Troughs

[johnw]

Paddy  - I sometimes put an old well-rotted piece of sod (and devoid of worms) on the bottom of the troughs for plants that like a little rich living deep down.  The Diapensia likes to get its feet down into it, doesn't inhibit the drainage so much.

johnw

[Lori S.]

...putting the troughs up on some sort of support. I think this is essential. Not much point in having the good compost, good drainage holes but leaving the trough on the ground where the drainage holes could be blocked up. I place  my troughs on small building bricks. For some purchased troughs I used small pottery feet to raise them.

The necessity of this point is a function of the how much rainfall one is having to offset by providing drainage... not essential in this dry climate (and as I mentioned, I'm not sure if it might even be detrimental not to have the troughs in contact with the mass of the ground in our extreme winters).*

(*NB  And I only mention this since the comment was aimed directly at Cohan, who shares this wretched climate, LOL!)

[Paddy Tobin]

Lori,

You describe two conditions which never occur here - extreme dryness and freezing winters. You have to work the troughs your own way.

Paddy

[johnw]

it might even be detrimental not to have the troughs in contact with the mass of the ground in our extreme winters).*

(*NB  And I only mention this since the comment was aimed directly at Cohan, who shares this wretched climate, LOL!)

Lori  - Even here in Zone 6 we move the troughs off their supports in December and set them on the ground to avoid freeze drying in particularly cold stretches.  We also put a clear plastic shield about 2 feet above them to avoid excess rain but the snow can still blow in and over the plants.  I don't know if this business is necessary here but until it becomes to much of an effort we won't take any chances.

Paddy - I have seen too many exploded styro troughs in this climate and don't like the look of styro in the trough mix or hypertufa mix. Troughs weigh a tonne as our mix seem toi be like yours, mainly sharp granite grit.

When Malcolm MacGregor was here we had a discussion about sharp versus rounded stone to improve drainage. He was in the round camp and I am definitely in the other. Knowing Malcolm he may only be challenging long held ideas or was it the ale?

johnw

[Lori S.]

Thank you for the comments, John... that's very interesting and helpful to hear, as it is something I've wondered about.  I have noticed less wintering-over success in the raised troughs I have, which are situated out in open areas, but have never been able to separate out the variables (less hardy plants?, too dry?, random chance?, unwitting offense to the gardening gods?)  

The highest possible permeability and porosity (i.e. the most perfect drainage) is achieved by having well sorted (i.e. all the same size) and the most rounded particles (with the ideal for rounding being perfect spheres).  (The foregoing is basic reservoir geology/geotechnical fact.)  Theoretically, and where it can actually be achieved (say, in sand beds or in real rock), rounded particles provide better perm/porosity, i.e. drainage.  

However, and this is a big however, in any mix that consists of a range of particle size, the smaller particles will fill the pore spaces between the larger ones... potentially (I'm strongly inclined say probably) rendering the argument largely moot!    

[Lesley Cox]

Interesting and helpful note John, about the Diapensia. Mine is growing nicely in a small plastic pot but I've been wondering what to do with it as it's past time it was moved out. I have some troughs ready to plant and a few rotted turfs too. (Any excuse to get rid of more grass ) Would the same regime be suitable for shortias do you think? (Incidentally, your little package arrived yesterday. So much seed in it. Will be sown today, a million thanks.)

[johnw]

Interesting and helpful note John, about the Diapensia. Mine is growing nicely in a small plastic pot but I've been wondering what to do with it as it's past time it was moved out. I have some troughs ready to plant and a few rotted turfs too. (Any excuse to get rid of more grass ) Would the same regime be suitable for shortias do you think? (Incidentally, your little package arrived yesterday. So much seed in it. Will be sown today, a million thanks.)

Lesley - The Diapensias in Newfoundland grow on hummocks of grit over very wet heavy soil. If you dig one up you find a long "root" and would think they require pure scree.  However upon deeper probing and closer examination the "root" is almost like a trunk that goes down a foot or so to heavy wet soil - a constant supply of moisture - and that's where the roots are, in the mud.  They are diifficult to dig as you get a plant with a ball of root mass in mud dangling a foot below.  Think carrying an old extra long dachsund.  The difficulty I presume with seedling is to make sure they don't dry out in the scree above until they get their roots down to the soil in the trough deep down. Under the scree I do a mix with coarse grit, gradually mixing with more & more soil until the sod. Have some now close to 18 years old and a sporadic flower every few months.  Plenty of wind for them.

Steve Doonan was the master of Shortias in North America and he recommended soil the seed on 50/50 fine granite grit/sand and coarse peat. It has worked for me when sown immediately and the pot placed in a plastic bag under lights at 20c. They come up in 3 weeks and can sometimes be desperately slow and finicky to harden off when they leave the bag when the size of the tip of your finger.   Steve's method is close to one a Danish shortia guru uses. I would like to hear of others methods.

johnw

[johnw]

Thank you for the comments, John... that's very interesting and helpful to hear, as it is something I've wondered about.  I have noticed less wintering-over success in the raised troughs I have, which are situated out in open areas, but have never been able to separate out the variables (less hardy plants?, too dry?, random chance?, unwitting offense to the gardening gods?)  

