicken coop so I'm going to start with that.

Woodchips are hard. I haven't had any luck with them. Usually I have to pay for them or drive a long way to pick them up myself. I'm hoping that maybe I can get lots of comfrey established in my field and do oodles of chop and drop. I'm also having the spaces between my trees mowed and leaving the grass to break down. Hopefully that will help. I'm hoping the hugelbeds will hold water into summer, and I've been planting comfrey, herbs, shrubs, and even some trees into them. I've also been experimenting with burying a little wood with or near my fruit and nut trees, always leaving space between the pieces so the roots can get a good grip

Very interesting,  your water situation.
Given your situation,the reasonable desire to avoid  irrigation infrastructure,  and a lack of outside sources of biomass,  you seem destined to grow your own.

I wonder if there  is a biomass crop that would thrive in between the trees during the wet season, capturing the high water table,  and holding it as mulch during the wet season.
Of course you are already doing this with the existing grasses, I'm not sure if any other plant would be better.
You mentioned comfrey, let me suggest sunchokes,which produce a crap load of biomass.
Sure they spread, but only from the roots.
I find them manageable,but my scale is tiny compared to yours.
Lately I've been mulching paths with the stems to good effect

Too bad about the pond restrictions.
The way you are adding pockets of wood seems as good a solution as any.
Paul had an idea for hidden ponds,  basically ponds filled with rocks, gravel and sand,  in a particular order.
It works out to a wicking bed in many ways.

Anybody else have handy ways to time shift precipitation?

There are two ways mother nature deals with your conditions and there are good examples probably near by in state or national forest areas that you could go and observe.
Your mulches should go a long way towards retaining moisture and I would do all I could to increase the thickness of those mulches.

Nature will either close space trees with no shrubs present except on the outer fringe area or there will be dense ground cover in the form of shrubs and grasses with widely spaced trees.

If you want to grow foods, then you can clump trees, bushes and shrubs by height and that will give some help in water retention.
Or, you could space those trees, bushes and shrubs so that you would have room for vegetables in between them, the vegetables can and should be planted at their "bare minimum" spacing from each other so they are covering the soil with their leaves.

If you want to enrich the soil with organic matter, use a wide variety mix of plants, the broader the spectrum of plant types the better.

Redhawk

In regards to the spacing of trees: it’s best to plant them taking into consideration their root zone and canopy at maturity. For example, a rule of thumb I use for most fruit trees is to plant them far enough apart so when they mature they form a continuous, unbroken canopy, shading the root zone. (For all intents and purposes, the canopy drip line is where the critical root zone exists – feeder roots.)

The spacing is true regardless of them being planted in rows, grids or in groves.

If you plant ‘companion’ plants (small shrubs, ground covers and vines) all at the same time, in the short term they will compete for nutrients and water, so this needs to be applied often during the early stages. However, as all the plants mature, they usually form a symbiosis and don’t need a lot of care afterwards. The understory plants, particularly ground covers, will act as an indicator when it’s too dry e.g. wilting, drooping so then it’s time for everything to receive deep watering.

Spacing of the ‘companion plants’ is fairly intuitive and should be far enough away from the major tree trunk to allow unhindered human access to everything and allow good air circulation for pest management.

Good soil preparation and mulching is critical to catch and hold water (nutrient retention). If you have graded land, swales would really improve things.

Although hugels are only meant for annuals, a pseudo hugel could be made beside each tree to encourage Mycorrhizal production – just a largish hole filled with a mixture of sticks, compost, dried lawn clippings, aged manure, etc.

hau F Agricola, I noticed some statements you made that I'm wondering where you got the information to make, please let me know where you read these.

1. "Canopy drip like is where the critical root zone exists."
* feeder roots are known to exist all along main roots even in mature trees, and it is also well known and documented that feeder roots extend far beyond the "drip line".

