Drainage material in pots.

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Umbrian

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Drainage material in pots.
« on: September 01, 2012, 07:30:52 AM »
I was very interested to read ,in the September issue of "The Garden", that recent research into the benefits of placing crocks or gravel in the base of pots to aid good drainage may well be counterproductive. The theory that water passes rapidly through coarse materials seems to be untrue, indeed "a sudden change in particle size causes water to remain in the soil above the drainage level....."
I have always placed one or two crocks in the base of pots and in recent years, with the universal use of plastic pots and subsequent lack of broken terra cotta pieces to use in this way, have been putting a generous layer of gravel in my pots before adding the compost. On several occasions I have noticed that the compost/soil in pots where the plant has started to fail is saturated and smells "sour" indicating that the drainage is poor. Since I always top-dress my pots with fine gravel to prevent undue evaporation it is not easy to see how damp or dry the pot is when watering.
The article concludes  (amongst other recommendations)that "a single shard to prevent roots from blocking the drainage hole" is more beneficial and so I shall revert to this practice. Do any other members of the Forum have ideas on this subject?
MGS member living and gardening in Umbria, Italy for past 19 years. Recently moved from my original house and now planning and planting a new small garden.

Daisy

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Re: Drainage material in pots.
« Reply #1 on: September 01, 2012, 08:21:59 AM »
I only put one shard over each drainage hole in pots. Then it is easier to makes sure it doesn't move out of position when adding the compost.

Umbrian. How do you have the September issue of The Garden already? I am still waiting for the August issue. ::) ::) ::)
Daisy :)
Amateur gardener, who has gardened in Surrey and Cornwall, England, but now has a tiny garden facing north west, near the coast in north east Crete. It is 300 meters above sea level. On a steep learning curve!!! Member of both MGS and RHS

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MikeHardman

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Re: Drainage material in pots.
« Reply #2 on: September 01, 2012, 09:30:29 AM »
I haven't read the article yet, but it is not a new discussion. If the material above the drainage layer is suitable, and if the drainage layer itself is suitable, water will be retained in just the same way that a kitchen sponge does despite your holding it in the air. The critical issue is those 'suitabilities'. Consider if the upper layer is the same material as the lower one - say all gravel - then it will all drain well. If it is all clay, it will drain badly (shrinkage aside). I think old clay crocks make a good drainage layer because the material is very easily wettable (by the layer above), and hence can 'conduct' water downwards. ...If the upper layer wants to let it go. If the upper layer is naturally water-retentive, a drainage layer below it won't help that much.
Back to conducting...
I think one has to think 'outside the pot', as it were. Below the drainage layer, there is the hole(s) in the pot and the pot base itself, and then whatever the pot is standing on (soil, concrete, legs, etc.). It is not much use to have a drainage layer that conducts water itself if the water has no easy way to leave the drainage layer. The gap constituted by the hole in the pot can be a barrier to flow! Yes, it will let water out under the pressure of its own weight, but it can stop the slower drainage driven by surface tension. If the pot is (unglazed) clay, resting on soil or capillary matting, it will conduct water away OK.

So, IMO, if you want to promote good drainage in a pot of soil:
1. Make sure the growing medium is itself at least fairly well drained.
2. Connect the growing medium to a water exit outside the pot, ie. use an unglazed pot resting on soil, with or without a drainage layer; if using a drainage layer, ensure it is not water repellent (eg. polystyrene); if it is not possible to rest the pot on the soil and/or not possible to use an unglazed pot, then put a strip of capillary matting through the hole and lead it down or across to some soil or other absorbent exit.
3. Allow for immediate drainage of excess water under its own weight by having the pot hole(s) open to air - which can mean lifting a pot off the soil (obviously contradictory to point 2), but capillary matting can make the connection again. I have some old clay pots which have the drainage holes at the sides rather than underneath - they are ideal for allowing pot-soil contact while keeping the holes open to air. But you see few of them these days.
4. Consider other factors. It can be a good idea to use perforated zinc over the drainage hole - to keep weevils, larvae and some other critters out. One can do so while still having a strip of capillary matting in place. Put a disc of capillary matting in the bottom of the pot in addition to the strip - helps gather water from across the pot and if the potting medium is prone to shrinkage (hence watering water going down the side), it can help re-wet the medium.
(Note: capillary matting can become less hydrophillic as it ages, partly due to the chemicals it has absorbed over time; it may need replacing. Also, roots can insinuate cap. matting, which might be an issue at repotting time.)

That's off the top of my head; my apologies if I have forgotten something obvious.
« Last Edit: September 01, 2012, 09:37:00 AM by MikeHardman »
Mike
Geologist by Uni training, IT consultant, Referee for Viola for Botanical Society of the British Isles, commissioned author and photographer on Viola for RHS (Enc. of Perennials, The Garden, The Plantsman).
I garden near Polis, Cyprus, 100m alt., on marl, but have gardened mainly in S.England

Jill S

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Re: Drainage material in pots.
« Reply #3 on: September 01, 2012, 10:30:39 AM »
I also have, for years, used a layer of gravel in the bottom of my pots. Having read the article I've begun to wonder if I should continue, but perhaps I've got away with it in the past because I havn't been generous with the amount. The potting compost itself has always been mixed with a good dollop of gravel/grit anyway. It's just a habit of very long standing, certainly my pots don't get waterlogged, if anything just the opposite, Oh for a happy median!!
Jill 
Member of RHS and MGS. Gardens in Surrey, UK and, whenever I get the chance, on Paros, Greece where the learning curve is not the only thing that's steep.