The highest possible permeability and porosity (i.e. the most perfect drainage) is achieved by having well sorted (i.e. all the same size) and the most rounded particles (with the ideal for rounding being perfect spheres).  (The foregoing is basic reservoir geology/geotechnical fact.)  Theoretically, and where it can actually be achieved (say, in sand beds or in real rock), rounded particles provide better perm/porosity, i.e. drainage.  

However, and this is a big however, in any mix that consists of a range of particle size, the smaller particles will fill the pore spaces between the larger ones... potentially (I'm strongly inclined say probably) rendering the argument largely moot!    

Lori  - This is opposite to what Steve Doonan had to say in his lecture about measuring and increasing drainage in potting mixes.  His lessons - the deeper the pot the faster the flow;  always sharp grit as water tends to slick around rounded stones, sort of like surface tension. Thoughts anyone?

I tried rounded roofing gravel once with disastrous results.

johnw

[Lori S.]

His lessons - the deeper the pot the faster the flow;  always sharp grit as water tends to slick around rounded stones, sort of like surface tension.

"The deeper the pot, the faster the flow."  Yes, no problem with that one (and nothing I said previously commented on this principle).  This is simply using the force of gravity to cause drainage (the higher you raise the level of the water in a sealed container, the more "head" or energy to flow there is when you pull the bung out of the bottom of the barrel)... exactly the same principle as setting a trough up on blocks, or, a more exaggerated example, on setting the town water tower up on high stilts.

"...always sharp grit as water tends to slick around rounded stones, sort of like surface tension..."  Sorry, I don't know of any physical principle that supports this notion (assuming I understand what it is trying to say).  Surface tension exists wherever  a fluid such as water contacts a solid, and applies to both rounded particles and to angular particles; because porosity (space between particles = pore space) and permeability are maximized with rounded particles of the same size, the effect of surface tension (I think it's being used here in the sense of holding water in place?) is minimized, and the amount of water that can drain through is maximized.    Spheres fit together with what is called "perfect packing"; each sphere contacts each other sphere at only a few discrete points (6?), meaning they create the maximum porosity and permeability, and the minimum ability to hold water by surface tension (due to those very few and tiny points of contact).  ("Sharp" means angular, as opposed to rounded.) By contrast, the angles on sharp particles project into the spaces between the adjacent similar particles (which would be void space if the particles were round) and thus they reduce the size of the pore spaces, and so reduce the porosity and hence the permeability.  Also, with sharp particles, those projecting bits cause a larger surface area of the grain to be in contact with other grains, so its ability to hold water by surface tension is increased, hence the ability of water to flow through it is decreased.

The other part of this to keep in mind is that the coarser the well-sorted (rounded) particles are, the higher the permeability (the ability to flow water through it).  Conversely, even if well-sorted, the finer the particles, the lower the permeability.

Here are some examples* of hydraulic conductivity/"permeability" measurement (using a particular standard formula) through sands with different grain sizes and sorting:

Very fine sand, poorly sorted         40-80 cm/s

Fine sand with appreciable fines     40-80

Medium sand, well sorted                80-120

Coarse sand, poorly sorted               80-120

Coarse sand, well sorted, clean        120-150

*http://www.geology.sdsu.edu/classes/geol552/grainsize/Kest.htm

So, the above example shows that the ability of a sand to flow water through it (i.e. to drain) improves with coarseness of the particles and with improved sorting (moving toward all particles being the same size).  (I don't have an example relating to rounding vs. angularity at hand, but can look for one if necessary.)

Anyway, this is a basic notion that gardeners have always had cocked up, so I hope this helps a bit to explain it.  If not, don't worry about it, because as I said, in any mix that consists of a range of particle size, the smaller particles will fill the pore spaces between the larger ones, decreasing the permeability, increasing the water held by surface tension, and "probably" IMO rendering the argument largely moot as to whether the particles should be sharp or round.  Theoretically, to preserve as much permeability as possible, given the poor sorting that we create in a mix, all particles should be as rounded as possible.
 
Note that all of the above talks about the flow of water through porous media, not about growing plants... I'm sure it's safe to conclude that the two are not precisely the same thing, LOL!

[johnw]

Lori - Thanks for that.  I recall Steve mentioned the granite grit he used for mixes was averaging 2-4 mm with some smaller and larger pieces but always well washed. back then granite grit was very difficult to find here despite this coast being granite.  A few years ago we found a source but their material needs sifting and washing.