2. "A pseudo hugel could be made beside each tree to encourage Mycorrhizal production."
* Mycorrhizae are very specialized and only found around living root systems or inside living root systems. That means that they aren't going to be found in sticks, compost, dried lawn clippings or aged manure.
The main way mycorrhizae spread is by root interaction with the exomycorrhizae from one root, spreading to those in close proximity or via sporulation by human or other animal transport. Endomycorrhizae tend to attach spores to bacteria that then travel along a fungal network.
The other method of a plants roots finding mycorrhizae is via in ground spores.

* to the best of my current knowledge

Redhawk

The other big player in your ecosystem, as well as mine, and one which makes big constraints on design, is wildfire.  In general wildfire resistant design gravitates toward wide spaceing, such that the canopies of mature trees don't touch, and minimal underbrush, so as to minimize the fire-ladder effect.  You want a ground fire to be able to go through the grass and mulch layer and stay there rather than get into the tree tops.....the bark of mature trees of many species can resist such a low-intensity burn.  In most parts of the West, it is not a question of avoiding or preventing all fire, but preparing for it when it comes.  The only exception to this would be a tight, dense planting that is under continuous heavy irrigation, preferably by overhead sprinklers that can be run before (and hopefully during, but don't rely on grid power to do this) a wildfire event.

Consider planting in basins to concentrate the rain around the trees and plants which need it most.  Be sure to put sensitive trees on a mound in the basin to avoid waterlogging during the wet season.



Truly drought-tolerant support trees can be planted between basins.

James, the way I approach this problem is to identify the "lowest common denominator". What is the limiting factor during your period of intended productivity?

Mine, at the moment, is water in mid-summer. We generally get a month in the peak of summer with little rain (I know, most other places are worse...) That is the limiting factor- at the moment.

Later as the system matures, it will be sunlight or root space or something else, and I will have to adapt the system.

If right now you have the same problem, I am a fan of planting for shade. I think the desert swales tend to be really packed in there in Geoff Lawton's look at the Tucson swales. This makes sense to me. Once the soil doesn't dry out in the middle of summer, you can build up some organics (you said you were having trouble with that) and eventually you can thin the overstory and produce in the understory. You could use drought-tolerant plants in the understory, but the selection of trees that are deep-rooted is just way higher than smaller plants. I especially like really heavy shade producers like fig and mulberry with big leaves in the hottest and dryest times. Then you have a spring and late fall crop time when the leaves are less of an issue. My mulberries seem to get pretty ratty by the end of summer and cast way less shade.

The advantage is also that those are both pretty low fire risk. If you keep the local humidity high (and some "deep swales" that oh no turned into pocket ponds) you can have pretty high humidity for a good part of the summer, especially if the ponds are shaded. If those same "deep swales" happened to get a layer of clay on the bottom it would be terrible, just awful.  

Alder Burns wrote:The other big player in your ecosystem, as well as mine, and one which makes big constraints on design, is wildfire.  In general wildfire resistant design gravitates toward wide spaceing, such that the canopies of mature trees don't touch, and minimal underbrush, so as to minimize the fire-ladder effect.  You want a ground fire to be able to go through the grass and mulch layer and stay there rather than get into the tree tops.....the bark of mature trees of many species can resist such a low-intensity burn.  In most parts of the West, it is not a question of avoiding or preventing all fire, but preparing for it when it comes.  The only exception to this would be a tight, dense planting that is under continuous heavy irrigation, preferably by overhead sprinklers that can be run before (and hopefully during, but don't rely on grid power to do this) a wildfire event.

That's true. I've often wondered if permaculture has a strategy for fire breaks. Fire is a big consideration for me, in that I plant my trees in a grid so I can keep things mowed (and also for general accessibility)

F Agricola wrote:
If you plant ‘companion’ plants (small shrubs, ground covers and vines) all at the same time, in the short term they will compete for nutrients and water, so this needs to be applied often during the early stages. However, as all the plants mature, they usually form a symbiosis and don’t need a lot of care afterwards. The understory plants, particularly ground covers, will act as an indicator when it’s too dry e.g. wilting, drooping so then it’s time for everything to receive deep watering.

Spacing of the ‘companion plants’ is fairly intuitive and should be far enough away from the major tree trunk to allow unhindered human access to everything and allow good air circulation for pest management.