Umbrian

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Re: Drainage material in pots.
« Reply #4 on: September 02, 2012, 07:16:44 AM »
Regarding your "add-on" about delivery of "The Garden". I too normally receive my copy very late in the month and could hardly believe it when the September issue came at the end of August. I have often thought of complaining about the late delivery but somehow have never got round to it but now it has been demonstrated that it is possible for us overseas to recieve it in good time I think I will try to find time to write and point this out! ???
MGS member living and gardening in Umbria, Italy for past 19 years. Recently moved from my original house and now planning and planting a new small garden.

Umbrian

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Re: Drainage material in pots.
« Reply #5 on: September 02, 2012, 07:23:04 AM »
Mike - as usual your comprehensive reply to my posting about drainage material in pots was mind boggling ;D
To have access to knowlegable minds like yours is what makes this forum so interesting and invaluable to members such as myself. Thank you for finding the time to write such postings. :)
MGS member living and gardening in Umbria, Italy for past 19 years. Recently moved from my original house and now planning and planting a new small garden.

Umbrian

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Re: Drainage material in pots.
« Reply #6 on: September 02, 2012, 07:30:19 AM »
Jill,
Like you I always make my own mix for potting up things and always add either fine grit or very coarse sand. Sometimes the quality of the compost I can buy here leaves a lot to be desired and my soil is too clayey to add whereas in the UK I always added some soil to the mixture too. I think that over the years I have become  too generous with the layer of grit in the bottom and like Daisy am going to revert to one shard over the hole. :)
PS. Are you "Oxford! Jill?
MGS member living and gardening in Umbria, Italy for past 19 years. Recently moved from my original house and now planning and planting a new small garden.

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Alisdair

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Re: Drainage material in pots.
« Reply #7 on: September 02, 2012, 10:38:04 AM »
Last year Fleur told this forum that her own tried and tested method for keeping rooted cuttings or nursery-bought plants in pots over the summer, and preventing them getting so pot-bound that it was hard to plant them out, was to put a layer of gravel in the bottom of the pot - click here to see her full description.
Since then Helena has been following Fleur's method with equally good results, and hasn't had any problems with the compost above the gravel getting either waterlogged or too dry.
In clay pots I originally used to put a layer of broken crocks in the bottom, and did have the problems this Garden article now describes. For that reason I switched first to a single crock over the drainage hole (better, but still some compacting or waterlogging problems unless the compost was very free-draining), and then to either a small patch of close-woven shade netting or, better still in my own experience, a firm little lump of semi-hardened organic material, either "clotted" leafmould or peat. (This doesn't keep out Mike's creepy-crawlies or worms, though!)
Gradually over time I've tended to switch away from clay to plastic pots, always with a very free-draining compost, and no drainage material in the bottom whatsoever.
An alternative approach to pots is Olivier Filippi's style - deep square-sided pots with indented sides and a mesh base, designed to direct roots downwards instead of going round and round. These are made by
Bamaplast of Italy (clicking on the Bamaplast link, you'll see that they come in a range of sizes from half to two and a half litres, and in five colours). They do work very well.
Alisdair Aird
Gardens in SE England (Sussex); also coastal Southern Greece, and (in a very small way) South West France; MGS member (and former president); vice chairman RHS Lily Group, past chairman Cyclamen Society

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ritamax

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Re: Drainage material in pots.
« Reply #8 on: September 02, 2012, 07:54:08 PM »
Linda Chalker-Scott has good arguments again, this time against the drainage in pots:  http://www.puyallup.wsu.edu/~linda%20chalker-scott/Horticultural%20Myths_files/Myths/Container%20drainage.pdf
Hobbygardener (MGS member) with a rooftop garden in Basel and a garden on heavy clay with sand 600m from seaside in Costa Blanca South (precipitation 300mm), learning to garden waterwise

Jill S

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Re: Drainage material in pots.
« Reply #9 on: September 02, 2012, 10:50:44 PM »
I think I'll continue to use my old routine, probably trying out the addition of Daisy's single sherd. Most plants seem happy enough with it, even though it means that I have to water pots perhaps more frequently than I would without my addition of gravel.

'Oxford' ??, Umbrian. Not for more years than I would care to shake a small, pointy, trowel at! What context?
Jill     
Member of RHS and MGS. Gardens in Surrey, UK and, whenever I get the chance, on Paros, Greece where the learning curve is not the only thing that's steep.

Umbrian

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Re: Drainage material in pots.
« Reply #10 on: September 03, 2012, 08:06:45 AM »
Just thought you might be the Jill I met quite some time ago now through the MGS and who's address I have lost. Sorry, nothing cryptic  :)
MGS member living and gardening in Umbria, Italy for past 19 years. Recently moved from my original house and now planning and planting a new small garden.

Jill S

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Re: Drainage material in pots.
« Reply #11 on: September 03, 2012, 09:38:55 AM »
Sorry it's not me!, perhaps the real Oxford Jill will join the forum, read this and get in touch. Luck, Jill
Member of RHS and MGS. Gardens in Surrey, UK and, whenever I get the chance, on Paros, Greece where the learning curve is not the only thing that's steep.