It would be interesting to hear the preferences of forum members re: coarse vs round stone.

johnw

[Rodger Whitlock]

Suddenly the discussion has gotten *really* interesting.

Drainage of water from a saturated soil depends on two forces: gravity, which drives the water downwards; and capillarity, which pulls it upwards. Capillarity in turn is controlled by the average spacing of the particles in the soil, that is, the void size. I will assert that in a first approximation the critical factor is the size of the finest component of the soil, as that controls the size of the voids between soil particles. A mix of 1" spheres and .001" spheres will have voids much the same size as purely .001" spheres and they will be much smaller than if you had nothing but 1" spheres.

At the bottom of a container full of soil, the water table is perched: it has nowhere to go to, and capillarity will keep it from dripping out. You can overcome this limitation in several ways, the choice depending on the circumstances.

• Provide a connection to the soil so it can wick away the excess moisture (capillarity again). IOW, put your pot or trough on the ground.
• A variant of the first method, wringing a pot into a bed of sand
• Inserting a length of twine or coarse string into the drain hole so as to lower the position of the perched water table. I have used this method in hanging baskets to ensure that they drain freely even if the soil mix in them is less than ideal. It looks a little odd, but it works.
• Using a deeper container (Steve Doonan's recommendation).

Another point is that spent potting soil has had much of the silt and other fines washed out of it, hence will drain better than freshly mixed stuff. My coldframes, which rest on a concrete slab, invariably develop a layer of fine silty mud in the bottom washed out of the pots as I water them. This year, I have recycled all my spent potting soil (it's full of expensive grit, perlite, and pumice!), and after screening and mixing, it impresses me as being grittier and less clumpy than when first mixed.

Finally, angular vs rounded aggregate. Assuming that we are comparing two samples of aggregate of the same average size, both quite uniform in size composition, I can well believe that irregular, angular particles will pack more tightly than rounded (i.e. near spherical) ones. Hence the angular aggregate would drain less freely than rounded aggregate that's otherwise comparable.

As it happens, I can conduct an experiment along these lines as I have a large supply of very evenly sized, fine, rounded gravel and can buy granite grit that is the same size and equally evenly sized, but which is angular. I will try to conduct the experiment within the next week and report back.

[olegKon]

Dear friends,
Thank you so much for nice words about the trough and good advice
Oleg

[Lesley Cox]

Without going into any of the scientific aspects, in general I prefer "gritty," or sharp edged grit and gravel, in both pots and troughs, not so much as an aid or otherwise to drainage, but because the roots of alpine plants seem to prefer it, the roots clustering around, apparently attracted to, the sharp edges. Cuttings too, root very strongly in sharp grit as opposed to rounded or worn grit.

[cohan]

Cohan,

I think Lori made two most important points re troughs - the compost must be really free draining, such that water poured from a watering can would run through it. I add loads of grit, to such an extent that the compost visually seems to have more grit than anything else. Secondly, the drainage holes must be good and large - no point in having good draining compost only for the water to lodge at the bottom of the trough.

Also, Lori mentioned putting the troughs up on some sort of support. I think this is essential. Not much point in having the good compost, good drainage holes but leaving the trough on the ground where the drainage holes could be blocked up. I place  my troughs on small building bricks. For some purchased troughs I used small pottery feet to raise them.
I'm not an enthusiast of using styrofoam in the mixture. This will have no value whatsoever for the plants. Granted the troughs are very heavy - it is important to place them where you want them before filling them even, perhaps, to construct them where or very near where you are going to place them. You can add to the drainage by building up stone, crevice style for instance, on top of the trough and then the plants will have a deeper root run and more compost under them.
Have fun.
Paddy

thanks for the input, paddy, i have experimenting ahead...
the styrofoam is not in the mix--just some large pieces in the bottom and along the sides (not right to the top)..i thought, if anything, i was using too much soil, especially after listening to friends with some very successful bowl type plantings of hardy and seasonal succulents--they fill much of the pot with onion bags of styrofoam peanuts, and 'soil' only for a couple of inches on top...however, they are in milder climates, which is why i did put much more soil than they do....

[cohan]

interesting discussion of permeability and particle shape..
in cactus and succulent circles, the conventional wisdom is that irregular particles are better, but it is actually quite logical that rounder particles will fit together less tightly...
i wonder if there is some reason, as lesley suggests, that plants like irregularly shaped particles, confusing growers into thinking that it was a drainage issue...

lori's point is also notable that particle sizes are unlikely to be uniform--i think bonsai growers very carefully screen for particle size... indoors, while cacti and succulents dont want to stay soggy, i have never had any problem with drainage, since water only comes when i apply it, which is never often enough..
outdoors the issues are very different, we arent 'normally' wet, but the last few years have certainly shown you cant expect weather/climate here to be remotely consistent! i hesitate to go super draining, as i am not fond of watering plants outdoors...lol