I've been wondering:
Longer term, I'd like to not have to water very often, or at all (say, maybe maximum one month deep watering). As I've mentioned, in summer things get REALLY hot and dry for quite a while (for me, "really hot" means 80's and up). Are there ground covers and shrubs that are drought tolerant enough to take that? I hesitate to do a lot of guild or companion planting as I don't want to have to irrigate the whole field forever and ever. I don't mind watering things to establishment, though. Do you have any ground covers or shrubs that you'd recommend?

William Bronson wrote:
I wonder if there  is a biomass crop that would thrive in between the trees during the wet season, capturing the high water table,  and holding it as mulch during the wet season.
Of course you are already doing this with the existing grasses, I'm not sure if any other plant would be better.
You mentioned comfrey, let me suggest sunchokes,which produce a crap load of biomass.
Sure they spread, but only from the roots.
I find them manageable,but my scale is tiny compared to yours.
Lately I've been mulching paths with the stems to good effect

Sunchokes are good for bees, right? I've been meaning to give them a try. I don't mind if they spread, so long as it's not too invasive. I want to plant things that grow like weeds once established

Tj Jefferson wrote: If those same "deep swales" happened to get a layer of clay on the bottom it would be terrible, just awful.  

Actually, my subsoil a few feet down is clay, so forming ponds, if I can get a way to do it without running afoul of an agency, should be pretty easy. The smallish one I'm digging now (the one I have permission for) stays full pretty well on its own. I'll probably use ducks to gley it at some point, just to be sure the seal is good

The ideal place for a tree is actually at the edge of a forest. On one side, ideally the south side, your tree gets sunshine. Behind it, there is cool, moist air, reducing stress on the tree, along with a layer of topsoil, and soil that doesn't bake and dry out in the hot dry sun. The tree gets the best of both worlds, cool moist forest air and soil and lots of sunlight to grow.

It's also a question of how big your trees are to start with and how long you intend to water them. In an open space, you'll often find seedlings growing underneath the shade of a mature tree, but not out in the open. The seedlings need the shade of the mature tree, but the mature tree shades the soil over its own roots and doesn't need any other trees nearby to survive. If you're going to buy 4 foot tall bareroot trees from a nursery and space them correctly based on the size the rootstock will allow, they might only need a few years of regular watering in the open before they shade their own soil and then need little or no care. If you're starting from seed that's going to be a bigger project and take a lot longer to get them to where they shade their own soil from the baking hot sun.

It's also a question of what you companion plant them with. For example, Pines will grow a deep taproot if the soil allows it and won't compete with other trees for the same water. Siberian Peashrub isn't know for exceptionally deep roots, but it fixes nitrogen making it available for other plants to help offset the cons of any water loss, and can form a lower level canopy below apples, cherries etc. which will help stop the soil from baking in the sun. I currently have many seedling apples, pears, and wild plums growing alongside Siberian Pea Shrub in the same containers under my grow lights, and will plant them outside together when they're big enough.

in relation to fire, creeping succulents are good (what Americans call iceplants). Plant a firebreak at least 3 yards wide around the forest and allow them to creep under trees, which will also help with shading the ground. Look out for local species, ours are often invasive in the US. You never need water once established and some bear edible fruits or have leaves with herbal applications. I chop mine every year just before the rains come, they make a lovely compost in the middle at the bottom of the heap - not where they can regenerate because then they will

Yeah, sunchokes are basically sunflowers with edible roots, I never remembered seeing bees on them , so I looked for information...and found a Permies  thread!
are jerusalem artichokes good for bees?

James,

You've mentioned a couple times that you eventually want to be able to leave your system alone without irrigation. That is precisely my goal as well. The main way to do this is to plant strictly native. Identify the food species that are native or at least adapted and just go with those.

I live in a climate that gets zero rain for eight months at a time--and temperatures well over 100. And I have a thriving food forest in my yard. I water some of the high value imports like lemon and guava, but there are plenty of native fruit trees I don't water.

Secondly, in answer to your question. Plant them close together. Make sure there are pioneer species among them that can be sacrificed when they start competing for space.

Think of it this way: the main thing is to accumulate organic material, both for fertility and moisture retention. How is that going to happen? You're going to grow it. Every square inch of your property is potential production space--that is, solar driven mulch factory. So fill it up. If you're filling it up with native species then you're not watering more than you want to and they can be sacrificed for the important stuff along the way.

One really helpful species for me is pigeon pea (cajanus cajan). They keep producing well over four months after the last drop of rain. They're a delicious protein, nitrogen fixer, mulcher, and semi-perennial (4 to 6 years). They reseed themselves, but are not hard to get rid of when you don't want them anymore. Find something like that for the first stage of your succession and you will be well on your way.

-Nathanael

hau James,
Nature has an entire set of plants, from trees to grasses that are fire resistant/ fire tolerant/ fire dependent and these are the plants one finds in areas where fire is the dominant disturbance.
Pines such as the ponderosa, Redwoods such as the Giant and Sequoia, and some of the firs, any species that drop their lower branches are tree species that live in fire prone areas for example.
Many grasses are fire dependent and their seeds only sprout after a fire event.

Now consider that permaculture is about working with nature or mimicking nature and thus we come up with a pine nut, or blue berry farm as a good fit for a fire prone area.

Redhawk

Spacing should be more dense and hight should be unrestricted, central leader pruning or no pruning at all. You want a large root system. Pruning the trees like a normal orchard reduces life span of trees and it affects how deep the roots can get. I have been experimenting with opuntia cacti as mulch. It is inflamable and holds water accessable any time of the year.

I would space long-lived perennials according to the low end of their expected size range, should I want a canopy soonest, and one that is completely closed.

This is not what I see as being the optimal situation for me.

I like the idea of alley cropping my food forests, wherein they are planted on-contour in wide hugels, with alleys of perennial pasture or pasture in rotation with field crops in the alleys, and market gardens closer to my zone 0.

This accords with the idea of creating texture on the land (tefa, if that's still a thing), reducing wind dessication and forming the land to slow the passage of fallen precipitation. I like the idea of using double-layers of large stone mulch, where available, to act as distributed air wells, so that moisture in warm air condenses out on hidden under-layers of stone, and I like to place them lower on the slopes of my food forest strips. This has the advantage of shading out the base of the mounds with stones, where the shade might fail, especially on the sunward slope.

My largest design thus far incorporates a path on the crown of each mound, mainly for access, but also to the end of providing shaded space up high and literally at the top of the system for more stacks or rows of larger stones to act as air wells. In areas with high humidity that occurs at times of decreased rainfall, as in Toronto most summers, for instance, air wells can really contribute to the water supply.

I would use self-seeding annuals or perennials with a lifespan shorter than the maturity of the tree species in question to fill space early on, until the young forest reaches a level of maturity that chokes other things out. I would also select based on fondness for sunlight. If a plant, especially a sun-loving food-bearing plant, is a perennial, I will fit it in based on the expected size of the trees around it at maturity. If it does well in the shade, like hazels or mulberries, I will put them in areas that will be shaded out soonest.

Shade can be a planning tool. If the temporary infill plants, even food-bearing ones, are sensitive to shade, the closing canopy will gradually remove them as they are no longer necessary.

-CK

Bryant RedHawk wrote:hau F Agricola, I noticed some statements you made that I'm wondering where you got the information to make, please let me know where you read these.

1. "Canopy drip like is where the critical root zone exists."
* feeder roots are known to exist all along main roots even in mature trees, and it is also well known and documented that feeder roots extend far beyond the "drip line".

2. "A pseudo hugel could be made beside each tree to encourage Mycorrhizal production."
* Mycorrhizae are very specialized and only found around living root systems or inside living root systems. That means that they aren't going to be found in sticks, compost, dried lawn clippings or aged manure.
The main way mycorrhizae spread is by root interaction with the exomycorrhizae from one root, spreading to those in close proximity or via sporulation by human or other animal transport. Endomycorrhizae tend to attach spores to bacteria that then travel along a fungal network.
The other method of a plants roots finding mycorrhizae is via in ground spores.

* to the best of my current knowledge

Redhawk

G'day Dr RH,

Apologies for not responding earlier - work (employment) is a drag sometimes!

Anyway, getting back to the discussion: in regards to the drip zone/feeder root matter; although my comment related predominantly to fruit trees, the major feeder roots exist between the trunk and the drip zone of the foliage. The majority of nutrients and water are taken up via these roots. A good example are citrus – although typically shallow rooted, deep watering and fertilizing within the drip zone maximises uptake and hopefully yield.

Sure, it’s not definitive. The notion has been around for a long time, most gardeners except it verbatim. Finding scholarly articles on the subject is somewhat of a challenge (though, probably based on my lack of enthusiasm to search for them). I did find:

Fertilizing Trees and Shrubs

Critical Root Zone

Uni California

In regards to Mycorrhiza, yep, I kinda screwed up in the delivery of the data, thanks for correcting the comment. A momentary lapse in sanity on my behalf! There is a lot of research involving the topic - when is Mycorrhiza and Saprophytic (wood decay) fungi not exactly Mycorrhiza or Saprophytic, and how do they change. It seems that wood can contain Mycorrhiza sp, and that plant roots can contain cross-over species that mimic it like Saprophytic fungi.

‘Apparently, the evolutionary step between saprophyte and mycorrhiza is relatively short, and it seems likely that some species presently are in the middle of taking this step.’

Uni Sweden

Microbial Ecology (A long read)

An interesting topic, seems nature isn’t as clear cut as first thought.

Regardless, it just goes to prove (if we needed validation) that the greater the volume of carbon rich stuff we put into the soil, the better the outcome.

[Man, some people are quick-draws with the 'thumbs down' decision!]

James Landreth wrote:I've been wondering:
Longer term, I'd like to not have to water very often, or at all (say, maybe maximum one month deep watering). As I've mentioned, in summer things get REALLY hot and dry for quite a while (for me, "really hot" means 80's and up). Are there ground covers and shrubs that are drought tolerant enough to take that? I hesitate to do a lot of guild or companion planting as I don't want to have to irrigate the whole field forever and ever. I don't mind watering things to establishment, though. Do you have any ground covers or shrubs that you'd recommend?

Had to smile about the definition of hot being 80F (26C) and up. That’s a nice winter’s day for us i.e. this summer has been a killer, with consecutive weeks above 40C (104F). I think it peaked at 48C (118F) in the shade.

In such vicious heat and humidity, evaporation is obviously extreme, so it’s Gardening 101, like others have mentioned: it’s all preparation work to begin with, condition the soil with organic matter (compost, wood chips, sticks, etc), apply a deep bed of mulch and keep it moist to get that ecology going.

Setting it up is the hard work. Ideally, once it’s established the work should reduce exponentially.

Note: if the summers are hot and dry, best to do the prep work and get the soil conditioned, leave till the next spring to plant. By that time the soil ecology should be buzzing along nicely. I assume your soil freezes over winter, so it may take a few seasons to get things rolling.

Nathanael’s suggestion is good if you have edible indigenous plant species. However, using a combination of local ones and those from a similar climate would give you more choice.

Plant suggestions for your area – ‘Western Washington State’? No idea whatsoever. Try Googling ‘Western Washington State vegetables/fruit’. At a complete guess, say: oregano/mint/chives/parsley as ground covers, carrots/beets/potatoes as root crops, cabbage/cauliflower, beans as a climber, apple/pears/stone fruit/cherry trees as middle and high canopies.

Possibly include dwarf varieties so you have that middle canopy layer beneath larger varieties. If you think about those choices and arranging them as companions/rotational, it may make pest management easier. E.g. pear and cherry slug (sawfly larvae).

Things like creating microclimates, etc come into play too.

When it all gets established, and depending on rainfall patterns, the occasional deep watering is probably all that it will need besides a bit of weeding, trimming and throwing around some compost, mulch and organic fertilizer.

Also, don't forget the humble chook, duck or geese - mobile manure and pest managers.